FIRDocumentReference.setData() blocks main thread on __psynch_mutexwait for seconds

[xaphod]

Description

Our app makes its Firestore setData() calls on main thread, because we understood that the SDK doesn't do heavy work on the main thread.

However we have multiple reports of the main thread being blocked for several seconds. Here's an example stacktrace:

  libsystem_kernel    0x1eb9c3f18  __psynch_mutexwait
  libsystem_pthread   0x224f42b64  _pthread_mutex_firstfit_lock_wait
  libsystem_pthread   0x224f42c94  _pthread_mutex_firstfit_lock_slow
  libc++.1            0x1ab4db404  std::__1::mutex::lock
  FirebaseFirestoreInternal0x104880568  std::__1::lock_guard<T>::lock_guard[abi:ne200100] (lock_guard.h:33)
  FirebaseFirestoreInternal0x104880568  std::__1::lock_guard<T>::lock_guard[abi:ne200100] (lock_guard.h:32)
  FirebaseFirestoreInternal0x104880568  firebase::firestore::util::AsyncQueue::is_running (async_queue.cc:107)
  FirebaseFirestoreInternal0x1048e15c4  firebase::firestore::core::FirestoreClient::is_terminated (firestore_client.cc:420)
  FirebaseFirestoreInternal0x1048e15c4  firebase::firestore::core::FirestoreClient::VerifyNotTerminated (firestore_client.cc:409)
  FirebaseFirestoreInternal0x1048e15c4  firebase::firestore::core::FirestoreClient::WriteMutations (firestore_client.cc:519)
  FirebaseFirestoreInternal0x1048afa84  firebase::firestore::api::DocumentReference::SetData (document_reference.cc:98)
  FirebaseFirestoreInternal0x1048cb66c  -[FIRDocumentReference setData:merge:completion:] (FIRDocumentReference.mm:168)
  FirebaseFirestore   0x1043574fc  FIRDocumentReference.setData<T> (DocumentReference+WriteEncodable.swift:66)
  <our app>               0x103016464  FirebaseBackedObject.persistChanges (FirebaseObject.swift:89)

Please can you advise me:

1. is this normal behavior or is something wrong?
2. if normal, then, what are the best threading practices for keeping main thread unblocked?

Thank you for your help.

Reproducing the issue

No response

Firebase SDK Version

12.5.0

Xcode Version

26.1

Installation Method

CocoaPods

Firebase Product(s)

Firestore

Targeted Platforms

macCatalyst, iOS

Relevant Log Output

If using Swift Package Manager, the project's Package.resolved

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If using CocoaPods, the project's Podfile.lock

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[google-oss-bot]

I couldn't figure out how to label this issue, so I've labeled it for a human to triage. Hang tight.

[xaphod]

6bbc955743b9497c80138df01c28a6cb-symbolicated.txt

Attaching the entire trace so you can see what all the threads are up to. Our internal ID is BOOTHAPP-IOS-3H

[rizafran]

Thanks for the report, @xaphod. Is it possible for you to provide a minimal reproduction code?

[xaphod]

@rizafran no, this is not easy to reproduce. We get reports of this in the wild but not here locally.

But let's just discuss what is happening for a moment, we don't need a repro for that.

In the stacktrace you can see that calling DocumentReference.setData() can block on a mutex::lock(). Either this is by design, or it is not.

If it's by design, what's the recommended approach for threading with the Firestore swift API -- should we be calling from any global thread instead of main, or our own serial queue?