121 lines
3.1 KiB
Go
121 lines
3.1 KiB
Go
// Package ctxio provides io.Reader and io.Writer wrappers that
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// respect context.Contexts. Use these at the interface between
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// your context code and your io.
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//
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// WARNING: read the code. see how writes and reads will continue
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// until you cancel the io. Maybe this package should provide
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// versions of io.ReadCloser and io.WriteCloser that automatically
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// call .Close when the context expires. But for now -- since in my
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// use cases I have long-lived connections with ephemeral io wrappers
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// -- this has yet to be a need.
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package ctxio
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import (
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"io"
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context "golang.org/x/net/context"
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)
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type ioret struct {
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n int
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err error
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}
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type Writer interface {
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io.Writer
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}
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type ctxWriter struct {
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w io.Writer
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ctx context.Context
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}
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// NewWriter wraps a writer to make it respect given Context.
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// If there is a blocking write, the returned Writer will return
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// whenever the context is cancelled (the return values are n=0
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// and err=ctx.Err().)
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//
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// Note well: this wrapper DOES NOT ACTUALLY cancel the underlying
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// write-- there is no way to do that with the standard go io
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// interface. So the read and write _will_ happen or hang. So, use
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// this sparingly, make sure to cancel the read or write as necesary
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// (e.g. closing a connection whose context is up, etc.)
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//
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// Furthermore, in order to protect your memory from being read
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// _after_ you've cancelled the context, this io.Writer will
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// first make a **copy** of the buffer.
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func NewWriter(ctx context.Context, w io.Writer) *ctxWriter {
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if ctx == nil {
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ctx = context.Background()
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}
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return &ctxWriter{ctx: ctx, w: w}
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}
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func (w *ctxWriter) Write(buf []byte) (int, error) {
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buf2 := make([]byte, len(buf))
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copy(buf2, buf)
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c := make(chan ioret, 1)
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go func() {
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n, err := w.w.Write(buf2)
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c <- ioret{n, err}
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close(c)
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}()
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select {
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case r := <-c:
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return r.n, r.err
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case <-w.ctx.Done():
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return 0, w.ctx.Err()
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}
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}
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type Reader interface {
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io.Reader
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}
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type ctxReader struct {
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r io.Reader
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ctx context.Context
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}
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// NewReader wraps a reader to make it respect given Context.
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// If there is a blocking read, the returned Reader will return
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// whenever the context is cancelled (the return values are n=0
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// and err=ctx.Err().)
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//
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// Note well: this wrapper DOES NOT ACTUALLY cancel the underlying
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// write-- there is no way to do that with the standard go io
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// interface. So the read and write _will_ happen or hang. So, use
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// this sparingly, make sure to cancel the read or write as necesary
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// (e.g. closing a connection whose context is up, etc.)
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//
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// Furthermore, in order to protect your memory from being read
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// _before_ you've cancelled the context, this io.Reader will
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// allocate a buffer of the same size, and **copy** into the client's
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// if the read succeeds in time.
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func NewReader(ctx context.Context, r io.Reader) *ctxReader {
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return &ctxReader{ctx: ctx, r: r}
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}
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func (r *ctxReader) Read(buf []byte) (int, error) {
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buf2 := make([]byte, len(buf))
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c := make(chan ioret, 1)
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go func() {
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n, err := r.r.Read(buf2)
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c <- ioret{n, err}
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close(c)
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}()
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select {
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case ret := <-c:
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copy(buf, buf2)
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return ret.n, ret.err
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case <-r.ctx.Done():
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return 0, r.ctx.Err()
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}
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}
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