Serve() should not return error on Stop() or GracefulStop() (#1485)

* Serve() should not return error on Stop() or GracefulStop()
* Block Serve() until Stop() or GracefulStop() returns

server_test.go

@@ -23,6 +23,7 @@ import (
 	"reflect"
 	"strings"
 	"testing"
+	"time"
 
 	"google.golang.org/grpc/test/leakcheck"
 )
@@ -53,6 +54,27 @@ func TestStopBeforeServe(t *testing.T) {
 	}
 }
 
+func TestGracefulStop(t *testing.T) {
+	defer leakcheck.Check(t)
+
+	lis, err := net.Listen("tcp", "localhost:0")
+	if err != nil {
+		t.Fatalf("failed to create listener: %v", err)
+	}
+
+	server := NewServer()
+	go func() {
+		// make sure Serve() is called
+		time.Sleep(time.Millisecond * 500)
+		server.GracefulStop()
+	}()
+
+	err = server.Serve(lis)
+	if err != nil {
+		t.Fatalf("Serve() returned non-nil error on GracefulStop: %v", err)
+	}
+}
+
 func TestGetServiceInfo(t *testing.T) {
 	defer leakcheck.Check(t)
 	testSd := ServiceDesc{