Crash Due to Dispatch Group Over-Release with Multiple fetchCompletionHandler Calls

[pletoss]

We’ve observed a crash when integrating FirebaseMessaging / GoogleUtilities with the Braze Unity SDK. The issue appears to stem from using a dispatch group in the swizzled AppDelegate: if fetchCompletionHandler is invoked more than once (a behavior not explicitly disallowed by Apple’s docs), an imbalance occurs calling dispatch_group_leave more times than dispatch_group_enter triggers an exception.

References

• GoogleUtilities Code (Line 874)
• Braze Unity SDK Issue #107

Related issues:

1. Intercom SDK Discussion
2. Firebase iOS SDK Issue #8442
3. Braze Unity SDK Issue #10

Suggested Resolution

Clarify (via Apple’s documentation or internal assumptions) whether fetchCompletionHandler is allowed to be called multiple times, or adjust GoogleUtilities to safely handle multiple invocations. This change would prevent not only the current crash but also avoid hard-to-debug issues with other libraries.

[ncooke3]

Thanks for the issue, @pletoss. Are you able to pinpoint when the duplicate fetchCompletionHandler call occurs?

[pletoss]

Thanks for the issue, @pletoss. Are you able to pinpoint when the duplicate fetchCompletionHandler call occurs?

Hi, almost, one happens in the BrazeKit closed-source Framework. So I only saw some unsymbolicated stacktraces. The other call is made by Unity's default UnityAppController.

However braze has an open source Unity Wrapper, where we can see a bit where it can stem from. I described it in this issue in the Braze repo: braze-inc/braze-unity-sdk#107

[hezippsun]

version:8.1.0