net/http: don't validate WriteHeader code if header's already been sent

Also vendors x/net/http git rev 42fe2e1c for:

    http2: don't check WriteHeader status if we've already sent the header
    https://golang.org/cl/86255

Fixes #23010

Change-Id: I4f3dd63acb52d5a34a0350aaf847a7a376d6968f
Reviewed-on: https://go-review.googlesource.com/86275Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/clientserver_test.go

@@ -1424,3 +1424,59 @@ func testWriteHeader0(t *testing.T, h2 bool) {
 		t.Error("expected panic in handler")
 	}
 }
+
+// Issue 23010: don't be super strict checking WriteHeader's code if
+// it's not even valid to call WriteHeader then anyway.
+func TestWriteHeaderNoCodeCheck_h1(t *testing.T)       { testWriteHeaderAfterWrite(t, h1Mode, false) }
+func TestWriteHeaderNoCodeCheck_h1hijack(t *testing.T) { testWriteHeaderAfterWrite(t, h1Mode, true) }
+func TestWriteHeaderNoCodeCheck_h2(t *testing.T)       { testWriteHeaderAfterWrite(t, h2Mode, false) }
+func testWriteHeaderAfterWrite(t *testing.T, h2, hijack bool) {
+	setParallel(t)
+	defer afterTest(t)
+
+	var errorLog lockedBytesBuffer
+	cst := newClientServerTest(t, h2, HandlerFunc(func(w ResponseWriter, r *Request) {
+		if hijack {
+			conn, _, _ := w.(Hijacker).Hijack()
+			defer conn.Close()
+			conn.Write([]byte("HTTP/1.1 200 OK\r\nContent-Length: 6\r\n\r\nfoo"))
+			w.WriteHeader(0) // verify this doesn't panic if there's already output; Issue 23010
+			conn.Write([]byte("bar"))
+			return
+		}
+		io.WriteString(w, "foo")
+		w.(Flusher).Flush()
+		w.WriteHeader(0) // verify this doesn't panic if there's already output; Issue 23010
+		io.WriteString(w, "bar")
+	}), func(ts *httptest.Server) {
+		ts.Config.ErrorLog = log.New(&errorLog, "", 0)
+	})
+	defer cst.close()
+	res, err := cst.c.Get(cst.ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer res.Body.Close()
+	body, err := ioutil.ReadAll(res.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := string(body), "foobar"; got != want {
+		t.Errorf("got = %q; want %q", got, want)
+	}
+
+	// Also check the stderr output:
+	if h2 {
+		// TODO: also emit this log message for HTTP/2?
+		// We historically haven't, so don't check.
+		return
+	}
+	gotLog := strings.TrimSpace(errorLog.String())
+	wantLog := "http: multiple response.WriteHeader calls"
+	if hijack {
+		wantLog = "http: response.WriteHeader on hijacked connection"
+	}
+	if gotLog != wantLog {
+		t.Errorf("stderr output = %q; want %q", gotLog, wantLog)
+	}
+}

src/net/http/h2_bundle.go

@@ -6204,7 +6204,6 @@ func http2checkWriteHeaderCode(code int) {
 }
 
 func (w *http2responseWriter) WriteHeader(code int) {
-	http2checkWriteHeaderCode(code)
 	rws := w.rws
 	if rws == nil {
 		panic("WriteHeader called after Handler finished")
@@ -6214,6 +6213,7 @@ func (w *http2responseWriter) WriteHeader(code int) {
 
 func (rws *http2responseWriterState) writeHeader(code int) {
 	if !rws.wroteHeader {
+		http2checkWriteHeaderCode(code)
 		rws.wroteHeader = true
 		rws.status = code
 		if len(rws.handlerHeader) > 0 {

src/net/http/server.go

@@ -1072,7 +1072,6 @@ func checkWriteHeaderCode(code int) {
 }
 
 func (w *response) WriteHeader(code int) {
-	checkWriteHeaderCode(code)
 	if w.conn.hijacked() {
 		w.conn.server.logf("http: response.WriteHeader on hijacked connection")
 		return
@@ -1081,6 +1080,7 @@ func (w *response) WriteHeader(code int) {
 		w.conn.server.logf("http: multiple response.WriteHeader calls")
 		return
 	}
+	checkWriteHeaderCode(code)
 	w.wroteHeader = true
 	w.status = code