xio: Add Pipe - io.Pipe analog that supports cancellation

I need this to support sysread(/head/watch) cancellation in WCFS
filesystem [1,2,3].

[1] wendelin.core@b17aeb8c
[2] wendelin.core@f05271b1
[3] wendelin.core@5ba816da

xio/pipe.go

+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE-go file.
+
+// Pipe adapter to connect code expecting an xio.Reader
+// with code expecting an xio.Writer.
+
+package xio
+
+import (
+	"context"
+	"io"
+	"sync"
+)
+
+// onceError is an object that will only store an error once.
+type onceError struct {
+	sync.Mutex // guards following
+	err        error
+}
+
+func (a *onceError) Store(err error) {
+	a.Lock()
+	defer a.Unlock()
+	if a.err != nil {
+		return
+	}
+	a.err = err
+}
+func (a *onceError) Load() error {
+	a.Lock()
+	defer a.Unlock()
+	return a.err
+}
+
+// A pipe is the shared pipe structure underlying PipeReader and PipeWriter.
+type pipe struct {
+	wrMu sync.Mutex // Serializes Write operations
+	wrCh chan []byte
+	rdCh chan int
+
+	once sync.Once // Protects closing done
+	done chan struct{}
+	rerr onceError
+	werr onceError
+}
+
+func (p *pipe) Read(ctx context.Context, b []byte) (n int, err error) {
+	select {
+	case <-p.done:
+		return 0, p.readCloseError()
+	case <-ctx.Done():
+		return 0, ctx.Err()
+	default:
+	}
+
+	select {
+	case bw := <-p.wrCh:
+		nr := copy(b, bw)
+		p.rdCh <- nr
+		return nr, nil
+	case <-p.done:
+		return 0, p.readCloseError()
+	case <-ctx.Done():
+		return 0, ctx.Err()
+	}
+}
+
+func (p *pipe) readCloseError() error {
+	rerr := p.rerr.Load()
+	if werr := p.werr.Load(); rerr == nil && werr != nil {
+		return werr
+	}
+	return io.ErrClosedPipe
+}
+
+func (p *pipe) CloseRead(err error) error {
+	if err == nil {
+		err = io.ErrClosedPipe
+	}
+	p.rerr.Store(err)
+	p.once.Do(func() { close(p.done) })
+	return nil
+}
+
+func (p *pipe) Write(ctx context.Context, b []byte) (n int, err error) {
+	select {
+	case <-p.done:
+		return 0, p.writeCloseError()
+	case <-ctx.Done():
+		return 0, ctx.Err()
+	default:
+		p.wrMu.Lock()
+		defer p.wrMu.Unlock()
+	}
+
+	for once := true; once || len(b) > 0; once = false {
+		select {
+		case p.wrCh <- b:
+			nw := <-p.rdCh
+			b = b[nw:]
+			n += nw
+		case <-p.done:
+			return n, p.writeCloseError()
+		case <-ctx.Done():
+			return n, ctx.Err()
+		}
+	}
+	return n, nil
+}
+
+func (p *pipe) writeCloseError() error {
+	werr := p.werr.Load()
+	if rerr := p.rerr.Load(); werr == nil && rerr != nil {
+		return rerr
+	}
+	return io.ErrClosedPipe
+}
+
+func (p *pipe) CloseWrite(err error) error {
+	if err == nil {
+		err = io.EOF
+	}
+	p.werr.Store(err)
+	p.once.Do(func() { close(p.done) })
+	return nil
+}
+
+// A PipeReader is the read half of a pipe.
+//
+// It is similar to io.PipeReader, but additionally provides cancellation support for Read.
+type PipeReader struct {
+	p *pipe
+}
+
+// Read implements xio.Reader interface:
+// it reads data from the pipe, blocking until a writer
+// arrives or the write end is closed.
+// If the write end is closed with an error, that error is
+// returned as err; otherwise err is EOF.
+func (r *PipeReader) Read(ctx context.Context, data []byte) (n int, err error) {
+	return r.p.Read(ctx, data)
+}
+
+// Close closes the reader; subsequent writes to the
+// write half of the pipe will return the error io.ErrClosedPipe.
+func (r *PipeReader) Close() error {
+	return r.CloseWithError(nil)
+}
+
+// CloseWithError closes the reader; subsequent writes
+// to the write half of the pipe will return the error err.
+//
+// CloseWithError never overwrites the previous error if it exists
+// and always returns nil.
+func (r *PipeReader) CloseWithError(err error) error {
+	return r.p.CloseRead(err)
+}
+
+// A PipeWriter is the write half of a pipe.
+//
+// It is similar to io.PipeWriter, but additionally provides cancellation support for Write.
+type PipeWriter struct {
+	p *pipe
+}
+
+// Write implements xio.Writer interface:
+// it writes data to the pipe, blocking until one or more readers
+// have consumed all the data or the read end is closed.
+// If the read end is closed with an error, that err is
+// returned as err; otherwise err is io.ErrClosedPipe.
+func (w *PipeWriter) Write(ctx context.Context, data []byte) (n int, err error) {
+	return w.p.Write(ctx, data)
+}
+
+// Close closes the writer; subsequent reads from the
+// read half of the pipe will return no bytes and EOF.
+func (w *PipeWriter) Close() error {
+	return w.CloseWithError(nil)
+}
+
+// CloseWithError closes the writer; subsequent reads from the
+// read half of the pipe will return no bytes and the error err,
+// or EOF if err is nil.
+//
+// CloseWithError never overwrites the previous error if it exists
+// and always returns nil.
+func (w *PipeWriter) CloseWithError(err error) error {
+	return w.p.CloseWrite(err)
+}
+
+// Pipe creates a synchronous in-memory pipe.
+// It can be used to connect code expecting a xio.Reader
+// with code expecting a xio.Writer.
+//
+// Reads and Writes on the pipe are matched one to one
+// except when multiple Reads are needed to consume a single Write.
+// That is, each Write to the PipeWriter blocks until it has satisfied
+// one or more Reads from the PipeReader that fully consume
+// the written data.
+// The data is copied directly from the Write to the corresponding
+// Read (or Reads); there is no internal buffering.
+//
+// It is safe to call Read and Write in parallel with each other or with Close.
+// Parallel calls to Read and parallel calls to Write are also safe:
+// the individual calls will be gated sequentially.
+//
+// Pipe is similar to io.Pipe but additionally provides cancellation support
+// for Read and Write.
+func Pipe() (*PipeReader, *PipeWriter) {
+	p := &pipe{
+		wrCh: make(chan []byte),
+		rdCh: make(chan int),
+		done: make(chan struct{}),
+	}
+	return &PipeReader{p}, &PipeWriter{p}
+}

xio/xio.go

@@ -26,6 +26,8 @@
 //	  It is the opposite operation for BindCtx, but for arbitrary io.X
 //	  returned xio.X handles context only on best-effort basis. In
 //	  particular IO cancellation is not reliably handled for os.File .
+//	- Pipe amends io.Pipe and creates synchronous in-memory pipe that
+//	  supports IO cancellation.
 //
 // Miscellaneous utilities:
 //