Close conn when HTTP/1.1 clients call bidi methods (#408)

If an HTTP/1.1 client calls a bidi RPC method, the caller has likely
written application-level code that expects a full-duplex stream. Unless
the server closes the TCP connection, the client code will often end up
blocked because it's trying to read the response before it closes the
request body.

connect_ext_test.go

@@ -1824,6 +1824,29 @@ func TestUnflushableResponseWriter(t *testing.T) {
 	}
 }
 
+func TestBidiOverHTTP1(t *testing.T) {
+	t.Parallel()
+	mux := http.NewServeMux()
+	mux.Handle(pingv1connect.NewPingServiceHandler(pingServer{}))
+	server := httptest.NewServer(mux)
+	t.Cleanup(server.Close)
+
+	// Clients expecting a full-duplex connection that end up with a simplex
+	// HTTP/1.1 connection shouldn't hang. Instead, the server should close the
+	// TCP connection.
+	client := pingv1connect.NewPingServiceClient(server.Client(), server.URL)
+	stream := client.CumSum(context.Background())
+	if err := stream.Send(&pingv1.CumSumRequest{Number: 2}); err != nil {
+		assert.ErrorIs(t, err, io.EOF)
+	}
+	_, err := stream.Receive()
+	assert.NotNil(t, err)
+	assert.Equal(t, connect.CodeOf(err), connect.CodeUnknown)
+	assert.Equal(t, err.Error(), "unknown: HTTP status 505 HTTP Version Not Supported")
+	assert.Nil(t, stream.CloseRequest())
+	assert.Nil(t, stream.CloseResponse())
+}
+
 type unflushableWriter struct {
 	w http.ResponseWriter
 }

handler.go

@@ -162,6 +162,10 @@ func (h *Handler) ServeHTTP(responseWriter http.ResponseWriter, request *http.Re
 	// okay if we can't re-use the connection.
 	isBidi := (h.spec.StreamType & StreamTypeBidi) == StreamTypeBidi
 	if isBidi && request.ProtoMajor < 2 {
+		// Clients coded to expect full-duplex connections may hang if they've
+		// mistakenly negotiated HTTP/1.1. To unblock them, we must close the
+		// underlying TCP connection.
+		responseWriter.Header().Set("Connection", "close")
 		responseWriter.WriteHeader(http.StatusHTTPVersionNotSupported)
 		return
 	}