Avoid panic on write after close in http

By adding a (*WriteFlusher).Close, we limit the Write calls to possibly
deallocated http response buffers to the lifetime of an http request.
Typically, this is seen as a very confusing panic, the cause is usually a
situation where an http.ResponseWriter is held after request completion. We
avoid the panic by disallowing further writes to the response writer after the
request is completed.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

ioutils/writeflusher.go

@@ -1,32 +1,54 @@
 package ioutils
 
 import (
+	"errors"
 	"io"
 	"net/http"
 	"sync"
 )
 
-// WriteFlusher wraps the Write and Flush operation.
+// 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 {
-	sync.Mutex
+	mu      sync.Mutex
 	w       io.Writer
 	flusher http.Flusher
 	flushed bool
+	closed  error
+
+	// TODO(stevvooe): Use channel for closed instead, remove mutex. Using a
+	// channel will allow one to properly order the operations.
 }
 
+var errWriteFlusherClosed = errors.New("writeflusher: closed")
+
 func (wf *WriteFlusher) Write(b []byte) (n int, err error) {
-	wf.Lock()
+	wf.mu.Lock()
-	defer wf.Unlock()
+	defer wf.mu.Unlock()
+	if wf.closed != nil {
+		return 0, wf.closed
+	}
+
 	n, err = wf.w.Write(b)
-	wf.flushed = true
+	wf.flush() // every write is a flush.
-	wf.flusher.Flush()
 	return n, err
 }
 
 // Flush the stream immediately.
 func (wf *WriteFlusher) Flush() {
-	wf.Lock()
+	wf.mu.Lock()
-	defer wf.Unlock()
+	defer wf.mu.Unlock()
+
+	wf.flush()
+}
+
+// flush the stream immediately without taking a lock. Used internally.
+func (wf *WriteFlusher) flush() {
+	if wf.closed != nil {
+		return
+	}
+
 	wf.flushed = true
 	wf.flusher.Flush()
 }
@@ -34,11 +56,30 @@ func (wf *WriteFlusher) Flush() {
 // Flushed returns the state of flushed.
 // If it's flushed, return true, or else it return false.
 func (wf *WriteFlusher) Flushed() bool {
-	wf.Lock()
+	// BUG(stevvooe): Remove this method. Its use is inherently racy. Seems to
-	defer wf.Unlock()
+	// be used to detect whether or a response code has been issued or not.
+	// Another hook should be used instead.
+	wf.mu.Lock()
+	defer wf.mu.Unlock()
+
 	return wf.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.mu.Lock()
+	defer wf.mu.Unlock()
+
+	if wf.closed != nil {
+		return wf.closed
+	}
+
+	wf.closed = errWriteFlusherClosed
+	return nil
+}
+
 // NewWriteFlusher returns a new WriteFlusher.
 func NewWriteFlusher(w io.Writer) *WriteFlusher {
 	var flusher http.Flusher