net/http: handle WriteHeader(101) as a non-informational header

Prior to Go 1.19 adding support for sending 1xx informational headers
with ResponseWriter.WriteHeader, WriteHeader(101) would send a 101
status and disable further writes to the response. This behavior
was not documented, but is intentional: Writing to the response
body explicitly checks to see if a 101 status has been sent before
writing.

Restore the pre-1.19 behavior when writing a 101 Switching Protocols
header: The header is sent, no subsequent headers are sent, and
subsequent writes to the response body fail.

For #59564

Change-Id: I72c116f88405b1ef5067b510f8c7cff0b36951ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/485775
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/net/http/serve_test.go

@@ -6431,6 +6431,75 @@ func testDisableKeepAliveUpgrade(t *testing.T, mode testMode) {
 	}
 }
 
+type tlogWriter struct{ t *testing.T }
+
+func (w tlogWriter) Write(p []byte) (int, error) {
+	w.t.Log(string(p))
+	return len(p), nil
+}
+
+func TestWriteHeaderSwitchingProtocols(t *testing.T) {
+	run(t, testWriteHeaderSwitchingProtocols, []testMode{http1Mode})
+}
+func testWriteHeaderSwitchingProtocols(t *testing.T, mode testMode) {
+	const wantBody = "want"
+	const wantUpgrade = "someProto"
+	ts := newClientServerTest(t, mode, HandlerFunc(func(w ResponseWriter, r *Request) {
+		w.Header().Set("Connection", "Upgrade")
+		w.Header().Set("Upgrade", wantUpgrade)
+		w.WriteHeader(StatusSwitchingProtocols)
+		NewResponseController(w).Flush()
+
+		// Writing headers or the body after sending a 101 header should fail.
+		w.WriteHeader(200)
+		if _, err := w.Write([]byte("x")); err == nil {
+			t.Errorf("Write to body after 101 Switching Protocols unexpectedly succeeded")
+		}
+
+		c, _, err := NewResponseController(w).Hijack()
+		if err != nil {
+			t.Errorf("Hijack: %v", err)
+			return
+		}
+		defer c.Close()
+		if _, err := c.Write([]byte(wantBody)); err != nil {
+			t.Errorf("Write to hijacked body: %v", err)
+		}
+	}), func(ts *httptest.Server) {
+		// Don't spam log with warning about superfluous WriteHeader call.
+		ts.Config.ErrorLog = log.New(tlogWriter{t}, "log: ", 0)
+	}).ts
+
+	conn, err := net.Dial("tcp", ts.Listener.Addr().String())
+	if err != nil {
+		t.Fatalf("net.Dial: %v", err)
+	}
+	_, err = conn.Write([]byte("GET / HTTP/1.1\r\nHost: foo\r\n\r\n"))
+	if err != nil {
+		t.Fatalf("conn.Write: %v", err)
+	}
+	defer conn.Close()
+
+	r := bufio.NewReader(conn)
+	res, err := ReadResponse(r, &Request{Method: "GET"})
+	if err != nil {
+		t.Fatal("ReadResponse error:", err)
+	}
+	if res.StatusCode != StatusSwitchingProtocols {
+		t.Errorf("Response StatusCode=%v, want 101", res.StatusCode)
+	}
+	if got := res.Header.Get("Upgrade"); got != wantUpgrade {
+		t.Errorf("Response Upgrade header = %q, want %q", got, wantUpgrade)
+	}
+	body, err := io.ReadAll(r)
+	if err != nil {
+		t.Error(err)
+	}
+	if string(body) != wantBody {
+		t.Errorf("Response body = %q, want %q", string(body), wantBody)
+	}
+}
+
 func TestMuxRedirectRelative(t *testing.T) {
 	setParallel(t)
 	req, err := ReadRequest(bufio.NewReader(strings.NewReader("GET http://example.com HTTP/1.1\r\nHost: test\r\n\r\n")))

src/net/http/server.go

@@ -1154,8 +1154,11 @@ func (w *response) WriteHeader(code int) {
 	}
 	checkWriteHeaderCode(code)
 
-	// Handle informational headers
-	if code >= 100 && code <= 199 {
+	// Handle informational headers.
+	//
+	// We shouldn't send any further headers after 101 Switching Protocols,
+	// so it takes the non-informational path.
+	if code >= 100 && code <= 199 && code != StatusSwitchingProtocols {
 		// Prevent a potential race with an automatically-sent 100 Continue triggered by Request.Body.Read()
 		if code == 100 && w.canWriteContinue.Load() {
 			w.writeContinueMu.Lock()