fix(GODT-3203): Crash in chunkDivide

If for some reason all the message we are trying to sync in a chunk are
deleted from another client it is possible that the input to the build
stage will be empty. This case is now handled correctly.

internal/services/syncservice/stage_build.go

@@ -101,6 +101,23 @@ func (b *BuildStage) run(ctx context.Context) {
 			continue
 		}
 
+		if len(req.batch) == 0 {
+			// it is possible that if one does a mass delete on another client an entire download batch fails,
+			// and we reach this point without any messages to build.
+			req.onStageCompleted(ctx)
+
+			if err := b.output.Produce(ctx, ApplyRequest{
+				childJob: req.childJob,
+				messages: nil,
+			}); err != nil {
+				err = fmt.Errorf("failed to produce output for next stage: %w", err)
+				logrus.Errorf(err.Error())
+				req.job.onError(err)
+			}
+
+			continue
+		}
+
 		err = req.job.messageBuilder.WithKeys(func(_ *crypto.KeyRing, addrKRs map[string]*crypto.KeyRing) error {
 			chunks := chunkSyncBuilderBatch(req.batch, b.maxBuildMem)
 

internal/services/syncservice/stage_build_test.go

@@ -321,3 +321,35 @@ func TestBuildStage_CancelledJobIsDiscarded(t *testing.T) {
 	_, err := output.Consume(context.Background())
 	require.ErrorIs(t, err, ErrNoMoreInput)
 }
+
+func TestTask_EmptyInputDoesNotCrash(t *testing.T) {
+	mockCtrl := gomock.NewController(t)
+
+	input := NewChannelConsumerProducer[BuildRequest]()
+	output := NewChannelConsumerProducer[ApplyRequest]()
+	reporter := mocks.NewMockReporter(mockCtrl)
+
+	labels := getTestLabels()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	tj := newTestJob(ctx, mockCtrl, "u", labels)
+
+	tj.syncReporter.EXPECT().OnProgress(gomock.Any(), gomock.Eq(int64(10)))
+
+	tj.job.begin()
+	childJob := tj.job.newChildJob("f", 10)
+	tj.job.end()
+
+	stage := NewBuildStage(input, output, 1024, &async.NoopPanicHandler{}, reporter)
+
+	go func() {
+		stage.run(ctx)
+	}()
+
+	require.NoError(t, input.Produce(ctx, BuildRequest{childJob: childJob, batch: []proton.FullMessage{}}))
+
+	req, err := output.Consume(ctx)
+	cancel()
+	require.NoError(t, err)
+	require.Len(t, req.messages, 0)
+}