xdsclient: fix panic on empty resource in ADS response (grpc#8970)

This PR fixes an intermittent panic in xdsclient `decodeResponse` that
occurs when processing an xDS ADS response containing an empty resource
name, while doing fuzz testing.

RELEASE NOTES: 
* xds: Fix a panic in the xDS client that can occur when processing an
ADS response containing an empty resource name.

Author: Pranjali-2501

internal/xds/clients/xdsclient/channel.go

@@ -214,12 +214,7 @@ func (xc *xdsChannel) onResponse(resp response, onDone func()) ([]string, error)
 		return nil, xdsresource.NewErrorf(xdsresource.ErrorTypeResourceTypeUnsupported, "Resource type URL %q unknown in response from server", resp.typeURL)
 	}
 
-	// Decode the resources and build the list of resource names to return.
-	opts := &DecodeOptions{
-		Config:       xc.clientConfig,
-		ServerConfig: xc.serverConfig,
-	}
-	updates, md, err := decodeResponse(opts, &rType, resp)
+	updates, md, err := xc.decodeResponse(&rType, resp)
 	var names []string
 	for name := range updates {
 		names = append(names, name)
@@ -243,12 +238,16 @@ func (xc *xdsChannel) onResponse(resp response, onDone func()) ([]string, error)
 // If there are any errors decoding the resources, the metadata will indicate
 // that the update was NACKed, and the returned error will contain information
 // about all errors encountered by this function.
-func decodeResponse(opts *DecodeOptions, rType *ResourceType, resp response) (map[string]dataAndErrTuple, xdsresource.UpdateMetadata, error) {
+func (xc *xdsChannel) decodeResponse(rType *ResourceType, resp response) (map[string]dataAndErrTuple, xdsresource.UpdateMetadata, error) {
 	timestamp := time.Now()
 	md := xdsresource.UpdateMetadata{
 		Version:   resp.version,
 		Timestamp: timestamp,
 	}
+	opts := &DecodeOptions{
+		Config:       xc.clientConfig,
+		ServerConfig: xc.serverConfig,
+	}
 
 	topLevelErrors := make([]error, 0)          // Tracks deserialization errors, where we don't have a resource name.
 	perResourceErrors := make(map[string]error) // Tracks resource validation errors, where we have a resource name.
@@ -268,6 +267,10 @@ func decodeResponse(opts *DecodeOptions, rType *ResourceType, resp response) (ma
 		// Name field of the result is left unpopulated only when resource
 		// deserialization fails.
 		name := ""
+		if result == nil && err == nil {
+			xc.logger.Errorf("Decode() returned nil result and nil error for resource: %v", r)
+			continue
+		}
 		if result != nil {
 			name = xdsresource.ParseName(result.Name).String()
 		}

internal/xds/clients/xdsclient/channel_test.go

@@ -793,7 +793,8 @@ func (s) TestDecodeResponse_PanicRecoveryEnabled(t *testing.T) {
 	resp := response{resources: []*anypb.Any{{Value: []byte("test")}}}
 	wantErr := "recovered from panic during resource parsing"
 
-	if _, _, err := decodeResponse(&DecodeOptions{}, rType, resp); err == nil || !strings.Contains(err.Error(), wantErr) {
+	xc := &xdsChannel{}
+	if _, _, err := xc.decodeResponse(rType, resp); err == nil || !strings.Contains(err.Error(), wantErr) {
 		t.Fatalf("decodeResponse() failed with err: %v, want %q", err, wantErr)
 	}
 }
@@ -809,11 +810,12 @@ func (s) TestDecodeResponse_PanicRecoveryDisabled(t *testing.T) {
 	}
 	resp := response{resources: []*anypb.Any{{Value: []byte("test")}}}
 	wantErr := "simulate panic"
+	xc := &xdsChannel{}
 
 	defer func() {
 		if r := recover(); r == nil || !strings.Contains(fmt.Sprint(r), wantErr) {
 			t.Fatalf("Expected panic in decodeResponse, got: %v, want: %q", r, wantErr)
 		}
 	}()
-	decodeResponse(&DecodeOptions{}, rType, resp)
+	xc.decodeResponse(rType, resp)
 }