server: allow PreparedMsgs to work for server streams

server.go

@@ -1521,6 +1521,8 @@ func (s *Server) processStreamingRPC(t transport.ServerTransport, stream *transp
 		}
 	}
 
+	ss.ctx = newContextWithRPCInfo(ss.ctx, false, ss.codec, ss.cp, ss.comp)
+
 	if trInfo != nil {
 		trInfo.tr.LazyLog(&trInfo.firstLine, false)
 	}

test/end2end_test.go

@@ -2248,6 +2248,61 @@ func testPreloaderClientSend(t *testing.T, e env) {
 	}
 }
 
+func (s) TestPreloaderSenderSend(t *testing.T) {
+	ss := &stubserver.StubServer{
+		FullDuplexCallF: func(stream testpb.TestService_FullDuplexCallServer) error {
+			for i := 0; i < 10; i++ {
+				preparedMsg := &grpc.PreparedMsg{}
+				err := preparedMsg.Encode(stream, &testpb.StreamingOutputCallResponse{
+					Payload: &testpb.Payload{
+						Body: []byte{'0' + uint8(i)},
+					},
+				})
+				if err != nil {
+					return err
+				}
+				stream.SendMsg(preparedMsg)
+			}
+			return nil
+		},
+	}
+	if err := ss.Start(nil); err != nil {
+		t.Fatalf("Error starting endpoint server: %v", err)
+	}
+	defer ss.Stop()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+
+	stream, err := ss.Client.FullDuplexCall(ctx)
+	if err != nil {
+		t.Fatalf("ss.Client.EmptyCall(_, _) = _, %v; want _, nil", err)
+	}
+
+	var ngot int
+	var buf bytes.Buffer
+	for {
+		reply, err := stream.Recv()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			t.Fatal(err)
+		}
+		ngot++
+		if buf.Len() > 0 {
+			buf.WriteByte(',')
+		}
+		buf.Write(reply.GetPayload().GetBody())
+	}
+	if want := 10; ngot != want {
+		t.Errorf("Got %d replies, want %d", ngot, want)
+	}
+	if got, want := buf.String(), "0,1,2,3,4,5,6,7,8,9"; got != want {
+		t.Errorf("Got replies %q; want %q", got, want)
+	}
+}
+
 func (s) TestMaxMsgSizeClientDefault(t *testing.T) {
 	for _, e := range listTestEnv() {
 		testMaxMsgSizeClientDefault(t, e)