internal/http3: return error when Write is used after status 304 is set

In our HTTP/1 and HTTP/2 implementations, calling Write in a server
handler after WriteHeader has been called with status 304 will return an
http.ErrBodyNotAllowed error. This change adds the same behavior for the
HTTP/3 server.

For golang/go#70914

Change-Id: I6be926412d51217a8b88b2ad4ce79935dd3e7af7
Reviewed-on: https://go-review.googlesource.com/c/net/+/751140
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: nicholashusin

internal/http3/server.go

@@ -463,6 +463,10 @@ func (rw *responseWriter) Write(b []byte) (n int, err error) {
 	rw.mu.Lock()
 	defer rw.mu.Unlock()
 
+	if rw.statusCode == http.StatusNotModified {
+		return 0, http.ErrBodyNotAllowed
+	}
+
 	b, trimmed := rw.trimWriteLocked(b)
 	if trimmed {
 		defer func() {

internal/http3/server_test.go

@@ -5,6 +5,7 @@
 package http3
 
 import (
+	"errors"
 	"io"
 	"maps"
 	"net/http"
@@ -912,6 +913,25 @@ func TestServer103EarlyHints(t *testing.T) {
 	})
 }
 
+func TestServer304NotModified(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		ts := newTestServer(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusNotModified)
+			if _, err := w.Write([]byte("body should not be allowed")); !errors.Is(err, http.ErrBodyNotAllowed) {
+				t.Errorf("got %v error when calling Write after WriteHeader(304), want %v error", err, http.ErrBodyNotAllowed)
+			}
+		}))
+		tc := ts.connect()
+		tc.greet()
+
+		reqStream := tc.newStream(streamTypeRequest)
+		reqStream.writeHeaders(requestHeader(nil))
+		synctest.Wait()
+		reqStream.wantSomeHeaders(http.Header{":status": {"304"}})
+		reqStream.wantClosed("request is complete")
+	})
+}
+
 type testServer struct {
 	t  testing.TB
 	s  *Server