Finish a transaction never returns on iOS after being in background

[NikHomann]

Platform

Flutter Mobile

Obfuscation

Disabled

Debug Info

Disabled

Doctor

[√] Flutter (Channel stable, 3.19.4, on Microsoft Windows [Version 10.0.19045.4291], locale de-DE)
• Flutter version 3.19.4 on channel stable at C:\Users\Niklas\fvm\versions\3.19.4
• Upstream repository https://github.com/flutter/flutter.git
• Framework revision 68bfaea224 (5 weeks ago), 2024-03-20 15:36:31 -0700
• Engine revision a5c24f538d
• Dart version 3.3.2
• DevTools version 2.31.1

Version

7.18.0

Steps to Reproduce

How to create a reproducible example:

1. Create new flutter project
2. Add sentry packages
3. Initialize Sentry for instance like here: https://docs.sentry.io/platforms/flutter/#configure
4. Add a await Sentry.startTransaction("test", "test").finish() somewhere, for instance in the basic flutter example before counter is incremented
5. Go through the steps below to see the problem

Steps to Reproduce:

1. Run an app with Sentry in release mode on an iOS device
2. Optionally, enable "Fast App Termination" in the developer settings to not have to wait for so long
3. Put the app in the background and wait for some time (~30min with "Fast App Termination")
4. Open the app and somehow start and finish a transaction

If you downgrade to version 7.10.1 (there might be others, but this is the version I was using before) it works as expected.

Expected Result

The transaction.finish(...) returns after a short amount of time and the transaction is logged in Sentry.

Actual Result

The transaction.finish(...) never returns and the transaction is not logged in Sentry.

Are you willing to submit a PR?

No

[buenaflor]

Hi thanks for raising this issue, before going deeper into this: do you know if this is also reproducible with a shorter background time?

[NikHomann]

Hi thanks for raising this issue, before going deeper into this: do you know if this is also reproducible with a shorter background time?

I am not sure and do not have exact numbers, but I think sometimes it happened also for shorter background times. However, the ~30min seemed to give me the problem reliably (nearly) every time.

[buenaflor]

Got it, thanks for the issue, we'll try to reproduce. If you have any more info lmk

[kuhnroyal]

I believe I found the culprit, the NativeAppStartIntegration or NativeAppStartEventProcessor hangs and the future here never completes because options.autoAppStart is set to false.

I believe the NativeAppStartIntegration is incorrectly added to active integrations in some cases.
If autoAppStart is false and SentryFlutter.setAppStartEnd is never called, then this gets stuck.

@NikHomann Not fully sure that this is the same reason for your case. I am manually starting a new isolate and initialize Sentry with autoAppStart = false.

Workaround:

// NativeAppStartIntegration is not exported
// ignore: implementation_imports
import 'package:sentry_flutter/src/integrations/native_app_start_integration.dart';

 options.removeIntegration(options.integrations.firstWhere((e) => e is NativeAppStartIntegration));

[buenaflor]

we didn't account for setAppStartEnd so we need to modify that, this should be it thx for investigating

[kuhnroyal]

I am still experiencing 30 sec timeouts here.
There should probably be a way to disable appStart tracking completely or the processor needs to just return null if the end time it is not set yet. The 30 secs default timeout are problematic.

[buenaflor]

hey, I'll take another look, sorry about that, I'll try to get to it today or tomorrow since next week will be a bit slower because of droid/fluttercon

[kuhnroyal]

No worries, see you in Berlin 👋

[kuhnroyal]

Not completely sure but I think the addPostFrameCallback is not triggering in my case because it is started from a background isolate (workmanager). I don't really need the appstart information but it is not possible to disable the integration.

[buenaflor]

I think you can do something like

options.integrations.removeWhere((integration) => 
      integration is IntegrationYouWantToRemove);

Which should be the NativeAppStartIntegration, I'm currently on my phone so I can't verify the correctness of the code but afaik it should be possible to remove

[buenaflor]

hey this code will help you remove/disable it:

final integration = options.integrations.firstWhere(
    (element) => element.toString().contains('NativeAppStartIntegration'));
options.removeIntegration(integration);

[kuhnroyal]

Yea, I restored my workaround from #2007 (comment) which I removed when 8.2.0 was released. Obviously I didn't test it  🤣

[buenaflor]

To me it looks like setAppStartEnd just adds more complexity all in all. imo it should be deprecated.

the flutter sdk is also the only SDK that has this API.

[buenaflor]

I am still experiencing 30 sec timeouts here.

by the way do have a way to reproduce it reliably? or is it just something that happens once in a while

[buenaflor]

#2140 I've added a fix that will not wait for getAppStartInfo if autoAppstarts is false and setAppStartEnd has not been called

[kuhnroyal]

I am still experiencing 30 sec timeouts here.

by the way do have a way to reproduce it reliably? or is it just something that happens once in a while

I am starting a new isolate via https://pub.dev/packages/flutter_isolate and call SentryFlutter.init. Since there is no UI, my assumption is, that the postFrameCallback is never triggered.

[buenaflor]

ah yeah looks like it

[shinayser]

I am also experiencing this issue on Android on some of my testings. Is a fix for it available?
The workaround is safe to use or may cause some undesirable issues?

[buenaflor]

@shinayser

the temporary workaround with disabling the app start integration is safe to use. this is only used to instrument the app start times

[shinayser]

@buenaflor This is exactly what I was trying to do when I figured out the bug 😢

[buenaflor]

@shinayser did you successfully remove the integration?

[shinayser]

@buenaflor So, my problem is that I am actually starting sentry in the native side (on Android, Application.kt class) and using the parameter options.autoInitializeNativeSdk=true on the flutter side.

Because of that, when removing the integration and caling the transation.fnish() throws an error on sentry saying that "Sentry sdk isn't started".

In resume, this workaround works but if you are using the native sdk startup you wil be in trouble and sentry will not be able to finish transactions.

[buenaflor]

@shinayser and you're using the SentryNavigatorObserver correct?

[shinayser]

@shinayser and you're using the SentryNavigatorObserver correct?

Yes