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>
(cherry picked from commit ec2289b2d9ac79fd5e0f69f56f023dfe8ee78bf8)

api/server/router/local/container.go

@@ -66,7 +66,9 @@ func (s *router) getContainersStats(ctx context.Context, w http.ResponseWriter,
 		w.Header().Set("Content-Type", "application/json")
 		out = w
 	} else {
-		out = ioutils.NewWriteFlusher(w)
+		wf := ioutils.NewWriteFlusher(w)
+		out = wf
+		defer wf.Close()
 	}
 
 	var closeNotifier <-chan bool
@@ -122,11 +124,16 @@ func (s *router) getContainersLogs(ctx context.Context, w http.ResponseWriter, r
 		return derr.ErrorCodeNoSuchContainer.WithArgs(containerName)
 	}
 
-	outStream := ioutils.NewWriteFlusher(w)
 	// write an empty chunk of data (this is to ensure that the
 	// HTTP Response is sent immediately, even if the container has
 	// not yet produced any data)
-	outStream.Write(nil)
+	w.WriteHeader(http.StatusOK)
+	if flusher, ok := w.(http.Flusher); ok {
+		flusher.Flush()
+	}
+
+	output := ioutils.NewWriteFlusher(w)
+	defer output.Close()
 
 	logsConfig := &daemon.ContainerLogsConfig{
 		Follow:     httputils.BoolValue(r, "follow"),
@@ -135,7 +142,7 @@ func (s *router) getContainersLogs(ctx context.Context, w http.ResponseWriter, r
 		Tail:       r.Form.Get("tail"),
 		UseStdout:  stdout,
 		UseStderr:  stderr,
-		OutStream:  outStream,
+		OutStream:  output,
 		Stop:       closeNotifier,
 	}
 

api/server/router/local/image.go

@@ -105,6 +105,7 @@ func (s *router) postImagesCreate(ctx context.Context, w http.ResponseWriter, r
 		err    error
 		output = ioutils.NewWriteFlusher(w)
 	)
+	defer output.Close()
 
 	w.Header().Set("Content-Type", "application/json")
 
@@ -188,6 +189,7 @@ func (s *router) postImagesPush(ctx context.Context, w http.ResponseWriter, r *h
 
 	name := vars["name"]
 	output := ioutils.NewWriteFlusher(w)
+	defer output.Close()
 	imagePushConfig := &graph.ImagePushConfig{
 		MetaHeaders: metaHeaders,
 		AuthConfig:  authConfig,
@@ -218,6 +220,7 @@ func (s *router) getImagesGet(ctx context.Context, w http.ResponseWriter, r *htt
 	w.Header().Set("Content-Type", "application/x-tar")
 
 	output := ioutils.NewWriteFlusher(w)
+	defer output.Close()
 	var names []string
 	if name, ok := vars["name"]; ok {
 		names = []string{name}
@@ -297,6 +300,7 @@ func (s *router) postBuild(ctx context.Context, w http.ResponseWriter, r *http.R
 
 	version := httputils.VersionFromContext(ctx)
 	output := ioutils.NewWriteFlusher(w)
+	defer output.Close()
 	sf := streamformatter.NewJSONStreamFormatter()
 	errf := func(err error) error {
 		// Do not write the error in the http output if it's still empty.

api/server/router/local/info.go

@@ -52,16 +52,6 @@ func (s *router) getInfo(ctx context.Context, w http.ResponseWriter, r *http.Req
 	return httputils.WriteJSON(w, http.StatusOK, info)
 }
 
-func buildOutputEncoder(w http.ResponseWriter) *json.Encoder {
-	w.Header().Set("Content-Type", "application/json")
-	outStream := ioutils.NewWriteFlusher(w)
-	// Write an empty chunk of data.
-	// This is to ensure that the HTTP status code is sent immediately,
-	// so that it will not block the receiver.
-	outStream.Write(nil)
-	return json.NewEncoder(outStream)
-}
-
 func (s *router) getEvents(ctx context.Context, w http.ResponseWriter, r *http.Request, vars map[string]string) error {
 	if err := httputils.ParseForm(r); err != nil {
 		return err
@@ -87,7 +77,19 @@ func (s *router) getEvents(ctx context.Context, w http.ResponseWriter, r *http.R
 		return err
 	}
 
-	enc := buildOutputEncoder(w)
+	w.Header().Set("Content-Type", "application/json")
+
+	// This is to ensure that the HTTP status code is sent immediately,
+	// so that it will not block the receiver.
+	w.WriteHeader(http.StatusOK)
+	if flusher, ok := w.(http.Flusher); ok {
+		flusher.Flush()
+	}
+
+	output := ioutils.NewWriteFlusher(w)
+	defer output.Close()
+
+	enc := json.NewEncoder(output)
 	d := s.daemon
 	es := d.EventsService
 	current, l := es.Subscribe()