stream: swallow Header errors as we used to; RecvMsg can still return…

… it (#6591)

stream.go

@@ -91,7 +91,9 @@ type Stream interface {
 // status package.
 type ClientStream interface {
 	// Header returns the header metadata received from the server if there
-	// is any. It blocks if the metadata is not ready to read.
+	// is any. It blocks if the metadata is not ready to read.  If the metadata
+	// is nil and the error is also nil, then the stream was terminated without
+	// headers, and the status can be discovered by calling RecvMsg.
 	Header() (metadata.MD, error)
 	// Trailer returns the trailer metadata from the server, if there is any.
 	// It must only be called after stream.CloseAndRecv has returned, or
@@ -802,7 +804,8 @@ func (cs *clientStream) Header() (metadata.MD, error) {
 
 	if err != nil {
 		cs.finish(err)
-		return nil, err
+		// Do not return the error.  The user should get it by calling Recv().
+		return nil, nil
 	}
 
 	if len(cs.binlogs) != 0 && !cs.serverHeaderBinlogged && m != nil {

stream_test.go

@@ -0,0 +1,67 @@
+/*
+ *
+ * Copyright 2023 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package grpc_test
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/internal/grpctest"
+	"google.golang.org/grpc/internal/stubserver"
+	"google.golang.org/grpc/interop/grpc_testing"
+	"google.golang.org/grpc/status"
+)
+
+const defaultTestTimeout = 10 * time.Second
+
+type s struct {
+	grpctest.Tester
+}
+
+func Test(t *testing.T) {
+	grpctest.RunSubTests(t, s{})
+}
+
+func (s) TestStream_Header_TrailersOnly(t *testing.T) {
+	ss := stubserver.StubServer{
+		FullDuplexCallF: func(stream grpc_testing.TestService_FullDuplexCallServer) error {
+			return status.Errorf(codes.NotFound, "a test error")
+		},
+	}
+	if err := ss.Start(nil); err != nil {
+		t.Fatal("Error starting server:", err)
+	}
+	defer ss.Stop()
+
+	ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
+	defer cancel()
+
+	s, err := ss.Client.FullDuplexCall(ctx)
+	if err != nil {
+		t.Fatal("Error staring call", err)
+	}
+	if md, err := s.Header(); md != nil || err != nil {
+		t.Fatalf("s.Header() = %v, %v; want nil, nil", md, err)
+	}
+	if _, err := s.Recv(); status.Code(err) != codes.NotFound {
+		t.Fatalf("s.Recv() = _, %v; want _, err.Code()=codes.NotFound", err)
+	}
+}