Fix a Firestore transaction crash if getDocument was called after terminateWithCompletion.

[dconeybe]

Fix a crash if [FIRTransaction getDocument] was called after [FIRFirestore terminateWithCompletion], and add a test for it.

The crash error message looked like this:

libc++abi: terminating with uncaught exception of type std::__1::bad_weak_ptr: bad_weak_ptr

The crash was due to terminateWithCompletion triggering the reference count of a shared_ptr to the Datastore object to reach zero, causing the object to be deleted. The Transaction class, however, stored a raw pointer to this Datastore object and calling [FIRTransaction getDocument] attempted to use this dangling pointer.

The fix was to change the Datastore* instance variable to a std::weak_ptr<Datastore> in the Transaction class and gracefully handle the situation where it got nulled out.

This bug also affects the C++ SDK and the Unity SDK and those SDKs will get this fix automatically once they upgrade their dependencies. Googlers can see b/201415845 for details.

[google-oss-bot]

Binary Size Report

Affected SDKs

• FirebaseFirestore

  Type	Base (a5859f9)	Head (0273aef)	Diff
  firebase-ios-sdk	4.96 MB	4.96 MB	-16 B (-0.0%)

Test Logs

• Base (a5859f9c): https://github.com/firebase/firebase-ios-sdk/actions/runs/1311749310
• Head (0273aef6): https://github.com/firebase/firebase-ios-sdk/actions/runs/1316865105

[google-oss-bot]

Coverage Report

Affected SDKs

• FirebaseFirestore-iOS-FirebaseFirestore.framework

  SDK overall coverage changed from 87.64% (a5859f9) to 87.68% (0273aef) by +0.04%.

  Filename	Base (a5859f9)	Head (0273aef)	Diff
  leveldb_key.cc	96.75%	97.99%	+1.24%
  task.cc	93.04%	94.78%	+1.74%
  transaction.cc	91.03%	89.17%	-1.86%

Test Logs

• Base (a5859f9c): https://github.com/firebase/firebase-ios-sdk/actions/runs/1311749310
• Head (0273aef6): https://github.com/firebase/firebase-ios-sdk/actions/runs/1316865105

[schmidt-sebastian]

Thanks for adding the test. LGTM.