[feature] Implement following hashtags (#3141)

* Implement followed tags API

* Insert statuses with followed tags into home timelines

* Test following and unfollowing tags

* Correct Swagger path params

* Trim conversation caches

* Migration for followed_tags table

* Followed tag caches and DB implementation

* Lint and tests

* Add missing tag info endpoint, reorganize tag API

* Unwrap boosts when timelining based on tags

* Apply visibility filters to tag followers

* Address review comments

docs/api/swagger.yaml

@@ -2916,6 +2916,12 @@ definitions:
         x-go-package: github.com/superseriousbusiness/gotosocial/internal/api/activitypub/users
     tag:
         properties:
+            following:
+                description: |-
+                    Following is true if the user is following this tag, false if they're not,
+                    and not present if there is no currently authenticated user.
+                type: boolean
+                x-go-name: Following
             history:
                 description: |-
                     History of this hashtag's usage.
@@ -6439,7 +6445,7 @@ paths:
                     - read:accounts
             summary: Get an array of all hashtags that you currently have featured on your profile.
             tags:
-                - featured_tags
+                - tags
     /api/v1/filters:
         get:
             operationId: filtersV1Get
@@ -6834,6 +6840,58 @@ paths:
             summary: Reject/deny follow request from the given account ID.
             tags:
                 - follow_requests
+    /api/v1/followed_tags:
+        get:
+            operationId: getFollowedTags
+            parameters:
+                - description: 'Return only followed tags *OLDER* than the given max ID. The followed tag with the specified ID will not be included in the response. NOTE: the ID is of the internal followed tag, NOT a tag name.'
+                  in: query
+                  name: max_id
+                  type: string
+                - description: 'Return only followed tags *NEWER* than the given since ID. The followed tag with the specified ID will not be included in the response. NOTE: the ID is of the internal followed tag, NOT a tag name.'
+                  in: query
+                  name: since_id
+                  type: string
+                - description: 'Return only followed tags *IMMEDIATELY NEWER* than the given min ID. The followed tag with the specified ID will not be included in the response. NOTE: the ID is of the internal followed tag, NOT a tag name.'
+                  in: query
+                  name: min_id
+                  type: string
+                - default: 100
+                  description: Number of followed tags to return.
+                  in: query
+                  maximum: 200
+                  minimum: 1
+                  name: limit
+                  type: integer
+            produces:
+                - application/json
+            responses:
+                "200":
+                    description: ""
+                    headers:
+                        Link:
+                            description: Links to the next and previous queries.
+                            type: string
+                    schema:
+                        items:
+                            $ref: '#/definitions/tag'
+                        type: array
+                "400":
+                    description: bad request
+                "401":
+                    description: unauthorized
+                "404":
+                    description: not found
+                "406":
+                    description: not acceptable
+                "500":
+                    description: internal server error
+            security:
+                - OAuth2 Bearer:
+                    - read:follows
+            summary: Get an array of all hashtags that you currently follow.
+            tags:
+                - tags
     /api/v1/instance:
         get:
             operationId: instanceGetV1
@@ -9072,6 +9130,103 @@ paths:
             summary: Initiate a websocket connection for live streaming of statuses and notifications.
             tags:
                 - streaming
+    /api/v1/tags/{tag_name}:
+        get:
+            description: If the tag does not exist, this method will not create it in the database.
+            operationId: getTag
+            parameters:
+                - description: Name of the tag (no leading `#`)
+                  in: path
+                  name: tag_name
+                  required: true
+                  type: string
+            produces:
+                - application/json
+            responses:
+                "200":
+                    description: Info about the tag.
+                    schema:
+                        $ref: '#/definitions/tag'
+                "400":
+                    description: bad request
+                "401":
+                    description: unauthorized
+                "404":
+                    description: not found
+                "406":
+                    description: not acceptable
+                "500":
+                    description: internal server error
+            security:
+                - OAuth2 Bearer:
+                    - read:follows
+            summary: Get details for a hashtag, including whether you currently follow it.
+            tags:
+                - tags
+    /api/v1/tags/{tag_name}/follow:
+        post:
+            description: 'Idempotent: if you are already following the tag, this call will still succeed.'
+            operationId: followTag
+            parameters:
+                - description: Name of the tag (no leading `#`)
+                  in: path
+                  name: tag_name
+                  required: true
+                  type: string
+            produces:
+                - application/json
+            responses:
+                "200":
+                    description: Info about the tag.
+                    schema:
+                        $ref: '#/definitions/tag'
+                "400":
+                    description: bad request
+                "401":
+                    description: unauthorized
+                "403":
+                    description: forbidden
+                "500":
+                    description: internal server error
+            security:
+                - OAuth2 Bearer:
+                    - write:follows
+            summary: Follow a hashtag.
+            tags:
+                - tags
+    /api/v1/tags/{tag_name}/unfollow:
+        post:
+            description: 'Idempotent: if you are not following the tag, this call will still succeed.'
+            operationId: unfollowTag
+            parameters:
+                - description: Name of the tag (no leading `#`)
+                  in: path
+                  name: tag_name
+                  required: true
+                  type: string
+            produces:
+                - application/json
+            responses:
+                "200":
+                    description: Info about the tag.
+                    schema:
+                        $ref: '#/definitions/tag'
+                "400":
+                    description: bad request
+                "401":
+                    description: unauthorized
+                "403":
+                    description: forbidden
+                "404":
+                    description: unauthorized
+                "500":
+                    description: internal server error
+            security:
+                - OAuth2 Bearer:
+                    - write:follows
+            summary: Unfollow a hashtag.
+            tags:
+                - tags
     /api/v1/timelines/home:
         get:
             description: |-

internal/api/client.go

@@ -32,6 +32,7 @@ import (
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/featuredtags"
 	filtersV1 "github.com/superseriousbusiness/gotosocial/internal/api/client/filters/v1"
 	filtersV2 "github.com/superseriousbusiness/gotosocial/internal/api/client/filters/v2"
+	"github.com/superseriousbusiness/gotosocial/internal/api/client/followedtags"
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/followrequests"
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/instance"
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/interactionpolicies"
@@ -46,6 +47,7 @@ import (
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/search"
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/statuses"
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/streaming"
+	"github.com/superseriousbusiness/gotosocial/internal/api/client/tags"
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/timelines"
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/user"
 	"github.com/superseriousbusiness/gotosocial/internal/db"
@@ -59,7 +61,7 @@ type Client struct {
 	processor *processing.Processor
 	db        db.DB
 
-	accounts            *accounts.Module            // api/v1/accounts
+	accounts            *accounts.Module            // api/v1/accounts, api/v1/profile
 	admin               *admin.Module               // api/v1/admin
 	apps                *apps.Module                // api/v1/apps
 	blocks              *blocks.Module              // api/v1/blocks
@@ -71,6 +73,7 @@ type Client struct {
 	filtersV1           *filtersV1.Module           // api/v1/filters
 	filtersV2           *filtersV2.Module           // api/v2/filters
 	followRequests      *followrequests.Module      // api/v1/follow_requests
+	followedTags        *followedtags.Module        // api/v1/followed_tags
 	instance            *instance.Module            // api/v1/instance
 	interactionPolicies *interactionpolicies.Module // api/v1/interaction_policies
 	lists               *lists.Module               // api/v1/lists
@@ -84,6 +87,7 @@ type Client struct {
 	search              *search.Module              // api/v1/search, api/v2/search
 	statuses            *statuses.Module            // api/v1/statuses
 	streaming           *streaming.Module           // api/v1/streaming
+	tags                *tags.Module                // api/v1/tags
 	timelines           *timelines.Module           // api/v1/timelines
 	user                *user.Module                // api/v1/user
 }
@@ -117,6 +121,7 @@ func (c *Client) Route(r *router.Router, m ...gin.HandlerFunc) {
 	c.filtersV1.Route(h)
 	c.filtersV2.Route(h)
 	c.followRequests.Route(h)
+	c.followedTags.Route(h)
 	c.instance.Route(h)
 	c.interactionPolicies.Route(h)
 	c.lists.Route(h)
@@ -130,6 +135,7 @@ func (c *Client) Route(r *router.Router, m ...gin.HandlerFunc) {
 	c.search.Route(h)
 	c.statuses.Route(h)
 	c.streaming.Route(h)
+	c.tags.Route(h)
 	c.timelines.Route(h)
 	c.user.Route(h)
 }
@@ -151,6 +157,7 @@ func NewClient(state *state.State, p *processing.Processor) *Client {
 		filtersV1:           filtersV1.New(p),
 		filtersV2:           filtersV2.New(p),
 		followRequests:      followrequests.New(p),
+		followedTags:        followedtags.New(p),
 		instance:            instance.New(p),
 		interactionPolicies: interactionpolicies.New(p),
 		lists:               lists.New(p),
@@ -164,6 +171,7 @@ func NewClient(state *state.State, p *processing.Processor) *Client {
 		search:              search.New(p),
 		statuses:            statuses.New(p),
 		streaming:           streaming.New(p, time.Second*30, 4096),
+		tags:                tags.New(p),
 		timelines:           timelines.New(p),
 		user:                user.New(p),
 	}

internal/api/client/featuredtags/get.go

@@ -34,7 +34,7 @@ import (
 //
 //	---
 //	tags:
-//	- featured_tags
+//	- tags
 //
 //	produces:
 //	- application/json

internal/api/client/followedtags/followedtags.go

@@ -0,0 +1,43 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package followedtags
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	"github.com/superseriousbusiness/gotosocial/internal/processing"
+)
+
+const (
+	BasePath = "/v1/followed_tags"
+)
+
+type Module struct {
+	processor *processing.Processor
+}
+
+func New(processor *processing.Processor) *Module {
+	return &Module{
+		processor: processor,
+	}
+}
+
+func (m *Module) Route(attachHandler func(method string, path string, f ...gin.HandlerFunc) gin.IRoutes) {
+	attachHandler(http.MethodGet, BasePath, m.FollowedTagsGETHandler)
+}

internal/api/client/followedtags/followedtags_test.go

@@ -0,0 +1,104 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package followedtags_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/suite"
+	"github.com/superseriousbusiness/gotosocial/internal/api/client/followedtags"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+	"github.com/superseriousbusiness/gotosocial/internal/email"
+	"github.com/superseriousbusiness/gotosocial/internal/federation"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/media"
+	"github.com/superseriousbusiness/gotosocial/internal/processing"
+	"github.com/superseriousbusiness/gotosocial/internal/state"
+	"github.com/superseriousbusiness/gotosocial/internal/storage"
+	"github.com/superseriousbusiness/gotosocial/testrig"
+)
+
+type FollowedTagsTestSuite struct {
+	suite.Suite
+	db           db.DB
+	storage      *storage.Driver
+	mediaManager *media.Manager
+	federator    *federation.Federator
+	processor    *processing.Processor
+	emailSender  email.Sender
+	sentEmails   map[string]string
+	state        state.State
+
+	// standard suite models
+	testTokens       map[string]*gtsmodel.Token
+	testClients      map[string]*gtsmodel.Client
+	testApplications map[string]*gtsmodel.Application
+	testUsers        map[string]*gtsmodel.User
+	testAccounts     map[string]*gtsmodel.Account
+	testTags         map[string]*gtsmodel.Tag
+
+	// module being tested
+	followedTagsModule *followedtags.Module
+}
+
+func (suite *FollowedTagsTestSuite) SetupSuite() {
+	suite.testTokens = testrig.NewTestTokens()
+	suite.testClients = testrig.NewTestClients()
+	suite.testApplications = testrig.NewTestApplications()
+	suite.testUsers = testrig.NewTestUsers()
+	suite.testAccounts = testrig.NewTestAccounts()
+	suite.testTags = testrig.NewTestTags()
+}
+
+func (suite *FollowedTagsTestSuite) SetupTest() {
+	suite.state.Caches.Init()
+	testrig.StartNoopWorkers(&suite.state)
+
+	testrig.InitTestConfig()
+	config.Config(func(cfg *config.Configuration) {
+		cfg.WebAssetBaseDir = "../../../../web/assets/"
+		cfg.WebTemplateBaseDir = "../../../../web/templates/"
+	})
+	testrig.InitTestLog()
+
+	suite.db = testrig.NewTestDB(&suite.state)
+	suite.state.DB = suite.db
+	suite.storage = testrig.NewInMemoryStorage()
+	suite.state.Storage = suite.storage
+
+	suite.mediaManager = testrig.NewTestMediaManager(&suite.state)
+	suite.federator = testrig.NewTestFederator(&suite.state, testrig.NewTestTransportController(&suite.state, testrig.NewMockHTTPClient(nil, "../../../../testrig/media")), suite.mediaManager)
+	suite.sentEmails = make(map[string]string)
+	suite.emailSender = testrig.NewEmailSender("../../../../web/template/", suite.sentEmails)
+	suite.processor = testrig.NewTestProcessor(&suite.state, suite.federator, suite.emailSender, suite.mediaManager)
+	suite.followedTagsModule = followedtags.New(suite.processor)
+
+	testrig.StandardDBSetup(suite.db, nil)
+	testrig.StandardStorageSetup(suite.storage, "../../../../testrig/media")
+}
+
+func (suite *FollowedTagsTestSuite) TearDownTest() {
+	testrig.StandardDBTeardown(suite.db)
+	testrig.StandardStorageTeardown(suite.storage)
+	testrig.StopWorkers(&suite.state)
+}
+
+func TestFollowedTagsTestSuite(t *testing.T) {
+	suite.Run(t, new(FollowedTagsTestSuite))
+}

internal/api/client/followedtags/get.go

@@ -0,0 +1,139 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package followedtags
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
+	"github.com/superseriousbusiness/gotosocial/internal/paging"
+)
+
+// FollowedTagsGETHandler swagger:operation GET /api/v1/followed_tags getFollowedTags
+//
+// Get an array of all hashtags that you currently follow.
+//
+//	---
+//	tags:
+//	- tags
+//
+//	produces:
+//	- application/json
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- read:follows
+//
+//	parameters:
+//	-
+//		name: max_id
+//		type: string
+//		description: >-
+//			Return only followed tags *OLDER* than the given max ID.
+//			The followed tag with the specified ID will not be included in the response.
+//			NOTE: the ID is of the internal followed tag, NOT a tag name.
+//		in: query
+//		required: false
+//	-
+//		name: since_id
+//		type: string
+//		description: >-
+//			Return only followed tags *NEWER* than the given since ID.
+//			The followed tag with the specified ID will not be included in the response.
+//			NOTE: the ID is of the internal followed tag, NOT a tag name.
+//		in: query
+//	-
+//		name: min_id
+//		type: string
+//		description: >-
+//			Return only followed tags *IMMEDIATELY NEWER* than the given min ID.
+//			The followed tag with the specified ID will not be included in the response.
+//			NOTE: the ID is of the internal followed tag, NOT a tag name.
+//		in: query
+//		required: false
+//	-
+//		name: limit
+//		type: integer
+//		description: Number of followed tags to return.
+//		default: 100
+//		minimum: 1
+//		maximum: 200
+//		in: query
+//		required: false
+//
+//	responses:
+//		'200':
+//			headers:
+//				Link:
+//					type: string
+//					description: Links to the next and previous queries.
+//			schema:
+//				type: array
+//				items:
+//					"$ref": "#/definitions/tag"
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'404':
+//			description: not found
+//		'406':
+//			description: not acceptable
+//		'500':
+//			description: internal server error
+func (m *Module) FollowedTagsGETHandler(c *gin.Context) {
+	authed, err := oauth.Authed(c, true, true, true, true)
+	if err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorUnauthorized(err, err.Error()), m.processor.InstanceGetV1)
+		return
+	}
+
+	if _, err := apiutil.NegotiateAccept(c, apiutil.JSONAcceptHeaders...); err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorNotAcceptable(err, err.Error()), m.processor.InstanceGetV1)
+		return
+	}
+
+	page, errWithCode := paging.ParseIDPage(c,
+		1,   // min limit
+		200, // max limit
+		100, // default limit
+	)
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	resp, errWithCode := m.processor.Tags().Followed(
+		c.Request.Context(),
+		authed.Account.ID,
+		page,
+	)
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	if resp.LinkHeader != "" {
+		c.Header("Link", resp.LinkHeader)
+	}
+
+	apiutil.JSON(c, http.StatusOK, resp.Items)
+}

internal/api/client/followedtags/get_test.go

@@ -0,0 +1,125 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package followedtags_test
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+
+	"github.com/superseriousbusiness/gotosocial/internal/api/client/followedtags"
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
+	"github.com/superseriousbusiness/gotosocial/testrig"
+)
+
+func (suite *FollowedTagsTestSuite) getFollowedTags(
+	accountFixtureName string,
+	expectedHTTPStatus int,
+	expectedBody string,
+) ([]apimodel.Tag, error) {
+	// instantiate recorder + test context
+	recorder := httptest.NewRecorder()
+	ctx, _ := testrig.CreateGinTestContext(recorder, nil)
+	ctx.Set(oauth.SessionAuthorizedAccount, suite.testAccounts[accountFixtureName])
+	ctx.Set(oauth.SessionAuthorizedToken, oauth.DBTokenToToken(suite.testTokens[accountFixtureName]))
+	ctx.Set(oauth.SessionAuthorizedApplication, suite.testApplications["application_1"])
+	ctx.Set(oauth.SessionAuthorizedUser, suite.testUsers[accountFixtureName])
+
+	// create the request
+	ctx.Request = httptest.NewRequest(http.MethodGet, config.GetProtocol()+"://"+config.GetHost()+"/api/"+followedtags.BasePath, nil)
+	ctx.Request.Header.Set("accept", "application/json")
+
+	// trigger the handler
+	suite.followedTagsModule.FollowedTagsGETHandler(ctx)
+
+	// read the response
+	result := recorder.Result()
+	defer result.Body.Close()
+
+	b, err := io.ReadAll(result.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	errs := gtserror.NewMultiError(2)
+
+	// check code + body
+	if resultCode := recorder.Code; expectedHTTPStatus != resultCode {
+		errs.Appendf("expected %d got %d", expectedHTTPStatus, resultCode)
+		if expectedBody == "" {
+			return nil, errs.Combine()
+		}
+	}
+
+	// if we got an expected body, return early
+	if expectedBody != "" {
+		if string(b) != expectedBody {
+			errs.Appendf("expected %s got %s", expectedBody, string(b))
+		}
+		return nil, errs.Combine()
+	}
+
+	resp := []apimodel.Tag{}
+	if err := json.Unmarshal(b, &resp); err != nil {
+		return nil, err
+	}
+
+	return resp, nil
+}
+
+// Test that we can list a user's followed tags.
+func (suite *FollowedTagsTestSuite) TestGet() {
+	accountFixtureName := "local_account_2"
+	testAccount := suite.testAccounts[accountFixtureName]
+	testTag := suite.testTags["welcome"]
+
+	// Follow an existing tag.
+	if err := suite.db.PutFollowedTag(context.Background(), testAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	followedTags, err := suite.getFollowedTags(accountFixtureName, http.StatusOK, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	if suite.Len(followedTags, 1) {
+		followedTag := followedTags[0]
+		suite.Equal(testTag.Name, followedTag.Name)
+		if suite.NotNil(followedTag.Following) {
+			suite.True(*followedTag.Following)
+		}
+	}
+}
+
+// Test that we can list a user's followed tags even if they don't have any.
+func (suite *FollowedTagsTestSuite) TestGetEmpty() {
+	accountFixtureName := "local_account_1"
+
+	followedTags, err := suite.getFollowedTags(accountFixtureName, http.StatusOK, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	suite.Len(followedTags, 0)
+}

internal/api/client/tags/follow.go

@@ -0,0 +1,92 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
+)
+
+// FollowTagPOSTHandler swagger:operation POST /api/v1/tags/{tag_name}/follow followTag
+//
+// Follow a hashtag.
+//
+// Idempotent: if you are already following the tag, this call will still succeed.
+//
+//	---
+//	tags:
+//	- tags
+//
+//	produces:
+//	- application/json
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- write:follows
+//
+//	parameters:
+//	-
+//		name: tag_name
+//		type: string
+//		description: Name of the tag (no leading `#`)
+//		in: path
+//		required: true
+//
+//	responses:
+//		'200':
+//			description: "Info about the tag."
+//			schema:
+//				"$ref": "#/definitions/tag"
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'403':
+//			description: forbidden
+//		'500':
+//			description: internal server error
+func (m *Module) FollowTagPOSTHandler(c *gin.Context) {
+	authed, err := oauth.Authed(c, true, true, true, true)
+	if err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorUnauthorized(err, err.Error()), m.processor.InstanceGetV1)
+		return
+	}
+
+	if authed.Account.IsMoving() {
+		apiutil.ForbiddenAfterMove(c)
+		return
+	}
+
+	name, errWithCode := apiutil.ParseTagName(c.Param(apiutil.TagNameKey))
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	apiTag, errWithCode := m.processor.Tags().Follow(c.Request.Context(), authed.Account, name)
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	apiutil.JSON(c, http.StatusOK, apiTag)
+}

internal/api/client/tags/follow_test.go

@@ -0,0 +1,82 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags_test
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/superseriousbusiness/gotosocial/internal/api/client/tags"
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+)
+
+func (suite *TagsTestSuite) follow(
+	accountFixtureName string,
+	tagName string,
+	expectedHTTPStatus int,
+	expectedBody string,
+) (*apimodel.Tag, error) {
+	return suite.tagAction(
+		accountFixtureName,
+		tagName,
+		http.MethodPost,
+		tags.FollowPath,
+		suite.tagsModule.FollowTagPOSTHandler,
+		expectedHTTPStatus,
+		expectedBody,
+	)
+}
+
+// Follow a tag we don't already follow.
+func (suite *TagsTestSuite) TestFollow() {
+	accountFixtureName := "local_account_2"
+	testTag := suite.testTags["welcome"]
+
+	apiTag, err := suite.follow(accountFixtureName, testTag.Name, http.StatusOK, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	suite.Equal(testTag.Name, apiTag.Name)
+	if suite.NotNil(apiTag.Following) {
+		suite.True(*apiTag.Following)
+	}
+}
+
+// When we follow a tag already followed by the account, it should succeed.
+func (suite *TagsTestSuite) TestFollowIdempotent() {
+	accountFixtureName := "local_account_2"
+	testAccount := suite.testAccounts[accountFixtureName]
+	testTag := suite.testTags["welcome"]
+
+	// Setup: follow an existing tag.
+	if err := suite.db.PutFollowedTag(context.Background(), testAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Follow it again through the API.
+	apiTag, err := suite.follow(accountFixtureName, testTag.Name, http.StatusOK, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	suite.Equal(testTag.Name, apiTag.Name)
+	if suite.NotNil(apiTag.Following) {
+		suite.True(*apiTag.Following)
+	}
+}

internal/api/client/tags/get.go

@@ -0,0 +1,89 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
+)
+
+// TagGETHandler swagger:operation GET /api/v1/tags/{tag_name} getTag
+//
+// Get details for a hashtag, including whether you currently follow it.
+//
+// If the tag does not exist, this method will not create it in the database.
+//
+//	---
+//	tags:
+//	- tags
+//
+//	produces:
+//	- application/json
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- read:follows
+//
+//	parameters:
+//	-
+//		name: tag_name
+//		type: string
+//		description: Name of the tag (no leading `#`)
+//		in: path
+//		required: true
+//
+//	responses:
+//		'200':
+//			description: "Info about the tag."
+//			schema:
+//				"$ref": "#/definitions/tag"
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'404':
+//			description: not found
+//		'406':
+//			description: not acceptable
+//		'500':
+//			description: internal server error
+func (m *Module) TagGETHandler(c *gin.Context) {
+	authed, err := oauth.Authed(c, true, true, true, true)
+	if err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorUnauthorized(err, err.Error()), m.processor.InstanceGetV1)
+		return
+	}
+
+	name, errWithCode := apiutil.ParseTagName(c.Param(apiutil.TagNameKey))
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	apiTag, errWithCode := m.processor.Tags().Get(c.Request.Context(), authed.Account, name)
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	apiutil.JSON(c, http.StatusOK, apiTag)
+}

internal/api/client/tags/get_test.go

@@ -0,0 +1,93 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags_test
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/superseriousbusiness/gotosocial/internal/api/client/tags"
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+)
+
+// tagAction follows or unfollows a tag.
+func (suite *TagsTestSuite) get(
+	accountFixtureName string,
+	tagName string,
+	expectedHTTPStatus int,
+	expectedBody string,
+) (*apimodel.Tag, error) {
+	return suite.tagAction(
+		accountFixtureName,
+		tagName,
+		http.MethodGet,
+		tags.TagPath,
+		suite.tagsModule.TagGETHandler,
+		expectedHTTPStatus,
+		expectedBody,
+	)
+}
+
+// Get a tag followed by the account.
+func (suite *TagsTestSuite) TestGetFollowed() {
+	accountFixtureName := "local_account_2"
+	testAccount := suite.testAccounts[accountFixtureName]
+	testTag := suite.testTags["welcome"]
+
+	// Setup: follow an existing tag.
+	if err := suite.db.PutFollowedTag(context.Background(), testAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Get it through the API.
+	apiTag, err := suite.get(accountFixtureName, testTag.Name, http.StatusOK, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	suite.Equal(testTag.Name, apiTag.Name)
+	if suite.NotNil(apiTag.Following) {
+		suite.True(*apiTag.Following)
+	}
+}
+
+// Get a tag not followed by the account.
+func (suite *TagsTestSuite) TestGetUnfollowed() {
+	accountFixtureName := "local_account_2"
+	testTag := suite.testTags["Hashtag"]
+
+	apiTag, err := suite.get(accountFixtureName, testTag.Name, http.StatusOK, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	suite.Equal(testTag.Name, apiTag.Name)
+	if suite.NotNil(apiTag.Following) {
+		suite.False(*apiTag.Following)
+	}
+}
+
+// Get a tag that does not exist, which should result in a 404.
+func (suite *TagsTestSuite) TestGetNotFound() {
+	accountFixtureName := "local_account_2"
+
+	_, err := suite.get(accountFixtureName, "THIS_TAG_DOES_NOT_EXIST", http.StatusNotFound, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+}

internal/api/client/tags/tags.go

@@ -0,0 +1,49 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
+	"github.com/superseriousbusiness/gotosocial/internal/processing"
+)
+
+const (
+	BasePath     = "/v1/tags"
+	TagPath      = BasePath + "/:" + apiutil.TagNameKey
+	FollowPath   = TagPath + "/follow"
+	UnfollowPath = TagPath + "/unfollow"
+)
+
+type Module struct {
+	processor *processing.Processor
+}
+
+func New(processor *processing.Processor) *Module {
+	return &Module{
+		processor: processor,
+	}
+}
+
+func (m *Module) Route(attachHandler func(method string, path string, f ...gin.HandlerFunc) gin.IRoutes) {
+	attachHandler(http.MethodGet, TagPath, m.TagGETHandler)
+	attachHandler(http.MethodPost, FollowPath, m.FollowTagPOSTHandler)
+	attachHandler(http.MethodPost, UnfollowPath, m.UnfollowTagPOSTHandler)
+}

internal/api/client/tags/tags_test.go

@@ -0,0 +1,179 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags_test
+
+import (
+	"encoding/json"
+	"io"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/suite"
+	"github.com/superseriousbusiness/gotosocial/internal/api/client/tags"
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+	"github.com/superseriousbusiness/gotosocial/internal/email"
+	"github.com/superseriousbusiness/gotosocial/internal/federation"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/media"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
+	"github.com/superseriousbusiness/gotosocial/internal/processing"
+	"github.com/superseriousbusiness/gotosocial/internal/state"
+	"github.com/superseriousbusiness/gotosocial/internal/storage"
+	"github.com/superseriousbusiness/gotosocial/testrig"
+)
+
+type TagsTestSuite struct {
+	suite.Suite
+	db           db.DB
+	storage      *storage.Driver
+	mediaManager *media.Manager
+	federator    *federation.Federator
+	processor    *processing.Processor
+	emailSender  email.Sender
+	sentEmails   map[string]string
+	state        state.State
+
+	// standard suite models
+	testTokens       map[string]*gtsmodel.Token
+	testClients      map[string]*gtsmodel.Client
+	testApplications map[string]*gtsmodel.Application
+	testUsers        map[string]*gtsmodel.User
+	testAccounts     map[string]*gtsmodel.Account
+	testTags         map[string]*gtsmodel.Tag
+
+	// module being tested
+	tagsModule *tags.Module
+}
+
+func (suite *TagsTestSuite) SetupSuite() {
+	suite.testTokens = testrig.NewTestTokens()
+	suite.testClients = testrig.NewTestClients()
+	suite.testApplications = testrig.NewTestApplications()
+	suite.testUsers = testrig.NewTestUsers()
+	suite.testAccounts = testrig.NewTestAccounts()
+	suite.testTags = testrig.NewTestTags()
+}
+
+func (suite *TagsTestSuite) SetupTest() {
+	suite.state.Caches.Init()
+	testrig.StartNoopWorkers(&suite.state)
+
+	testrig.InitTestConfig()
+	config.Config(func(cfg *config.Configuration) {
+		cfg.WebAssetBaseDir = "../../../../web/assets/"
+		cfg.WebTemplateBaseDir = "../../../../web/templates/"
+	})
+	testrig.InitTestLog()
+
+	suite.db = testrig.NewTestDB(&suite.state)
+	suite.state.DB = suite.db
+	suite.storage = testrig.NewInMemoryStorage()
+	suite.state.Storage = suite.storage
+
+	suite.mediaManager = testrig.NewTestMediaManager(&suite.state)
+	suite.federator = testrig.NewTestFederator(&suite.state, testrig.NewTestTransportController(&suite.state, testrig.NewMockHTTPClient(nil, "../../../../testrig/media")), suite.mediaManager)
+	suite.sentEmails = make(map[string]string)
+	suite.emailSender = testrig.NewEmailSender("../../../../web/template/", suite.sentEmails)
+	suite.processor = testrig.NewTestProcessor(&suite.state, suite.federator, suite.emailSender, suite.mediaManager)
+	suite.tagsModule = tags.New(suite.processor)
+
+	testrig.StandardDBSetup(suite.db, nil)
+	testrig.StandardStorageSetup(suite.storage, "../../../../testrig/media")
+}
+
+func (suite *TagsTestSuite) TearDownTest() {
+	testrig.StandardDBTeardown(suite.db)
+	testrig.StandardStorageTeardown(suite.storage)
+	testrig.StopWorkers(&suite.state)
+}
+
+// tagAction gets, follows, or unfollows a tag, returning the tag.
+func (suite *TagsTestSuite) tagAction(
+	accountFixtureName string,
+	tagName string,
+	method string,
+	path string,
+	handler func(c *gin.Context),
+	expectedHTTPStatus int,
+	expectedBody string,
+) (*apimodel.Tag, error) {
+	// instantiate recorder + test context
+	recorder := httptest.NewRecorder()
+	ctx, _ := testrig.CreateGinTestContext(recorder, nil)
+	ctx.Set(oauth.SessionAuthorizedAccount, suite.testAccounts[accountFixtureName])
+	ctx.Set(oauth.SessionAuthorizedToken, oauth.DBTokenToToken(suite.testTokens[accountFixtureName]))
+	ctx.Set(oauth.SessionAuthorizedApplication, suite.testApplications["application_1"])
+	ctx.Set(oauth.SessionAuthorizedUser, suite.testUsers[accountFixtureName])
+
+	// create the request
+	url := config.GetProtocol() + "://" + config.GetHost() + "/api/" + path
+	ctx.Request = httptest.NewRequest(
+		method,
+		strings.Replace(url, ":tag_name", tagName, 1),
+		nil,
+	)
+	ctx.Request.Header.Set("accept", "application/json")
+
+	ctx.AddParam("tag_name", tagName)
+
+	// trigger the handler
+	handler(ctx)
+
+	// read the response
+	result := recorder.Result()
+	defer result.Body.Close()
+
+	b, err := io.ReadAll(result.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	errs := gtserror.NewMultiError(2)
+
+	// check code + body
+	if resultCode := recorder.Code; expectedHTTPStatus != resultCode {
+		errs.Appendf("expected %d got %d", expectedHTTPStatus, resultCode)
+		if expectedBody == "" {
+			return nil, errs.Combine()
+		}
+	}
+
+	// if we got an expected body, return early
+	if expectedBody != "" {
+		if string(b) != expectedBody {
+			errs.Appendf("expected %s got %s", expectedBody, string(b))
+		}
+		return nil, errs.Combine()
+	}
+
+	resp := &apimodel.Tag{}
+	if err := json.Unmarshal(b, resp); err != nil {
+		return nil, err
+	}
+
+	return resp, nil
+}
+
+func TestTagsTestSuite(t *testing.T) {
+	suite.Run(t, new(TagsTestSuite))
+}

internal/api/client/tags/unfollow.go

@@ -0,0 +1,94 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
+)
+
+// UnfollowTagPOSTHandler swagger:operation POST /api/v1/tags/{tag_name}/unfollow unfollowTag
+//
+// Unfollow a hashtag.
+//
+// Idempotent: if you are not following the tag, this call will still succeed.
+//
+//	---
+//	tags:
+//	- tags
+//
+//	produces:
+//	- application/json
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- write:follows
+//
+//	parameters:
+//	-
+//		name: tag_name
+//		type: string
+//		description: Name of the tag (no leading `#`)
+//		in: path
+//		required: true
+//
+//	responses:
+//		'200':
+//			description: "Info about the tag."
+//			schema:
+//				"$ref": "#/definitions/tag"
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'403':
+//			description: forbidden
+//		'404':
+//			description: unauthorized
+//		'500':
+//			description: internal server error
+func (m *Module) UnfollowTagPOSTHandler(c *gin.Context) {
+	authed, err := oauth.Authed(c, true, true, true, true)
+	if err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorUnauthorized(err, err.Error()), m.processor.InstanceGetV1)
+		return
+	}
+
+	if authed.Account.IsMoving() {
+		apiutil.ForbiddenAfterMove(c)
+		return
+	}
+
+	name, errWithCode := apiutil.ParseTagName(c.Param(apiutil.TagNameKey))
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	apiTag, errWithCode := m.processor.Tags().Unfollow(c.Request.Context(), authed.Account, name)
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	apiutil.JSON(c, http.StatusOK, apiTag)
+}

internal/api/client/tags/unfollow_test.go

@@ -0,0 +1,82 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags_test
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/superseriousbusiness/gotosocial/internal/api/client/tags"
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+)
+
+func (suite *TagsTestSuite) unfollow(
+	accountFixtureName string,
+	tagName string,
+	expectedHTTPStatus int,
+	expectedBody string,
+) (*apimodel.Tag, error) {
+	return suite.tagAction(
+		accountFixtureName,
+		tagName,
+		http.MethodPost,
+		tags.UnfollowPath,
+		suite.tagsModule.UnfollowTagPOSTHandler,
+		expectedHTTPStatus,
+		expectedBody,
+	)
+}
+
+// Unfollow a tag that we follow.
+func (suite *TagsTestSuite) TestUnfollow() {
+	accountFixtureName := "local_account_2"
+	testAccount := suite.testAccounts[accountFixtureName]
+	testTag := suite.testTags["welcome"]
+
+	// Setup: follow an existing tag.
+	if err := suite.db.PutFollowedTag(context.Background(), testAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Unfollow it through the API.
+	apiTag, err := suite.unfollow(accountFixtureName, testTag.Name, http.StatusOK, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	suite.Equal(testTag.Name, apiTag.Name)
+	if suite.NotNil(apiTag.Following) {
+		suite.False(*apiTag.Following)
+	}
+}
+
+// When we unfollow a tag not followed by the account, it should succeed.
+func (suite *TagsTestSuite) TestUnfollowIdempotent() {
+	accountFixtureName := "local_account_2"
+	testTag := suite.testTags["Hashtag"]
+
+	apiTag, err := suite.unfollow(accountFixtureName, testTag.Name, http.StatusOK, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	suite.Equal(testTag.Name, apiTag.Name)
+	if suite.NotNil(apiTag.Following) {
+		suite.False(*apiTag.Following)
+	}
+}

internal/api/model/tag.go

@@ -31,4 +31,7 @@ type Tag struct {
 	// Currently just a stub, if provided will always be an empty array.
 	// example: []
 	History *[]any `json:"history,omitempty"`
+	// Following is true if the user is following this tag, false if they're not,
+	// and not present if there is no currently authenticated user.
+	Following *bool `json:"following,omitempty"`
 }

internal/cache/cache.go

@@ -57,6 +57,7 @@ func (c *Caches) Init() {
 	log.Infof(nil, "init: %p", c)
 
 	c.initAccount()
+	c.initAccountIDsFollowingTag()
 	c.initAccountNote()
 	c.initAccountSettings()
 	c.initAccountStats()
@@ -98,6 +99,7 @@ func (c *Caches) Init() {
 	c.initStatusFave()
 	c.initStatusFaveIDs()
 	c.initTag()
+	c.initTagIDsFollowedByAccount()
 	c.initThreadMute()
 	c.initToken()
 	c.initTombstone()
@@ -134,6 +136,7 @@ func (c *Caches) Stop() {
 // significant overhead to all cache writes.
 func (c *Caches) Sweep(threshold float64) {
 	c.DB.Account.Trim(threshold)
+	c.DB.AccountIDsFollowingTag.Trim(threshold)
 	c.DB.AccountNote.Trim(threshold)
 	c.DB.AccountSettings.Trim(threshold)
 	c.DB.AccountStats.Trim(threshold)
@@ -142,6 +145,8 @@ func (c *Caches) Sweep(threshold float64) {
 	c.DB.BlockIDs.Trim(threshold)
 	c.DB.BoostOfIDs.Trim(threshold)
 	c.DB.Client.Trim(threshold)
+	c.DB.Conversation.Trim(threshold)
+	c.DB.ConversationLastStatusIDs.Trim(threshold)
 	c.DB.Emoji.Trim(threshold)
 	c.DB.EmojiCategory.Trim(threshold)
 	c.DB.Filter.Trim(threshold)
@@ -171,6 +176,7 @@ func (c *Caches) Sweep(threshold float64) {
 	c.DB.StatusFave.Trim(threshold)
 	c.DB.StatusFaveIDs.Trim(threshold)
 	c.DB.Tag.Trim(threshold)
+	c.DB.TagIDsFollowedByAccount.Trim(threshold)
 	c.DB.ThreadMute.Trim(threshold)
 	c.DB.Token.Trim(threshold)
 	c.DB.Tombstone.Trim(threshold)

internal/cache/db.go

@@ -29,6 +29,9 @@ type DBCaches struct {
 	// Account provides access to the gtsmodel Account database cache.
 	Account StructCache[*gtsmodel.Account]
 
+	// AccountIDsFollowingTag caches account IDs following a given tag ID.
+	AccountIDsFollowingTag SliceCache[string]
+
 	// AccountNote provides access to the gtsmodel Note database cache.
 	AccountNote StructCache[*gtsmodel.AccountNote]
 
@@ -160,6 +163,9 @@ type DBCaches struct {
 	// Tag provides access to the gtsmodel Tag database cache.
 	Tag StructCache[*gtsmodel.Tag]
 
+	// TagIDsFollowedByAccount caches tag IDs followed by a given account ID.
+	TagIDsFollowedByAccount SliceCache[string]
+
 	// ThreadMute provides access to the gtsmodel ThreadMute database cache.
 	ThreadMute StructCache[*gtsmodel.ThreadMute]
 
@@ -234,6 +240,17 @@ func (c *Caches) initAccount() {
 	})
 }
 
+func (c *Caches) initAccountIDsFollowingTag() {
+	// Calculate maximum cache size.
+	cap := calculateSliceCacheMax(
+		config.GetCacheAccountIDsFollowingTagMemRatio(),
+	)
+
+	log.Infof(nil, "cache size = %d", cap)
+
+	c.DB.AccountIDsFollowingTag.Init(0, cap)
+}
+
 func (c *Caches) initAccountNote() {
 	// Calculate maximum cache size.
 	cap := calculateResultCacheMax(
@@ -1317,6 +1334,17 @@ func (c *Caches) initTag() {
 	})
 }
 
+func (c *Caches) initTagIDsFollowedByAccount() {
+	// Calculate maximum cache size.
+	cap := calculateSliceCacheMax(
+		config.GetCacheTagIDsFollowedByAccountMemRatio(),
+	)
+
+	log.Infof(nil, "cache size = %d", cap)
+
+	c.DB.TagIDsFollowedByAccount.Init(0, cap)
+}
+
 func (c *Caches) initThreadMute() {
 	cap := calculateResultCacheMax(
 		sizeofThreadMute(), // model in-mem size.

internal/config/config.go

@@ -193,6 +193,7 @@ type HTTPClientConfiguration struct {
 type CacheConfiguration struct {
 	MemoryTarget                      bytesize.Size `name:"memory-target"`
 	AccountMemRatio                   float64       `name:"account-mem-ratio"`
+	AccountIDsFollowingTagMemRatio    float64       `name:"account-ids-following-tag-mem-ratio"`
 	AccountNoteMemRatio               float64       `name:"account-note-mem-ratio"`
 	AccountSettingsMemRatio           float64       `name:"account-settings-mem-ratio"`
 	AccountStatsMemRatio              float64       `name:"account-stats-mem-ratio"`
@@ -232,6 +233,7 @@ type CacheConfiguration struct {
 	StatusFaveMemRatio                float64       `name:"status-fave-mem-ratio"`
 	StatusFaveIDsMemRatio             float64       `name:"status-fave-ids-mem-ratio"`
 	TagMemRatio                       float64       `name:"tag-mem-ratio"`
+	TagIDsFollowedByAccountMemRatio   float64       `name:"tag-ids-followed-by-account-mem-ratio"`
 	ThreadMuteMemRatio                float64       `name:"thread-mute-mem-ratio"`
 	TokenMemRatio                     float64       `name:"token-mem-ratio"`
 	TombstoneMemRatio                 float64       `name:"tombstone-mem-ratio"`

internal/config/defaults.go

@@ -157,6 +157,7 @@ var Defaults = Configuration{
 		// file have been addressed, these should
 		// be able to make some more sense :D
 		AccountMemRatio:                   5,
+		AccountIDsFollowingTagMemRatio:    1,
 		AccountNoteMemRatio:               1,
 		AccountSettingsMemRatio:           0.1,
 		AccountStatsMemRatio:              2,
@@ -196,6 +197,7 @@ var Defaults = Configuration{
 		StatusFaveMemRatio:                2,
 		StatusFaveIDsMemRatio:             3,
 		TagMemRatio:                       2,
+		TagIDsFollowedByAccountMemRatio:   1,
 		ThreadMuteMemRatio:                0.2,
 		TokenMemRatio:                     0.75,
 		TombstoneMemRatio:                 0.5,

internal/config/helpers.gen.go

@@ -2775,6 +2775,37 @@ func GetCacheAccountMemRatio() float64 { return global.GetCacheAccountMemRatio()
 // SetCacheAccountMemRatio safely sets the value for global configuration 'Cache.AccountMemRatio' field
 func SetCacheAccountMemRatio(v float64) { global.SetCacheAccountMemRatio(v) }
 
+// GetCacheAccountIDsFollowingTagMemRatio safely fetches the Configuration value for state's 'Cache.AccountIDsFollowingTagMemRatio' field
+func (st *ConfigState) GetCacheAccountIDsFollowingTagMemRatio() (v float64) {
+	st.mutex.RLock()
+	v = st.config.Cache.AccountIDsFollowingTagMemRatio
+	st.mutex.RUnlock()
+	return
+}
+
+// SetCacheAccountIDsFollowingTagMemRatio safely sets the Configuration value for state's 'Cache.AccountIDsFollowingTagMemRatio' field
+func (st *ConfigState) SetCacheAccountIDsFollowingTagMemRatio(v float64) {
+	st.mutex.Lock()
+	defer st.mutex.Unlock()
+	st.config.Cache.AccountIDsFollowingTagMemRatio = v
+	st.reloadToViper()
+}
+
+// CacheAccountIDsFollowingTagMemRatioFlag returns the flag name for the 'Cache.AccountIDsFollowingTagMemRatio' field
+func CacheAccountIDsFollowingTagMemRatioFlag() string {
+	return "cache-account-ids-following-tag-mem-ratio"
+}
+
+// GetCacheAccountIDsFollowingTagMemRatio safely fetches the value for global configuration 'Cache.AccountIDsFollowingTagMemRatio' field
+func GetCacheAccountIDsFollowingTagMemRatio() float64 {
+	return global.GetCacheAccountIDsFollowingTagMemRatio()
+}
+
+// SetCacheAccountIDsFollowingTagMemRatio safely sets the value for global configuration 'Cache.AccountIDsFollowingTagMemRatio' field
+func SetCacheAccountIDsFollowingTagMemRatio(v float64) {
+	global.SetCacheAccountIDsFollowingTagMemRatio(v)
+}
+
 // GetCacheAccountNoteMemRatio safely fetches the Configuration value for state's 'Cache.AccountNoteMemRatio' field
 func (st *ConfigState) GetCacheAccountNoteMemRatio() (v float64) {
 	st.mutex.RLock()
@@ -3758,6 +3789,37 @@ func GetCacheTagMemRatio() float64 { return global.GetCacheTagMemRatio() }
 // SetCacheTagMemRatio safely sets the value for global configuration 'Cache.TagMemRatio' field
 func SetCacheTagMemRatio(v float64) { global.SetCacheTagMemRatio(v) }
 
+// GetCacheTagIDsFollowedByAccountMemRatio safely fetches the Configuration value for state's 'Cache.TagIDsFollowedByAccountMemRatio' field
+func (st *ConfigState) GetCacheTagIDsFollowedByAccountMemRatio() (v float64) {
+	st.mutex.RLock()
+	v = st.config.Cache.TagIDsFollowedByAccountMemRatio
+	st.mutex.RUnlock()
+	return
+}
+
+// SetCacheTagIDsFollowedByAccountMemRatio safely sets the Configuration value for state's 'Cache.TagIDsFollowedByAccountMemRatio' field
+func (st *ConfigState) SetCacheTagIDsFollowedByAccountMemRatio(v float64) {
+	st.mutex.Lock()
+	defer st.mutex.Unlock()
+	st.config.Cache.TagIDsFollowedByAccountMemRatio = v
+	st.reloadToViper()
+}
+
+// CacheTagIDsFollowedByAccountMemRatioFlag returns the flag name for the 'Cache.TagIDsFollowedByAccountMemRatio' field
+func CacheTagIDsFollowedByAccountMemRatioFlag() string {
+	return "cache-tag-ids-followed-by-account-mem-ratio"
+}
+
+// GetCacheTagIDsFollowedByAccountMemRatio safely fetches the value for global configuration 'Cache.TagIDsFollowedByAccountMemRatio' field
+func GetCacheTagIDsFollowedByAccountMemRatio() float64 {
+	return global.GetCacheTagIDsFollowedByAccountMemRatio()
+}
+
+// SetCacheTagIDsFollowedByAccountMemRatio safely sets the value for global configuration 'Cache.TagIDsFollowedByAccountMemRatio' field
+func SetCacheTagIDsFollowedByAccountMemRatio(v float64) {
+	global.SetCacheTagIDsFollowedByAccountMemRatio(v)
+}
+
 // GetCacheThreadMuteMemRatio safely fetches the Configuration value for state's 'Cache.ThreadMuteMemRatio' field
 func (st *ConfigState) GetCacheThreadMuteMemRatio() (v float64) {
 	st.mutex.RLock()

internal/db/bundb/migrations/20240725211933_add_followed_tags.go

@@ -0,0 +1,51 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package migrations
+
+import (
+	"context"
+
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/uptrace/bun"
+)
+
+func init() {
+	up := func(ctx context.Context, db *bun.DB) error {
+		return db.RunInTx(ctx, nil, func(ctx context.Context, tx bun.Tx) error {
+			if _, err := tx.
+				NewCreateTable().
+				Model(&gtsmodel.FollowedTag{}).
+				IfNotExists().
+				Exec(ctx); err != nil {
+				return err
+			}
+
+			return nil
+		})
+	}
+
+	down := func(ctx context.Context, db *bun.DB) error {
+		return db.RunInTx(ctx, nil, func(ctx context.Context, tx bun.Tx) error {
+			return nil
+		})
+	}
+
+	if err := Migrations.Register(up, down); err != nil {
+		panic(err)
+	}
+}

internal/db/bundb/tag.go

@@ -19,9 +19,13 @@ package bundb
 
 import (
 	"context"
+	"errors"
 	"strings"
 
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
 	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/paging"
 	"github.com/superseriousbusiness/gotosocial/internal/state"
 	"github.com/superseriousbusiness/gotosocial/internal/util"
 	"github.com/uptrace/bun"
@@ -131,3 +135,158 @@ func (t *tagDB) PutTag(ctx context.Context, tag *gtsmodel.Tag) error {
 
 	return nil
 }
+
+func (t *tagDB) GetFollowedTags(ctx context.Context, accountID string, page *paging.Page) ([]*gtsmodel.Tag, error) {
+	tagIDs, err := t.getTagIDsFollowedByAccount(ctx, accountID, page)
+	if err != nil {
+		return nil, err
+	}
+
+	tags, err := t.GetTags(ctx, tagIDs)
+	if err != nil {
+		return nil, err
+	}
+
+	return tags, nil
+}
+
+func (t *tagDB) getTagIDsFollowedByAccount(ctx context.Context, accountID string, page *paging.Page) ([]string, error) {
+	return loadPagedIDs(&t.state.Caches.DB.TagIDsFollowedByAccount, accountID, page, func() ([]string, error) {
+		var tagIDs []string
+
+		// Tag IDs not in cache. Perform DB query.
+		if _, err := t.db.
+			NewSelect().
+			Model((*gtsmodel.FollowedTag)(nil)).
+			Column("tag_id").
+			Where("? = ?", bun.Ident("account_id"), accountID).
+			OrderExpr("? DESC", bun.Ident("tag_id")).
+			Exec(ctx, &tagIDs); // nocollapse
+		err != nil && !errors.Is(err, db.ErrNoEntries) {
+			return nil, gtserror.Newf("error getting tag IDs followed by account %s: %w", accountID, err)
+		}
+
+		return tagIDs, nil
+	})
+}
+
+func (t *tagDB) getAccountIDsFollowingTag(ctx context.Context, tagID string) ([]string, error) {
+	return loadPagedIDs(&t.state.Caches.DB.AccountIDsFollowingTag, tagID, nil, func() ([]string, error) {
+		var accountIDs []string
+
+		// Account IDs not in cache. Perform DB query.
+		if _, err := t.db.
+			NewSelect().
+			Model((*gtsmodel.FollowedTag)(nil)).
+			Column("account_id").
+			Where("? = ?", bun.Ident("tag_id"), tagID).
+			OrderExpr("? DESC", bun.Ident("account_id")).
+			Exec(ctx, &accountIDs); // nocollapse
+		err != nil && !errors.Is(err, db.ErrNoEntries) {
+			return nil, gtserror.Newf("error getting account IDs following tag %s: %w", tagID, err)
+		}
+
+		return accountIDs, nil
+	})
+}
+
+func (t *tagDB) IsAccountFollowingTag(ctx context.Context, accountID string, tagID string) (bool, error) {
+	accountTagIDs, err := t.getTagIDsFollowedByAccount(ctx, accountID, nil)
+	if err != nil {
+		return false, err
+	}
+
+	for _, accountTagID := range accountTagIDs {
+		if accountTagID == tagID {
+			return true, nil
+		}
+	}
+
+	return false, nil
+}
+
+func (t *tagDB) PutFollowedTag(ctx context.Context, accountID string, tagID string) error {
+	// Insert the followed tag.
+	result, err := t.db.NewInsert().
+		Model(&gtsmodel.FollowedTag{
+			AccountID: accountID,
+			TagID:     tagID,
+		}).
+		On("CONFLICT (?, ?) DO NOTHING", bun.Ident("account_id"), bun.Ident("tag_id")).
+		Exec(ctx)
+	if err != nil {
+		return gtserror.Newf("error inserting followed tag: %w", err)
+	}
+
+	// If it fails because that account already follows that tag, that's fine, and we're done.
+	rows, err := result.RowsAffected()
+	if err != nil {
+		return gtserror.Newf("error getting inserted row count: %w", err)
+	}
+	if rows == 0 {
+		return nil
+	}
+
+	// Otherwise, this is a new followed tag, so we invalidate caches related to it.
+	t.state.Caches.DB.AccountIDsFollowingTag.Invalidate(tagID)
+	t.state.Caches.DB.TagIDsFollowedByAccount.Invalidate(accountID)
+
+	return nil
+}
+
+func (t *tagDB) DeleteFollowedTag(ctx context.Context, accountID string, tagID string) error {
+	result, err := t.db.NewDelete().
+		Model((*gtsmodel.FollowedTag)(nil)).
+		Where("? = ?", bun.Ident("account_id"), accountID).
+		Where("? = ?", bun.Ident("tag_id"), tagID).
+		Exec(ctx)
+	if err != nil {
+		return gtserror.Newf("error deleting followed tag %s for account %s: %w", tagID, accountID, err)
+	}
+
+	rows, err := result.RowsAffected()
+	if err != nil {
+		return gtserror.Newf("error getting inserted row count: %w", err)
+	}
+	if rows == 0 {
+		return nil
+	}
+
+	// If we deleted anything, invalidate caches related to it.
+	t.state.Caches.DB.AccountIDsFollowingTag.Invalidate(tagID)
+	t.state.Caches.DB.TagIDsFollowedByAccount.Invalidate(accountID)
+
+	return err
+}
+
+func (t *tagDB) DeleteFollowedTagsByAccountID(ctx context.Context, accountID string) error {
+	// Delete followed tags from the database, returning the list of tag IDs affected.
+	tagIDs := []string{}
+	if err := t.db.NewDelete().
+		Model((*gtsmodel.FollowedTag)(nil)).
+		Where("? = ?", bun.Ident("account_id"), accountID).
+		Returning("?", bun.Ident("tag_id")).
+		Scan(ctx, &tagIDs); // nocollapse
+	err != nil {
+		return gtserror.Newf("error deleting followed tags for account %s: %w", accountID, err)
+	}
+
+	// Invalidate account ID caches for the account and those tags.
+	t.state.Caches.DB.TagIDsFollowedByAccount.Invalidate(accountID)
+	t.state.Caches.DB.AccountIDsFollowingTag.Invalidate(tagIDs...)
+
+	return nil
+}
+
+func (t *tagDB) GetAccountIDsFollowingTagIDs(ctx context.Context, tagIDs []string) ([]string, error) {
+	// Accounts might be following multiple tags in this list, but we only want to return each account once.
+	accountIDs := []string{}
+	for _, tagID := range tagIDs {
+		tagAccountIDs, err := t.getAccountIDsFollowingTag(ctx, tagID)
+		if err != nil {
+			return nil, err
+		}
+		accountIDs = append(accountIDs, tagAccountIDs...)
+	}
+	return util.UniqueStrings(accountIDs), nil
+}

internal/db/tag.go

@@ -21,6 +21,7 @@ import (
 	"context"
 
 	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/paging"
 )
 
 // Tag contains functions for getting/creating tags in the database.
@@ -36,4 +37,24 @@ type Tag interface {
 
 	// GetTags gets multiple tags.
 	GetTags(ctx context.Context, ids []string) ([]*gtsmodel.Tag, error)
+
+	// GetFollowedTags gets the user's followed tags.
+	GetFollowedTags(ctx context.Context, accountID string, page *paging.Page) ([]*gtsmodel.Tag, error)
+
+	// IsAccountFollowingTag returns whether the account follows the given tag.
+	IsAccountFollowingTag(ctx context.Context, accountID string, tagID string) (bool, error)
+
+	// PutFollowedTag creates a new followed tag for a the given user.
+	// If it already exists, it returns without an error.
+	PutFollowedTag(ctx context.Context, accountID string, tagID string) error
+
+	// DeleteFollowedTag deletes a followed tag for a the given user.
+	// If no such followed tag exists, it returns without an error.
+	DeleteFollowedTag(ctx context.Context, accountID string, tagID string) error
+
+	// DeleteFollowedTagsByAccountID deletes all of an account's followed tags.
+	DeleteFollowedTagsByAccountID(ctx context.Context, accountID string) error
+
+	// GetAccountIDsFollowingTagIDs returns the account IDs of any followers of the given tag IDs.
+	GetAccountIDsFollowingTagIDs(ctx context.Context, tagIDs []string) ([]string, error)
 }

internal/gtsmodel/tag.go

@@ -29,3 +29,12 @@ type Tag struct {
 	Listable  *bool     `bun:",nullzero,notnull,default:true"`                              // Tagged statuses can be listed on this instance.
 	Href      string    `bun:"-"`                                                           // Href of the hashtag. Will only be set on freshly-extracted hashtags from remote AP messages. Not stored in the database.
 }
+
+// FollowedTag represents a user following a tag.
+type FollowedTag struct {
+	// ID of the account that follows the tag.
+	AccountID string `bun:"type:CHAR(26),pk,nullzero"`
+
+	// ID of the tag.
+	TagID string `bun:"type:CHAR(26),pk,nullzero"`
+}

internal/processing/account/delete.go

@@ -474,6 +474,12 @@ func (p *Processor) deleteAccountPeripheral(ctx context.Context, account *gtsmod
 		return gtserror.Newf("error deleting poll votes by account: %w", err)
 	}
 
+	// Delete all followed tags owned by given account.
+	if err := p.state.DB.DeleteFollowedTagsByAccountID(ctx, account.ID); // nocollapse
+	err != nil && !errors.Is(err, db.ErrNoEntries) {
+		return gtserror.Newf("error deleting followed tags by account: %w", err)
+	}
+
 	// Delete account stats model.
 	if err := p.state.DB.DeleteAccountStats(ctx, account.ID); err != nil {
 		return gtserror.Newf("error deleting stats for account: %w", err)

internal/processing/processor.go

@@ -42,6 +42,7 @@ import (
 	"github.com/superseriousbusiness/gotosocial/internal/processing/search"
 	"github.com/superseriousbusiness/gotosocial/internal/processing/status"
 	"github.com/superseriousbusiness/gotosocial/internal/processing/stream"
+	"github.com/superseriousbusiness/gotosocial/internal/processing/tags"
 	"github.com/superseriousbusiness/gotosocial/internal/processing/timeline"
 	"github.com/superseriousbusiness/gotosocial/internal/processing/user"
 	"github.com/superseriousbusiness/gotosocial/internal/processing/workers"
@@ -88,6 +89,7 @@ type Processor struct {
 	search             search.Processor
 	status             status.Processor
 	stream             stream.Processor
+	tags               tags.Processor
 	timeline           timeline.Processor
 	user               user.Processor
 	workers            workers.Processor
@@ -153,6 +155,10 @@ func (p *Processor) Stream() *stream.Processor {
 	return &p.stream
 }
 
+func (p *Processor) Tags() *tags.Processor {
+	return &p.tags
+}
+
 func (p *Processor) Timeline() *timeline.Processor {
 	return &p.timeline
 }
@@ -207,6 +213,7 @@ func NewProcessor(
 	processor.markers = markers.New(state, converter)
 	processor.polls = polls.New(&common, state, converter)
 	processor.report = report.New(state, converter)
+	processor.tags = tags.New(state, converter)
 	processor.timeline = timeline.New(state, converter, visFilter)
 	processor.search = search.New(state, federator, converter, visFilter)
 	processor.status = status.New(state, &common, &processor.polls, federator, converter, visFilter, intFilter, parseMentionFunc)

internal/processing/search/util.go

@@ -146,7 +146,7 @@ func (p *Processor) packageHashtags(
 	} else {
 		// If API not version 1, provide slice of full tags.
 		rangeF = func(tag *gtsmodel.Tag) {
-			apiTag, err := p.converter.TagToAPITag(ctx, tag, true)
+			apiTag, err := p.converter.TagToAPITag(ctx, tag, true, nil)
 			if err != nil {
 				log.Debugf(
 					ctx,

internal/processing/tags/follow.go

@@ -0,0 +1,67 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags
+
+import (
+	"context"
+	"errors"
+
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/id"
+)
+
+// Follow follows the tag with the given name as the given account.
+// If there is no tag with that name, it creates a tag.
+func (p *Processor) Follow(
+	ctx context.Context,
+	account *gtsmodel.Account,
+	name string,
+) (*apimodel.Tag, gtserror.WithCode) {
+	// Try to get an existing tag with that name.
+	tag, err := p.state.DB.GetTagByName(ctx, name)
+	if err != nil && !errors.Is(err, db.ErrNoEntries) {
+		return nil, gtserror.NewErrorInternalError(
+			gtserror.Newf("DB error getting tag with name %s: %w", name, err),
+		)
+	}
+
+	// If there is no such tag, create it.
+	if tag == nil {
+		tag = &gtsmodel.Tag{
+			ID:   id.NewULID(),
+			Name: name,
+		}
+		if err := p.state.DB.PutTag(ctx, tag); err != nil {
+			return nil, gtserror.NewErrorInternalError(
+				gtserror.Newf("DB error creating tag with name %s: %w", name, err),
+			)
+		}
+	}
+
+	// Follow the tag.
+	if err := p.state.DB.PutFollowedTag(ctx, account.ID, tag.ID); err != nil {
+		return nil, gtserror.NewErrorInternalError(
+			gtserror.Newf("DB error following tag %s: %w", tag.ID, err),
+		)
+	}
+
+	return p.apiTag(ctx, tag, true)
+}

internal/processing/tags/followed.go

@@ -0,0 +1,73 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags
+
+import (
+	"context"
+	"errors"
+
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/log"
+	"github.com/superseriousbusiness/gotosocial/internal/paging"
+	"github.com/superseriousbusiness/gotosocial/internal/util"
+)
+
+// Followed gets the user's list of followed tags.
+func (p *Processor) Followed(
+	ctx context.Context,
+	accountID string,
+	page *paging.Page,
+) (*apimodel.PageableResponse, gtserror.WithCode) {
+	tags, err := p.state.DB.GetFollowedTags(ctx,
+		accountID,
+		page,
+	)
+	if err != nil && !errors.Is(err, db.ErrNoEntries) {
+		return nil, gtserror.NewErrorInternalError(
+			gtserror.Newf("DB error getting followed tags for account %s: %w", accountID, err),
+		)
+	}
+
+	count := len(tags)
+	if len(tags) == 0 {
+		return util.EmptyPageableResponse(), nil
+	}
+
+	lo := tags[count-1].ID
+	hi := tags[0].ID
+
+	items := make([]interface{}, 0, count)
+	following := util.Ptr(true)
+	for _, tag := range tags {
+		apiTag, err := p.converter.TagToAPITag(ctx, tag, true, following)
+		if err != nil {
+			log.Errorf(ctx, "error converting tag %s to API representation: %v", tag.ID, err)
+			continue
+		}
+		items = append(items, apiTag)
+	}
+
+	return paging.PackageResponse(paging.ResponseParams{
+		Items: items,
+		Path:  "/api/v1/followed_tags",
+		Next:  page.Next(lo, hi),
+		Prev:  page.Prev(lo, hi),
+	}), nil
+}

internal/processing/tags/followedtags.go

@@ -0,0 +1,53 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags
+
+import (
+	"context"
+
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/state"
+	"github.com/superseriousbusiness/gotosocial/internal/typeutils"
+)
+
+type Processor struct {
+	state     *state.State
+	converter *typeutils.Converter
+}
+
+func New(state *state.State, converter *typeutils.Converter) Processor {
+	return Processor{
+		state:     state,
+		converter: converter,
+	}
+}
+
+// apiTag is a shortcut to return the API version of the given tag,
+// or return an appropriate error if conversion fails.
+func (p *Processor) apiTag(ctx context.Context, tag *gtsmodel.Tag, following bool) (*apimodel.Tag, gtserror.WithCode) {
+	apiTag, err := p.converter.TagToAPITag(ctx, tag, true, &following)
+	if err != nil {
+		return nil, gtserror.NewErrorInternalError(
+			gtserror.Newf("error converting tag %s to API representation: %w", tag.Name, err),
+		)
+	}
+
+	return &apiTag, nil
+}

internal/processing/tags/get.go

@@ -0,0 +1,57 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags
+
+import (
+	"context"
+	"errors"
+
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+)
+
+// Get gets the tag with the given name, including whether it's followed by the given account.
+func (p *Processor) Get(
+	ctx context.Context,
+	account *gtsmodel.Account,
+	name string,
+) (*apimodel.Tag, gtserror.WithCode) {
+	// Try to get an existing tag with that name.
+	tag, err := p.state.DB.GetTagByName(ctx, name)
+	if err != nil && !errors.Is(err, db.ErrNoEntries) {
+		return nil, gtserror.NewErrorInternalError(
+			gtserror.Newf("DB error getting tag with name %s: %w", name, err),
+		)
+	}
+	if tag == nil {
+		return nil, gtserror.NewErrorNotFound(
+			gtserror.Newf("couldn't find tag with name %s: %w", name, err),
+		)
+	}
+
+	following, err := p.state.DB.IsAccountFollowingTag(ctx, account.ID, tag.ID)
+	if err != nil {
+		return nil, gtserror.NewErrorInternalError(
+			gtserror.Newf("DB error checking whether account %s follows tag %s: %w", account.ID, tag.ID, err),
+		)
+	}
+
+	return p.apiTag(ctx, tag, following)
+}

internal/processing/tags/unfollow.go

@@ -0,0 +1,58 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags
+
+import (
+	"context"
+	"errors"
+
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+)
+
+// Unfollow unfollows the tag with the given name as the given account.
+// If there is no tag with that name, it creates a tag.
+func (p *Processor) Unfollow(
+	ctx context.Context,
+	account *gtsmodel.Account,
+	name string,
+) (*apimodel.Tag, gtserror.WithCode) {
+	// Try to get an existing tag with that name.
+	tag, err := p.state.DB.GetTagByName(ctx, name)
+	if err != nil && !errors.Is(err, db.ErrNoEntries) {
+		return nil, gtserror.NewErrorInternalError(
+			gtserror.Newf("DB error getting tag with name %s: %w", name, err),
+		)
+	}
+	if tag == nil {
+		return nil, gtserror.NewErrorNotFound(
+			gtserror.Newf("couldn't find tag with name %s: %w", name, err),
+		)
+	}
+
+	// Unfollow the tag.
+	if err := p.state.DB.DeleteFollowedTag(ctx, account.ID, tag.ID); err != nil {
+		return nil, gtserror.NewErrorInternalError(
+			gtserror.Newf("DB error unfollowing tag %s: %w", tag.ID, err),
+		)
+	}
+
+	return p.apiTag(ctx, tag, false)
+}

internal/processing/workers/fromclientapi_test.go

@@ -52,6 +52,7 @@ func (suite *FromClientAPITestSuite) newStatus(
 	boostOfStatus *gtsmodel.Status,
 	mentionedAccounts []*gtsmodel.Account,
 	createThread bool,
+	tagIDs []string,
 ) *gtsmodel.Status {
 	var (
 		protocol = config.GetProtocol()
@@ -65,6 +66,7 @@ func (suite *FromClientAPITestSuite) newStatus(
 		URI:                 protocol + "://" + host + "/users/" + account.Username + "/statuses/" + statusID,
 		URL:                 protocol + "://" + host + "/@" + account.Username + "/statuses/" + statusID,
 		Content:             "pee pee poo poo",
+		TagIDs:              tagIDs,
 		Local:               util.Ptr(true),
 		AccountURI:          account.URI,
 		AccountID:           account.ID,
@@ -256,6 +258,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusWithNotification() {
 			nil,
 			nil,
 			false,
+			nil,
 		)
 	)
 
@@ -367,6 +370,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusReply() {
 			nil,
 			nil,
 			false,
+			nil,
 		)
 	)
 
@@ -428,6 +432,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusReplyMuted() {
 			nil,
 			nil,
 			false,
+			nil,
 		)
 		threadMute = &gtsmodel.ThreadMute{
 			ID:        "01HD3KRMBB1M85QRWHD912QWRE",
@@ -488,6 +493,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusBoostMuted() {
 			suite.testStatuses["local_account_1_status_1"],
 			nil,
 			false,
+			nil,
 		)
 		threadMute = &gtsmodel.ThreadMute{
 			ID:        "01HD3KRMBB1M85QRWHD912QWRE",
@@ -553,6 +559,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusListRepliesPolicyLis
 			nil,
 			nil,
 			false,
+			nil,
 		)
 	)
 
@@ -628,6 +635,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusListRepliesPolicyLis
 			nil,
 			nil,
 			false,
+			nil,
 		)
 	)
 
@@ -708,6 +716,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusReplyListRepliesPoli
 			nil,
 			nil,
 			false,
+			nil,
 		)
 	)
 
@@ -780,6 +789,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusBoost() {
 			suite.testStatuses["local_account_2_status_1"],
 			nil,
 			false,
+			nil,
 		)
 	)
 
@@ -843,6 +853,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusBoostNoReblogs() {
 			suite.testStatuses["local_account_2_status_1"],
 			nil,
 			false,
+			nil,
 		)
 	)
 
@@ -912,6 +923,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusWhichBeginsConversat
 			nil,
 			[]*gtsmodel.Account{receivingAccount},
 			true,
+			nil,
 		)
 	)
 
@@ -997,6 +1009,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusWhichShouldNotCreate
 			nil,
 			[]*gtsmodel.Account{receivingAccount},
 			true,
+			nil,
 		)
 	)
 
@@ -1038,6 +1051,555 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusWhichShouldNotCreate
 	)
 }
 
+// A public status with a hashtag followed by a local user who does not otherwise follow the author
+// should end up in the tag-following user's home timeline.
+func (suite *FromClientAPITestSuite) TestProcessCreateStatusWithFollowedHashtag() {
+	testStructs := suite.SetupTestStructs()
+	defer suite.TearDownTestStructs(testStructs)
+
+	var (
+		ctx              = context.Background()
+		postingAccount   = suite.testAccounts["admin_account"]
+		receivingAccount = suite.testAccounts["local_account_2"]
+		streams          = suite.openStreams(ctx,
+			testStructs.Processor,
+			receivingAccount,
+			nil,
+		)
+		homeStream = streams[stream.TimelineHome]
+		testTag    = suite.testTags["welcome"]
+
+		// postingAccount posts a new public status not mentioning anyone but using testTag.
+		status = suite.newStatus(
+			ctx,
+			testStructs.State,
+			postingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			nil,
+			nil,
+			false,
+			[]string{testTag.ID},
+		)
+	)
+
+	// Check precondition: receivingAccount does not follow postingAccount.
+	following, err := testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: receivingAccount does not block postingAccount or vice versa.
+	blocking, err := testStructs.State.DB.IsEitherBlocked(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Setup: receivingAccount follows testTag.
+	if err := testStructs.State.DB.PutFollowedTag(ctx, receivingAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Process the new status.
+	if err := testStructs.Processor.Workers().ProcessFromClientAPI(
+		ctx,
+		&messages.FromClientAPI{
+			APObjectType:   ap.ObjectNote,
+			APActivityType: ap.ActivityCreate,
+			GTSModel:       status,
+			Origin:         postingAccount,
+		},
+	); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Check status in home stream.
+	suite.checkStreamed(
+		homeStream,
+		true,
+		"",
+		stream.EventTypeUpdate,
+	)
+}
+
+// A public status with a hashtag followed by a local user who does not otherwise follow the author
+// should not end up in the tag-following user's home timeline
+// if the user has the author blocked.
+func (suite *FromClientAPITestSuite) TestProcessCreateStatusWithFollowedHashtagAndBlock() {
+	testStructs := suite.SetupTestStructs()
+	defer suite.TearDownTestStructs(testStructs)
+
+	var (
+		ctx              = context.Background()
+		postingAccount   = suite.testAccounts["remote_account_1"]
+		receivingAccount = suite.testAccounts["local_account_2"]
+		streams          = suite.openStreams(ctx,
+			testStructs.Processor,
+			receivingAccount,
+			nil,
+		)
+		homeStream = streams[stream.TimelineHome]
+		testTag    = suite.testTags["welcome"]
+
+		// postingAccount posts a new public status not mentioning anyone but using testTag.
+		status = suite.newStatus(
+			ctx,
+			testStructs.State,
+			postingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			nil,
+			nil,
+			false,
+			[]string{testTag.ID},
+		)
+	)
+
+	// Check precondition: receivingAccount does not follow postingAccount.
+	following, err := testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: postingAccount does not block receivingAccount.
+	blocking, err := testStructs.State.DB.IsBlocked(ctx, postingAccount.ID, receivingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Check precondition: receivingAccount blocks postingAccount.
+	blocking, err = testStructs.State.DB.IsBlocked(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.True(blocking)
+
+	// Setup: receivingAccount follows testTag.
+	if err := testStructs.State.DB.PutFollowedTag(ctx, receivingAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Process the new status.
+	if err := testStructs.Processor.Workers().ProcessFromClientAPI(
+		ctx,
+		&messages.FromClientAPI{
+			APObjectType:   ap.ObjectNote,
+			APActivityType: ap.ActivityCreate,
+			GTSModel:       status,
+			Origin:         postingAccount,
+		},
+	); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Check status in home stream.
+	suite.checkStreamed(
+		homeStream,
+		false,
+		"",
+		"",
+	)
+}
+
+// A boost of a public status with a hashtag followed by a local user
+// who does not otherwise follow the author or booster
+// should end up in the tag-following user's home timeline as the original status.
+func (suite *FromClientAPITestSuite) TestProcessCreateBoostWithFollowedHashtag() {
+	testStructs := suite.SetupTestStructs()
+	defer suite.TearDownTestStructs(testStructs)
+
+	var (
+		ctx              = context.Background()
+		postingAccount   = suite.testAccounts["remote_account_2"]
+		boostingAccount  = suite.testAccounts["admin_account"]
+		receivingAccount = suite.testAccounts["local_account_2"]
+		streams          = suite.openStreams(ctx,
+			testStructs.Processor,
+			receivingAccount,
+			nil,
+		)
+		homeStream = streams[stream.TimelineHome]
+		testTag    = suite.testTags["welcome"]
+
+		// postingAccount posts a new public status not mentioning anyone but using testTag.
+		status = suite.newStatus(
+			ctx,
+			testStructs.State,
+			postingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			nil,
+			nil,
+			false,
+			[]string{testTag.ID},
+		)
+
+		// boostingAccount boosts that status.
+		boost = suite.newStatus(
+			ctx,
+			testStructs.State,
+			boostingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			status,
+			nil,
+			false,
+			nil,
+		)
+	)
+
+	// Check precondition: receivingAccount does not follow postingAccount.
+	following, err := testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: receivingAccount does not block postingAccount or vice versa.
+	blocking, err := testStructs.State.DB.IsEitherBlocked(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Check precondition: receivingAccount does not follow boostingAccount.
+	following, err = testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, boostingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: receivingAccount does not block boostingAccount or vice versa.
+	blocking, err = testStructs.State.DB.IsEitherBlocked(ctx, receivingAccount.ID, boostingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Setup: receivingAccount follows testTag.
+	if err := testStructs.State.DB.PutFollowedTag(ctx, receivingAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Process the boost.
+	if err := testStructs.Processor.Workers().ProcessFromClientAPI(
+		ctx,
+		&messages.FromClientAPI{
+			APObjectType:   ap.ActivityAnnounce,
+			APActivityType: ap.ActivityCreate,
+			GTSModel:       boost,
+			Origin:         postingAccount,
+		},
+	); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Check status in home stream.
+	suite.checkStreamed(
+		homeStream,
+		true,
+		"",
+		stream.EventTypeUpdate,
+	)
+}
+
+// A boost of a public status with a hashtag followed by a local user
+// who does not otherwise follow the author or booster
+// should not end up in the tag-following user's home timeline
+// if the user has the author blocked.
+func (suite *FromClientAPITestSuite) TestProcessCreateBoostWithFollowedHashtagAndBlock() {
+	testStructs := suite.SetupTestStructs()
+	defer suite.TearDownTestStructs(testStructs)
+
+	var (
+		ctx              = context.Background()
+		postingAccount   = suite.testAccounts["remote_account_1"]
+		boostingAccount  = suite.testAccounts["admin_account"]
+		receivingAccount = suite.testAccounts["local_account_2"]
+		streams          = suite.openStreams(ctx,
+			testStructs.Processor,
+			receivingAccount,
+			nil,
+		)
+		homeStream = streams[stream.TimelineHome]
+		testTag    = suite.testTags["welcome"]
+
+		// postingAccount posts a new public status not mentioning anyone but using testTag.
+		status = suite.newStatus(
+			ctx,
+			testStructs.State,
+			postingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			nil,
+			nil,
+			false,
+			[]string{testTag.ID},
+		)
+
+		// boostingAccount boosts that status.
+		boost = suite.newStatus(
+			ctx,
+			testStructs.State,
+			boostingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			status,
+			nil,
+			false,
+			nil,
+		)
+	)
+
+	// Check precondition: receivingAccount does not follow postingAccount.
+	following, err := testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: postingAccount does not block receivingAccount.
+	blocking, err := testStructs.State.DB.IsBlocked(ctx, postingAccount.ID, receivingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Check precondition: receivingAccount blocks postingAccount.
+	blocking, err = testStructs.State.DB.IsBlocked(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.True(blocking)
+
+	// Check precondition: receivingAccount does not follow boostingAccount.
+	following, err = testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, boostingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: receivingAccount does not block boostingAccount or vice versa.
+	blocking, err = testStructs.State.DB.IsEitherBlocked(ctx, receivingAccount.ID, boostingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Setup: receivingAccount follows testTag.
+	if err := testStructs.State.DB.PutFollowedTag(ctx, receivingAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Process the boost.
+	if err := testStructs.Processor.Workers().ProcessFromClientAPI(
+		ctx,
+		&messages.FromClientAPI{
+			APObjectType:   ap.ActivityAnnounce,
+			APActivityType: ap.ActivityCreate,
+			GTSModel:       boost,
+			Origin:         postingAccount,
+		},
+	); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Check status in home stream.
+	suite.checkStreamed(
+		homeStream,
+		false,
+		"",
+		"",
+	)
+}
+
+// A boost of a public status with a hashtag followed by a local user
+// who does not otherwise follow the author or booster
+// should not end up in the tag-following user's home timeline
+// if the user has the booster blocked.
+func (suite *FromClientAPITestSuite) TestProcessCreateBoostWithFollowedHashtagAndBlockedBoost() {
+	testStructs := suite.SetupTestStructs()
+	defer suite.TearDownTestStructs(testStructs)
+
+	var (
+		ctx              = context.Background()
+		postingAccount   = suite.testAccounts["admin_account"]
+		boostingAccount  = suite.testAccounts["remote_account_1"]
+		receivingAccount = suite.testAccounts["local_account_2"]
+		streams          = suite.openStreams(ctx,
+			testStructs.Processor,
+			receivingAccount,
+			nil,
+		)
+		homeStream = streams[stream.TimelineHome]
+		testTag    = suite.testTags["welcome"]
+
+		// postingAccount posts a new public status not mentioning anyone but using testTag.
+		status = suite.newStatus(
+			ctx,
+			testStructs.State,
+			postingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			nil,
+			nil,
+			false,
+			[]string{testTag.ID},
+		)
+
+		// boostingAccount boosts that status.
+		boost = suite.newStatus(
+			ctx,
+			testStructs.State,
+			boostingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			status,
+			nil,
+			false,
+			nil,
+		)
+	)
+
+	// Check precondition: receivingAccount does not follow postingAccount.
+	following, err := testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: receivingAccount does not block postingAccount or vice versa.
+	blocking, err := testStructs.State.DB.IsEitherBlocked(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Check precondition: receivingAccount does not follow boostingAccount.
+	following, err = testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, boostingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: boostingAccount does not block receivingAccount.
+	blocking, err = testStructs.State.DB.IsBlocked(ctx, boostingAccount.ID, receivingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Check precondition: receivingAccount blocks boostingAccount.
+	blocking, err = testStructs.State.DB.IsBlocked(ctx, receivingAccount.ID, boostingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.True(blocking)
+
+	// Setup: receivingAccount follows testTag.
+	if err := testStructs.State.DB.PutFollowedTag(ctx, receivingAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Process the boost.
+	if err := testStructs.Processor.Workers().ProcessFromClientAPI(
+		ctx,
+		&messages.FromClientAPI{
+			APObjectType:   ap.ActivityAnnounce,
+			APActivityType: ap.ActivityCreate,
+			GTSModel:       boost,
+			Origin:         postingAccount,
+		},
+	); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Check status in home stream.
+	suite.checkStreamed(
+		homeStream,
+		false,
+		"",
+		"",
+	)
+}
+
+// Updating a public status with a hashtag followed by a local user who does not otherwise follow the author
+// should stream a status update to the tag-following user's home timeline.
+func (suite *FromClientAPITestSuite) TestProcessUpdateStatusWithFollowedHashtag() {
+	testStructs := suite.SetupTestStructs()
+	defer suite.TearDownTestStructs(testStructs)
+
+	var (
+		ctx              = context.Background()
+		postingAccount   = suite.testAccounts["admin_account"]
+		receivingAccount = suite.testAccounts["local_account_2"]
+		streams          = suite.openStreams(ctx,
+			testStructs.Processor,
+			receivingAccount,
+			nil,
+		)
+		homeStream = streams[stream.TimelineHome]
+		testTag    = suite.testTags["welcome"]
+
+		// postingAccount posts a new public status not mentioning anyone but using testTag.
+		status = suite.newStatus(
+			ctx,
+			testStructs.State,
+			postingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			nil,
+			nil,
+			false,
+			[]string{testTag.ID},
+		)
+	)
+
+	// Check precondition: receivingAccount does not follow postingAccount.
+	following, err := testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: receivingAccount does not block postingAccount or vice versa.
+	blocking, err := testStructs.State.DB.IsEitherBlocked(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Setup: receivingAccount follows testTag.
+	if err := testStructs.State.DB.PutFollowedTag(ctx, receivingAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Update the status.
+	if err := testStructs.Processor.Workers().ProcessFromClientAPI(
+		ctx,
+		&messages.FromClientAPI{
+			APObjectType:   ap.ObjectNote,
+			APActivityType: ap.ActivityUpdate,
+			GTSModel:       status,
+			Origin:         postingAccount,
+		},
+	); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Check status in home stream.
+	suite.checkStreamed(
+		homeStream,
+		true,
+		"",
+		stream.EventTypeStatusUpdate,
+	)
+}
+
 func (suite *FromClientAPITestSuite) TestProcessStatusDelete() {
 	testStructs := suite.SetupTestStructs()
 	defer suite.TearDownTestStructs(testStructs)

internal/processing/workers/surfacetimeline.go

@@ -30,10 +30,12 @@ import (
 	"github.com/superseriousbusiness/gotosocial/internal/log"
 	"github.com/superseriousbusiness/gotosocial/internal/stream"
 	"github.com/superseriousbusiness/gotosocial/internal/timeline"
+	"github.com/superseriousbusiness/gotosocial/internal/util"
 )
 
 // timelineAndNotifyStatus inserts the given status into the HOME
-// and LIST timelines of accounts that follow the status author.
+// and LIST timelines of accounts that follow the status author,
+// as well as the HOME timelines of accounts that follow tags used by the status.
 //
 // It will also handle notifications for any mentions attached to
 // the account, notifications for any local accounts that want
@@ -56,18 +58,24 @@ func (s *Surface) timelineAndNotifyStatus(ctx context.Context, status *gtsmodel.
 		follows = append(follows, &gtsmodel.Follow{
 			AccountID:   status.AccountID,
 			Account:     status.Account,
-			Notify:      func() *bool { b := false; return &b }(), // Account shouldn't notify itself.
+			Notify:      util.Ptr(false), // Account shouldn't notify itself.
-			ShowReblogs: func() *bool { b := true; return &b }(),  // Account should show own reblogs.
+			ShowReblogs: util.Ptr(true),  // Account should show own reblogs.
 		})
 	}
 
 	// Timeline the status for each local follower of this account.
 	// This will also handle notifying any followers with notify
 	// set to true on their follow.
-	if err := s.timelineAndNotifyStatusForFollowers(ctx, status, follows); err != nil {
+	homeTimelinedAccountIDs, err := s.timelineAndNotifyStatusForFollowers(ctx, status, follows)
+	if err != nil {
 		return gtserror.Newf("error timelining status %s for followers: %w", status.ID, err)
 	}
 
+	// Timeline the status for each local account who follows a tag used by this status.
+	if err := s.timelineAndNotifyStatusForTagFollowers(ctx, status, homeTimelinedAccountIDs); err != nil {
+		return gtserror.Newf("error timelining status %s for tag followers: %w", status.ID, err)
+	}
+
 	// Notify each local account that's mentioned by this status.
 	if err := s.notifyMentions(ctx, status); err != nil {
 		return gtserror.Newf("error notifying status mentions for status %s: %w", status.ID, err)
@@ -90,15 +98,18 @@ func (s *Surface) timelineAndNotifyStatus(ctx context.Context, status *gtsmodel.
 // adding the status to list timelines + home timelines of each
 // follower, as appropriate, and notifying each follower of the
 // new status, if the status is eligible for notification.
+//
+// Returns a list of accounts which had this status inserted into their home timelines.
 func (s *Surface) timelineAndNotifyStatusForFollowers(
 	ctx context.Context,
 	status *gtsmodel.Status,
 	follows []*gtsmodel.Follow,
-) error {
+) ([]string, error) {
 	var (
 		errs                    gtserror.MultiError
 		boost                   = status.BoostOfID != ""
 		reply                   = status.InReplyToURI != ""
+		homeTimelinedAccountIDs = []string{}
 	)
 
 	for _, follow := range follows {
@@ -122,17 +133,12 @@ func (s *Surface) timelineAndNotifyStatusForFollowers(
 			continue
 		}
 
-		filters, err := s.State.DB.GetFiltersForAccountID(ctx, follow.AccountID)
+		filters, mutes, err := s.getFiltersAndMutes(ctx, follow.AccountID)
 		if err != nil {
-			return gtserror.Newf("couldn't retrieve filters for account %s: %w", follow.AccountID, err)
+			errs.Append(err)
+			continue
 		}
 
-		mutes, err := s.State.DB.GetAccountMutes(gtscontext.SetBarebones(ctx), follow.AccountID, nil)
-		if err != nil {
-			return gtserror.Newf("couldn't retrieve mutes for account %s: %w", follow.AccountID, err)
-		}
-		compiledMutes := usermute.NewCompiledUserMuteList(mutes)
-
 		// Add status to any relevant lists
 		// for this follow, if applicable.
 		s.listTimelineStatusForFollow(
@@ -141,7 +147,7 @@ func (s *Surface) timelineAndNotifyStatusForFollowers(
 			follow,
 			&errs,
 			filters,
-			compiledMutes,
+			mutes,
 		)
 
 		// Add status to home timeline for owner
@@ -154,7 +160,7 @@ func (s *Surface) timelineAndNotifyStatusForFollowers(
 			status,
 			stream.TimelineHome,
 			filters,
-			compiledMutes,
+			mutes,
 		)
 		if err != nil {
 			errs.Appendf("error home timelining status: %w", err)
@@ -166,6 +172,7 @@ func (s *Surface) timelineAndNotifyStatusForFollowers(
 			// timeline, we shouldn't notify it.
 			continue
 		}
+		homeTimelinedAccountIDs = append(homeTimelinedAccountIDs, follow.AccountID)
 
 		if !*follow.Notify {
 			// This follower doesn't have notifs
@@ -196,7 +203,7 @@ func (s *Surface) timelineAndNotifyStatusForFollowers(
 		}
 	}
 
-	return errs.Combine()
+	return homeTimelinedAccountIDs, errs.Combine()
 }
 
 // listTimelineStatusForFollow puts the given status
@@ -259,6 +266,22 @@ func (s *Surface) listTimelineStatusForFollow(
 	}
 }
 
+// getFiltersAndMutes returns an account's filters and mutes.
+func (s *Surface) getFiltersAndMutes(ctx context.Context, accountID string) ([]*gtsmodel.Filter, *usermute.CompiledUserMuteList, error) {
+	filters, err := s.State.DB.GetFiltersForAccountID(ctx, accountID)
+	if err != nil {
+		return nil, nil, gtserror.Newf("couldn't retrieve filters for account %s: %w", accountID, err)
+	}
+
+	mutes, err := s.State.DB.GetAccountMutes(gtscontext.SetBarebones(ctx), accountID, nil)
+	if err != nil {
+		return nil, nil, gtserror.Newf("couldn't retrieve mutes for account %s: %w", accountID, err)
+	}
+	compiledMutes := usermute.NewCompiledUserMuteList(mutes)
+
+	return filters, compiledMutes, err
+}
+
 // listEligible checks if the given status is eligible
 // for inclusion in the list that that the given listEntry
 // belongs to, based on the replies policy of the list.
@@ -391,6 +414,138 @@ func (s *Surface) timelineStatus(
 	return true, nil
 }
 
+// timelineAndNotifyStatusForTagFollowers inserts the status into the
+// home timeline of each local account which follows a useable tag from the status,
+// skipping accounts for which it would have already been inserted.
+func (s *Surface) timelineAndNotifyStatusForTagFollowers(
+	ctx context.Context,
+	status *gtsmodel.Status,
+	alreadyHomeTimelinedAccountIDs []string,
+) error {
+	tagFollowerAccounts, err := s.tagFollowersForStatus(ctx, status, alreadyHomeTimelinedAccountIDs)
+	if err != nil {
+		return err
+	}
+
+	if status.BoostOf != nil {
+		// Unwrap boost and work with the original status.
+		status = status.BoostOf
+	}
+
+	// Insert the status into the home timeline of each tag follower.
+	errs := gtserror.MultiError{}
+	for _, tagFollowerAccount := range tagFollowerAccounts {
+		filters, mutes, err := s.getFiltersAndMutes(ctx, tagFollowerAccount.ID)
+		if err != nil {
+			errs.Append(err)
+			continue
+		}
+
+		if _, err := s.timelineStatus(
+			ctx,
+			s.State.Timelines.Home.IngestOne,
+			tagFollowerAccount.ID, // home timelines are keyed by account ID
+			tagFollowerAccount,
+			status,
+			stream.TimelineHome,
+			filters,
+			mutes,
+		); err != nil {
+			errs.Appendf(
+				"error inserting status %s into home timeline for account %s: %w",
+				status.ID,
+				tagFollowerAccount.ID,
+				err,
+			)
+		}
+	}
+	return errs.Combine()
+}
+
+// tagFollowersForStatus gets local accounts which follow any useable tags from the status,
+// skipping any with IDs in the provided list, and any that shouldn't be able to see it due to blocks.
+func (s *Surface) tagFollowersForStatus(
+	ctx context.Context,
+	status *gtsmodel.Status,
+	skipAccountIDs []string,
+) ([]*gtsmodel.Account, error) {
+	// If the status is a boost, look at the tags from the boosted status.
+	taggedStatus := status
+	if status.BoostOf != nil {
+		taggedStatus = status.BoostOf
+	}
+
+	if taggedStatus.Visibility != gtsmodel.VisibilityPublic || len(taggedStatus.Tags) == 0 {
+		// Only public statuses with tags are eligible for tag processing.
+		return nil, nil
+	}
+
+	// Build list of useable tag IDs.
+	useableTagIDs := make([]string, 0, len(taggedStatus.Tags))
+	for _, tag := range taggedStatus.Tags {
+		if *tag.Useable {
+			useableTagIDs = append(useableTagIDs, tag.ID)
+		}
+	}
+	if len(useableTagIDs) == 0 {
+		return nil, nil
+	}
+
+	// Get IDs for all accounts who follow one or more of the useable tags from this status.
+	allTagFollowerAccountIDs, err := s.State.DB.GetAccountIDsFollowingTagIDs(ctx, useableTagIDs)
+	if err != nil {
+		return nil, gtserror.Newf("DB error getting followers for tags of status %s: %w", taggedStatus.ID, err)
+	}
+	if len(allTagFollowerAccountIDs) == 0 {
+		return nil, nil
+	}
+
+	// Build set for faster lookup of account IDs to skip.
+	skipAccountIDSet := make(map[string]struct{}, len(skipAccountIDs))
+	for _, accountID := range skipAccountIDs {
+		skipAccountIDSet[accountID] = struct{}{}
+	}
+
+	// Build list of tag follower account IDs,
+	// except those which have already had this status inserted into their timeline.
+	tagFollowerAccountIDs := make([]string, 0, len(allTagFollowerAccountIDs))
+	for _, accountID := range allTagFollowerAccountIDs {
+		if _, skip := skipAccountIDSet[accountID]; skip {
+			continue
+		}
+		tagFollowerAccountIDs = append(tagFollowerAccountIDs, accountID)
+	}
+	if len(tagFollowerAccountIDs) == 0 {
+		return nil, nil
+	}
+
+	// Retrieve accounts for remaining tag followers.
+	tagFollowerAccounts, err := s.State.DB.GetAccountsByIDs(ctx, tagFollowerAccountIDs)
+	if err != nil {
+		return nil, gtserror.Newf("DB error getting accounts for followers of tags of status %s: %w", taggedStatus.ID, err)
+	}
+
+	// Check the visibility of the *input* status for each account.
+	// This accounts for the visibility of the boost as well as the original, if the input status is a boost.
+	errs := gtserror.MultiError{}
+	visibleTagFollowerAccounts := make([]*gtsmodel.Account, 0, len(tagFollowerAccounts))
+	for _, account := range tagFollowerAccounts {
+		visible, err := s.VisFilter.StatusVisible(ctx, account, status)
+		if err != nil {
+			errs.Appendf(
+				"error checking visibility of status %s to account %s",
+				status.ID,
+				account.ID,
+			)
+		}
+		if visible {
+			visibleTagFollowerAccounts = append(visibleTagFollowerAccounts, account)
+		}
+	}
+
+	return visibleTagFollowerAccounts, errs.Combine()
+}
+
 // deleteStatusFromTimelines completely removes the given status from all timelines.
 // It will also stream deletion of the status to all open streams.
 func (s *Surface) deleteStatusFromTimelines(ctx context.Context, statusID string) error {
@@ -425,7 +580,7 @@ func (s *Surface) invalidateStatusFromTimelines(ctx context.Context, statusID st
 }
 
 // timelineStatusUpdate looks up HOME and LIST timelines of accounts
-// that follow the the status author and pushes edit messages into any
+// that follow the the status author or tags and pushes edit messages into any
 // active streams.
 // Note that calling invalidateStatusFromTimelines takes care of the
 // state in general, we just need to do this for any streams that are
@@ -454,10 +609,15 @@ func (s *Surface) timelineStatusUpdate(ctx context.Context, status *gtsmodel.Sta
 	}
 
 	// Push to streams for each local follower of this account.
-	if err := s.timelineStatusUpdateForFollowers(ctx, status, follows); err != nil {
+	homeTimelinedAccountIDs, err := s.timelineStatusUpdateForFollowers(ctx, status, follows)
+	if err != nil {
 		return gtserror.Newf("error timelining status %s for followers: %w", status.ID, err)
 	}
 
+	if err := s.timelineStatusUpdateForTagFollowers(ctx, status, homeTimelinedAccountIDs); err != nil {
+		return gtserror.Newf("error timelining status %s for tag followers: %w", status.ID, err)
+	}
+
 	return nil
 }
 
@@ -465,13 +625,16 @@ func (s *Surface) timelineStatusUpdate(ctx context.Context, status *gtsmodel.Sta
 // slice of followers of the account that posted the given status,
 // pushing update messages into open list/home streams of each
 // follower.
+//
+// Returns a list of accounts which had this status updated in their home timelines.
 func (s *Surface) timelineStatusUpdateForFollowers(
 	ctx context.Context,
 	status *gtsmodel.Status,
 	follows []*gtsmodel.Follow,
-) error {
+) ([]string, error) {
 	var (
 		errs                    gtserror.MultiError
+		homeTimelinedAccountIDs = []string{}
 	)
 
 	for _, follow := range follows {
@@ -495,17 +658,12 @@ func (s *Surface) timelineStatusUpdateForFollowers(
 			continue
 		}
 
-		filters, err := s.State.DB.GetFiltersForAccountID(ctx, follow.AccountID)
+		filters, mutes, err := s.getFiltersAndMutes(ctx, follow.AccountID)
 		if err != nil {
-			return gtserror.Newf("couldn't retrieve filters for account %s: %w", follow.AccountID, err)
+			errs.Append(err)
+			continue
 		}
 
-		mutes, err := s.State.DB.GetAccountMutes(gtscontext.SetBarebones(ctx), follow.AccountID, nil)
-		if err != nil {
-			return gtserror.Newf("couldn't retrieve mutes for account %s: %w", follow.AccountID, err)
-		}
-		compiledMutes := usermute.NewCompiledUserMuteList(mutes)
-
 		// Add status to any relevant lists
 		// for this follow, if applicable.
 		s.listTimelineStatusUpdateForFollow(
@@ -514,26 +672,30 @@ func (s *Surface) timelineStatusUpdateForFollowers(
 			follow,
 			&errs,
 			filters,
-			compiledMutes,
+			mutes,
 		)
 
 		// Add status to home timeline for owner
 		// of this follow, if applicable.
-		err = s.timelineStreamStatusUpdate(
+		homeTimelined, err := s.timelineStreamStatusUpdate(
 			ctx,
 			follow.Account,
 			status,
 			stream.TimelineHome,
 			filters,
-			compiledMutes,
+			mutes,
 		)
 		if err != nil {
 			errs.Appendf("error home timelining status: %w", err)
 			continue
 		}
+
+		if homeTimelined {
+			homeTimelinedAccountIDs = append(homeTimelinedAccountIDs, follow.AccountID)
+		}
 	}
 
-	return errs.Combine()
+	return homeTimelinedAccountIDs, errs.Combine()
 }
 
 // listTimelineStatusUpdateForFollow pushes edits of the given status
@@ -580,7 +742,7 @@ func (s *Surface) listTimelineStatusUpdateForFollow(
 		// At this point we are certain this status
 		// should be included in the timeline of the
 		// list that this list entry belongs to.
-		if err := s.timelineStreamStatusUpdate(
+		if _, err := s.timelineStreamStatusUpdate(
 			ctx,
 			follow.Account,
 			status,
@@ -596,6 +758,8 @@ func (s *Surface) listTimelineStatusUpdateForFollow(
 
 // timelineStatusUpdate streams the edited status to the user using the
 // given streamType.
+//
+// Returns whether it was actually streamed.
 func (s *Surface) timelineStreamStatusUpdate(
 	ctx context.Context,
 	account *gtsmodel.Account,
@@ -603,16 +767,62 @@ func (s *Surface) timelineStreamStatusUpdate(
 	streamType string,
 	filters []*gtsmodel.Filter,
 	mutes *usermute.CompiledUserMuteList,
-) error {
+) (bool, error) {
 	apiStatus, err := s.Converter.StatusToAPIStatus(ctx, status, account, statusfilter.FilterContextHome, filters, mutes)
 	if errors.Is(err, statusfilter.ErrHideStatus) {
 		// Don't put this status in the stream.
-		return nil
+		return false, nil
 	}
 	if err != nil {
 		err = gtserror.Newf("error converting status %s to frontend representation: %w", status.ID, err)
-		return err
+		return false, err
 	}
 	s.Stream.StatusUpdate(ctx, account, apiStatus, streamType)
-	return nil
+	return true, nil
+}
+
+// timelineStatusUpdateForTagFollowers streams update notifications to the
+// home timeline of each local account which follows a tag used by the status,
+// skipping accounts for which it would have already been streamed.
+func (s *Surface) timelineStatusUpdateForTagFollowers(
+	ctx context.Context,
+	status *gtsmodel.Status,
+	alreadyHomeTimelinedAccountIDs []string,
+) error {
+	tagFollowerAccounts, err := s.tagFollowersForStatus(ctx, status, alreadyHomeTimelinedAccountIDs)
+	if err != nil {
+		return err
+	}
+
+	if status.BoostOf != nil {
+		// Unwrap boost and work with the original status.
+		status = status.BoostOf
+	}
+
+	// Stream the update to the home timeline of each tag follower.
+	errs := gtserror.MultiError{}
+	for _, tagFollowerAccount := range tagFollowerAccounts {
+		filters, mutes, err := s.getFiltersAndMutes(ctx, tagFollowerAccount.ID)
+		if err != nil {
+			errs.Append(err)
+			continue
+		}
+
+		if _, err := s.timelineStreamStatusUpdate(
+			ctx,
+			tagFollowerAccount,
+			status,
+			stream.TimelineHome,
+			filters,
+			mutes,
+		); err != nil {
+			errs.Appendf(
+				"error updating status %s on home timeline for account %s: %w",
+				status.ID,
+				tagFollowerAccount.ID,
+				err,
+			)
+		}
+	}
+	return errs.Combine()
 }

internal/typeutils/internaltofrontend.go

@@ -740,7 +740,8 @@ func (c *Converter) EmojiCategoryToAPIEmojiCategory(ctx context.Context, categor
 
 // TagToAPITag converts a gts model tag into its api (frontend) representation for serialization on the API.
 // If stubHistory is set to 'true', then the 'history' field of the tag will be populated with a pointer to an empty slice, for API compatibility reasons.
-func (c *Converter) TagToAPITag(ctx context.Context, t *gtsmodel.Tag, stubHistory bool) (apimodel.Tag, error) {
+// following is an optional flag marking whether the currently authenticated user (if there is one) is following the tag.
+func (c *Converter) TagToAPITag(ctx context.Context, t *gtsmodel.Tag, stubHistory bool, following *bool) (apimodel.Tag, error) {
 	return apimodel.Tag{
 		Name: strings.ToLower(t.Name),
 		URL:  uris.URIForTag(t.Name),
@@ -752,6 +753,7 @@ func (c *Converter) TagToAPITag(ctx context.Context, t *gtsmodel.Tag, stubHistor
 			h := make([]any, 0)
 			return &h
 		}(),
+		Following: following,
 	}, nil
 }
 
@@ -2347,7 +2349,7 @@ func (c *Converter) convertTagsToAPITags(ctx context.Context, tags []*gtsmodel.T
 
 	// Convert GTS models to frontend models
 	for _, tag := range tags {
-		apiTag, err := c.TagToAPITag(ctx, tag, false)
+		apiTag, err := c.TagToAPITag(ctx, tag, false, nil)
 		if err != nil {
 			errs.Appendf("error converting tag %s to api tag: %w", tag.ID, err)
 			continue

test/envparsing.sh

@@ -23,6 +23,7 @@ EXPECT=$(cat << "EOF"
     "application-name": "gts",
     "bind-address": "127.0.0.1",
     "cache": {
+        "account-ids-following-tag-mem-ratio": 1,
         "account-mem-ratio": 5,
         "account-note-mem-ratio": 1,
         "account-settings-mem-ratio": 0.1,
@@ -63,6 +64,7 @@ EXPECT=$(cat << "EOF"
         "status-fave-ids-mem-ratio": 3,
         "status-fave-mem-ratio": 2,
         "status-mem-ratio": 5,
+        "tag-ids-followed-by-account-mem-ratio": 1,
         "tag-mem-ratio": 2,
         "thread-mute-mem-ratio": 0.2,
         "token-mem-ratio": 0.75,