Use known issue IID to generate new PR index number when migrating from GitLab (#28616)

Fix #13884

services/migrations/gitlab.go

@@ -55,6 +55,23 @@ func (f *GitlabDownloaderFactory) GitServiceType() structs.GitServiceType {
 	return structs.GitlabService
 }
 
+type gitlabIIDResolver struct {
+	maxIssueIID int64
+	frozen      bool
+}
+
+func (r *gitlabIIDResolver) recordIssueIID(issueIID int) {
+	if r.frozen {
+		panic("cannot record issue IID after pull request IID generation has started")
+	}
+	r.maxIssueIID = max(r.maxIssueIID, int64(issueIID))
+}
+
+func (r *gitlabIIDResolver) generatePullRequestNumber(mrIID int) int64 {
+	r.frozen = true
+	return r.maxIssueIID + int64(mrIID)
+}
+
 // GitlabDownloader implements a Downloader interface to get repository information
 // from gitlab via go-gitlab
 // - issueCount is incremented in GetIssues() to ensure PR and Issue numbers do not overlap,
@@ -66,7 +83,7 @@ type GitlabDownloader struct {
 	baseURL     string
 	repoID      int
 	repoName    string
-	issueCount int64
+	iidResolver gitlabIIDResolver
 	maxPerPage  int
 }
 
@@ -450,8 +467,8 @@ func (g *GitlabDownloader) GetIssues(page, perPage int) ([]*base.Issue, bool, er
 			Context:      gitlabIssueContext{IsMergeRequest: false},
 		})
 
-		// increment issueCount, to be used in GetPullRequests()
+		// record the issue IID, to be used in GetPullRequests()
-		g.issueCount++
+		g.iidResolver.recordIssueIID(issue.IID)
 	}
 
 	return allIssues, len(issues) < perPage, nil
@@ -607,8 +624,8 @@ func (g *GitlabDownloader) GetPullRequests(page, perPage int) ([]*base.PullReque
 			awardPage++
 		}
 
-		// Add the PR ID to the Issue Count because PR and Issues share ID space in Gitea
+		// Generate new PR Numbers by the known Issue Numbers, because they share the same number space in Gitea, but they are independent in Gitlab
-		newPRNumber := g.issueCount + int64(pr.IID)
+		newPRNumber := g.iidResolver.generatePullRequestNumber(pr.IID)
 
 		allPRs = append(allPRs, &base.PullRequest{
 			Title:          pr.Title,

services/migrations/gitlab_test.go

@@ -516,3 +516,20 @@ func TestAwardsToReactions(t *testing.T) {
 		},
 	}, reactions)
 }
+
+func TestGitlabIIDResolver(t *testing.T) {
+	r := gitlabIIDResolver{}
+	r.recordIssueIID(1)
+	r.recordIssueIID(2)
+	r.recordIssueIID(3)
+	r.recordIssueIID(2)
+	assert.EqualValues(t, 4, r.generatePullRequestNumber(1))
+	assert.EqualValues(t, 13, r.generatePullRequestNumber(10))
+
+	assert.Panics(t, func() {
+		r := gitlabIIDResolver{}
+		r.recordIssueIID(1)
+		assert.EqualValues(t, 2, r.generatePullRequestNumber(1))
+		r.recordIssueIID(3) // the generation procedure has been started, it shouldn't accept any new issue IID, so it panics
+	})
+}