moby/pkg/ioutils/writeflusher.go

package ioutils

import (
	"io"
	"sync"
)

// WriteFlusher wraps the Write and Flush operation ensuring that every write
// is a flush. In addition, the Close method can be called to intercept
// Read/Write calls if the targets lifecycle has already ended.
type WriteFlusher struct {
	w           io.Writer
	flusher     flusher
	flushed     chan struct{}
	flushedOnce sync.Once
	closed      chan struct{}
	closeLock   sync.Mutex
}

type flusher interface {
	Flush()
}

func (wf *WriteFlusher) Write(b []byte) (int, error) {
	select {
	case <-wf.closed:
		return 0, io.EOF
	default:
	}

	n, err := wf.w.Write(b)
	wf.Flush() // every write is a flush.
	return n, err
}

// Flush the stream immediately.
func (wf *WriteFlusher) Flush() {
	select {
	case <-wf.closed:
		return
	default:
	}

	wf.flushedOnce.Do(func() {
		close(wf.flushed)
	})
	wf.flusher.Flush()
}

// Flushed returns the state of flushed.
// If it's flushed, return true, or else it return false.
func (wf *WriteFlusher) Flushed() bool {
	// BUG(stevvooe): Remove this method. Its use is inherently racy. Seems to
	// be used to detect whether or a response code has been issued or not.
	// Another hook should be used instead.
	var flushed bool
	select {
	case <-wf.flushed:
		flushed = true
	default:
	}
	return flushed
}

// Close closes the write flusher, disallowing any further writes to the
// target. After the flusher is closed, all calls to write or flush will
// result in an error.
func (wf *WriteFlusher) Close() error {
	wf.closeLock.Lock()
	defer wf.closeLock.Unlock()

	select {
	case <-wf.closed:
		return io.EOF
	default:
		close(wf.closed)
	}
	return nil
}

// nopFlusher represents a type which flush operation is nop.
type nopFlusher struct{}

// Flush is a nop operation.
func (f *nopFlusher) Flush() {}

// NewWriteFlusher returns a new WriteFlusher.
func NewWriteFlusher(w io.Writer) *WriteFlusher {
	var fl flusher
	if f, ok := w.(flusher); ok {
		fl = f
	} else {
		fl = &nopFlusher{}
	}
	return &WriteFlusher{w: w, flusher: fl, closed: make(chan struct{}), flushed: make(chan struct{})}
}