feat(GODT-2848): Decouple IMAP service from Event Loop

The IMAP service no longer blocks the event loop from progressing. When
syncing the event loop continues as usual, allow other parts of the
bridge to continue updating.

Once the sync is complete, it resets the event id back to the last
handled id before sync started and then we play catch up.

internal/services/userevents/service_test.go

@@ -76,9 +76,11 @@ func TestService_EventIDLoadStore(t *testing.T) {
 		time.Second,
 		async.NoopPanicHandler{},
 	)
-	require.NoError(t, service.Start(context.Background(), group))
+
+	_, err := service.Start(context.Background(), group)
+	require.NoError(t, err)
+
 	service.Resume()
-	service.ResumeIMAP()
 	group.Wait()
 }
 
@@ -131,9 +133,10 @@ func TestService_RetryEventOnNonCatastrophicFailure(t *testing.T) {
 	)
 	service.Subscribe(NewCallbackSubscriber("foo", EventHandler{MessageHandler: subscriber}))
 
-	require.NoError(t, service.Start(context.Background(), group))
+	_, err := service.Start(context.Background(), group)
+	require.NoError(t, err)
+
 	service.Resume()
-	service.ResumeIMAP()
 	group.Wait()
 }
 
@@ -194,9 +197,11 @@ func TestService_OnBadEventServiceIsPaused(t *testing.T) {
 	})
 
 	service.Subscribe(NewCallbackSubscriber("foo", EventHandler{MessageHandler: subscriber}))
-	require.NoError(t, service.Start(context.Background(), group))
+
+	_, err := service.Start(context.Background(), group)
+	require.NoError(t, err)
+
 	service.Resume()
-	service.ResumeIMAP()
 	group.Wait()
 }
 
@@ -251,9 +256,11 @@ func TestService_UnsubscribeDuringEventHandlingDoesNotCauseDeadlock(t *testing.T
 	})
 
 	service.Subscribe(subscription)
-	require.NoError(t, service.Start(context.Background(), group))
+
+	_, err := service.Start(context.Background(), group)
+	require.NoError(t, err)
+
 	service.Resume()
-	service.ResumeIMAP()
 	group.Wait()
 }
 
@@ -310,9 +317,11 @@ func TestService_UnsubscribeBeforeHandlingEventIsNotConsideredError(t *testing.T
 	})
 
 	service.Subscribe(subscription)
-	require.NoError(t, service.Start(context.Background(), group))
+
+	_, err := service.Start(context.Background(), group)
+	require.NoError(t, err)
+
 	service.Resume()
-	service.ResumeIMAP()
 	group.Wait()
 }
 
@@ -373,9 +382,69 @@ func TestService_WaitOnEventPublishAfterPause(t *testing.T) {
 	})
 
 	service.Subscribe(NewCallbackSubscriber("foo", EventHandler{MessageHandler: subscriber}))
-	require.NoError(t, service.Start(context.Background(), group))
+
+	_, err := service.Start(context.Background(), group)
+	require.NoError(t, err)
+
+	service.Resume()
+	group.Wait()
+}
+
+func TestService_EventRewind(t *testing.T) {
+	group := orderedtasks.NewOrderedCancelGroup(async.NoopPanicHandler{})
+	mockCtrl := gomock.NewController(t)
+	eventPublisher := mocks2.NewMockEventPublisher(mockCtrl)
+	eventIDStore := mocks.NewMockEventIDStore(mockCtrl)
+	eventSource := mocks.NewMockEventSource(mockCtrl)
+
+	firstEventID := "EVENT01"
+	secondEventID := "EVENT02"
+	messageEvents := []proton.MessageEvent{
+		{
+			EventItem: proton.EventItem{ID: "Message"},
+		},
+	}
+	secondEvent := []proton.Event{{
+		EventID:  secondEventID,
+		Messages: messageEvents,
+	}}
+
+	// Return Second Event from id store, but then reset to event 1
+
+	// Event id store expectations.
+	store1 := eventIDStore.EXPECT().Load(gomock.Any()).Times(1).Return(secondEventID, nil)
+	eventIDStore.EXPECT().Store(gomock.Any(), gomock.Eq(firstEventID)).Times(1).Return(nil).After(store1)
+	eventIDStore.EXPECT().Store(gomock.Any(), gomock.Eq(secondEventID)).Times(1).Return(nil)
+
+	// Event Source expectations.
+	eventSource.EXPECT().GetEvent(gomock.Any(), gomock.Eq(firstEventID)).MinTimes(1).DoAndReturn(
+		func(_ context.Context, _ string) ([]proton.Event, bool, error) {
+			group.Cancel()
+			return secondEvent, false, nil
+		},
+	)
+
+	// Subscriber expectations.
+
+	service := NewService(
+		"foo",
+		eventSource,
+		eventIDStore,
+		eventPublisher,
+		time.Millisecond,
+		time.Millisecond,
+		time.Second,
+		async.NoopPanicHandler{},
+	)
+
+	_, err := service.Start(context.Background(), group)
+	require.NoError(t, err)
+
+	go func() {
+		require.NoError(t, service.RewindEventID(context.Background(), firstEventID))
+	}()
+
 	service.Resume()
-	service.ResumeIMAP()
 	group.Wait()
 }