net/http, doc/go1.13.html: revert TimeoutHandler.Flush

Also added a test to ensure that any interactions
between TimeoutHandler and Flusher result in the
correct status code and body, but also that we don't
get superfluous logs from stray writes as was seen
in the bug report.

Fixes #34439.

Change-Id: I4af62db256742326f9353f98a2fcb5f71d2a5fd9
Reviewed-on: https://go-review.googlesource.com/c/go/+/197659
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

doc/go1.13.html

@@ -821,7 +821,7 @@ godoc
     <p><!-- CL 154383 -->
       <a href="/pkg/net/http/#TimeoutHandler"><code>TimeoutHandler</code></a>'s
       <a href="/pkg/net/http/#ResponseWriter"><code>ResponseWriter</code></a> now implements the
-      <a href="/pkg/net/http/#Pusher"><code>Pusher</code></a> and <a href="/pkg/net/http/#Flusher"><code>Flusher</code></a> interfaces.
+      <a href="/pkg/net/http/#Pusher"><code>Pusher</code></a> interface.
     </p>
 
     <p><!-- CL 157339 -->

src/net/http/serve_test.go

@@ -6161,6 +6161,54 @@ func TestUnsupportedTransferEncodingsReturn501(t *testing.T) {
 	}
 }
 
+// Issue 34439: ensure that TimeoutHandler doesn't implement Flusher
+// and that any interaction with Flusher won't affect TimeoutHandler's behavior.
+func TestTimeoutHandlerAndFlusher(t *testing.T) {
+	timeout := 50 * time.Millisecond
+
+	handler := HandlerFunc(func(w ResponseWriter, r *Request) {
+		w.WriteHeader(StatusTeapot)
+		w.Write([]byte("line1\n"))
+		fl, ok := w.(Flusher)
+		if ok {
+			fl.Flush()
+		}
+		time.Sleep(timeout * 2)
+		w.Write([]byte("line2\n"))
+	})
+
+	cst := httptest.NewUnstartedServer(TimeoutHandler(handler, timeout, "TIMED OUT\n"))
+	// Provide a logger that will report an error on any superfluous log.
+	cst.Config.ErrorLog = log.New(&errorOnWrite{t: t}, "", 0)
+	cst.Start()
+	defer cst.Close()
+
+	res, err := cst.Client().Get(cst.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer res.Body.Close()
+
+	if g, w := res.StatusCode, StatusServiceUnavailable; g != w {
+		t.Errorf("Status code mismatch\ngot:  %d\nwant: %d", g, w)
+	}
+
+	slurp, _ := ioutil.ReadAll(res.Body)
+	if g, w := string(slurp), "TIMED OUT\n"; g != w {
+		t.Fatalf("Body mismatch\ngot:  %q\nwant: %q", g, w)
+	}
+}
+
+// errorOnWrite will invoke t.Error on any attempted write.
+type errorOnWrite struct {
+	t *testing.T
+}
+
+func (ew *errorOnWrite) Write(b []byte) (int, error) {
+	ew.t.Errorf("Unexpected write: %s\n", b)
+	return len(b), nil
+}
+
 // fetchWireResponse is a helper for dialing to host,
 // sending http1ReqBody as the payload and retrieving
 // the response as it was sent on the wire.

src/net/http/server.go

@@ -3272,7 +3272,6 @@ type timeoutWriter struct {
 }
 
 var _ Pusher = (*timeoutWriter)(nil)
-var _ Flusher = (*timeoutWriter)(nil)
 
 // Push implements the Pusher interface.
 func (tw *timeoutWriter) Push(target string, opts *PushOptions) error {
@@ -3282,14 +3281,6 @@ func (tw *timeoutWriter) Push(target string, opts *PushOptions) error {
 	return ErrNotSupported
 }
 
-// Flush implements the Flusher interface.
-func (tw *timeoutWriter) Flush() {
-	f, ok := tw.w.(Flusher)
-	if ok {
-		f.Flush()
-	}
-}
-
 func (tw *timeoutWriter) Header() Header { return tw.h }
 
 func (tw *timeoutWriter) Write(p []byte) (int, error) {