client: consider service config invalid if loadBalancingConfig… (#3034)

Author: dfawley

Diff for: service_config.go

@@ -310,6 +310,14 @@ func parseServiceConfig(js string) (*ServiceConfig, error) {
 			}
 			break
 		}
+		if sc.lbConfig == nil {
+			// We had a loadBalancingConfig field but did not encounter a
+			// supported policy.  The config is considered invalid in this
+			// case.
+			err := fmt.Errorf("invalid loadBalancingConfig: no supported policies found")
+			grpclog.Warningf(err.Error())
+			return nil, err
+		}
 	}
 
 	if rsc.MethodConfig == nil {

Diff for: service_config_test.go

@@ -92,6 +92,23 @@ func (s) TestParseLBConfig(t *testing.T) {
 	runParseTests(t, testcases)
 }
 
+func (s) TestParseNoLBConfigSupported(t *testing.T) {
+	// We have a loadBalancingConfig field but will not encounter a supported
+	// policy.  The config will be considered invalid in this case.
+	testcases := []parseTestCase{
+		{
+			scjs: `{
+    "loadBalancingConfig": [{"not_a_balancer1": {} }, {"not_a_balancer2": {}}]
+}`,
+			wantErr: true,
+		}, {
+			scjs:    `{"loadBalancingConfig": []}`,
+			wantErr: true,
+		},
+	}
+	runParseTests(t, testcases)
+}
+
 func (s) TestParseLoadBalancer(t *testing.T) {
 	testcases := []parseTestCase{
 		{