net/http: allow Handlers to test Hijacked conn without spamming error log

Make a zero-byte write to a hijacked connection not log anything, so handlers
can test whether a connection is hacked by doing a Write(nil).

Fixes #16456

Change-Id: Id56caf822c8592067bd8422672f0c1aec89e866c
Reviewed-on: https://go-review.googlesource.com/30812Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/net/http/serve_test.go

@@ -2252,6 +2252,51 @@ func testHandlerPanic(t *testing.T, withHijack, h2 bool, panicValue interface{})
 	}
 }
 
+type terrorWriter struct{ t *testing.T }
+
+func (w terrorWriter) Write(p []byte) (int, error) {
+	w.t.Errorf("%s", p)
+	return len(p), nil
+}
+
+// Issue 16456: allow writing 0 bytes on hijacked conn to test hijack
+// without any log spam.
+func TestServerWriteHijackZeroBytes(t *testing.T) {
+	defer afterTest(t)
+	done := make(chan struct{})
+	ts := httptest.NewUnstartedServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		defer close(done)
+		w.(Flusher).Flush()
+		conn, _, err := w.(Hijacker).Hijack()
+		if err != nil {
+			t.Errorf("Hijack: %v", err)
+			return
+		}
+		defer conn.Close()
+		_, err = w.Write(nil)
+		if err != ErrHijacked {
+			t.Errorf("Write error = %v; want ErrHijacked", err)
+		}
+	}))
+	ts.Config.ErrorLog = log.New(terrorWriter{t}, "Unexpected write: ", 0)
+	ts.Start()
+	defer ts.Close()
+
+	tr := &Transport{}
+	defer tr.CloseIdleConnections()
+	c := &Client{Transport: tr}
+	res, err := c.Get(ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	res.Body.Close()
+	select {
+	case <-done:
+	case <-time.After(5 * time.Second):
+		t.Fatal("timeout")
+	}
+}
+
 func TestServerNoDate_h1(t *testing.T)        { testServerNoHeader(t, h1Mode, "Date") }
 func TestServerNoDate_h2(t *testing.T)        { testServerNoHeader(t, h2Mode, "Date") }
 func TestServerNoContentType_h1(t *testing.T) { testServerNoHeader(t, h1Mode, "Content-Type") }

src/net/http/server.go

@@ -1318,7 +1318,9 @@ func (w *response) WriteString(data string) (n int, err error) {
 // either dataB or dataS is non-zero.
 func (w *response) write(lenData int, dataB []byte, dataS string) (n int, err error) {
 	if w.conn.hijacked() {
-		w.conn.server.logf("http: response.Write on hijacked connection")
+		if lenData > 0 {
+			w.conn.server.logf("http: response.Write on hijacked connection")
+		}
 		return 0, ErrHijacked
 	}
 	if !w.wroteHeader {