fix: serialize updateLastUseDate with messageQueue on background entry

[nickolas-dimitrakas]

Background

• Crash report showed EXC_BAD_ACCESS KERN_INVALID_ADDRESS in objc_retain during CFPrefsSource mergeIntoDictionary, triggered by NSUserDefaults objectForKey:.
• Root cause: when UIApplicationDidEnterBackgroundNotification fires, MPBackendController dispatches its background work (which reads UserDefaults via MPPersistenceController.mpId) to the serial messageQueue, while MPStateMachine called updateLastUseDate: synchronously on the main thread — writing to the same UserDefaults. The concurrent read + write on CFPrefsSource's internal dictionary caused the crash.
• This is the same class of bug fixed in v8.44.2 (fix: serialize backgroundTimeRemaining with cancellation check #667) — thread-unsafe access during app backgrounding — but at a different point: the instant of entering background rather than during the background time check loop.

What Has Changed

• mParticle-Apple-SDK/MPStateMachine.m — handleApplicationDidEnterBackground: now dispatches updateLastUseDate: to messageQueue via dispatch_async, serializing the write with MPBackendController's concurrent read. The _launchDate ivar is captured into a local before the block to avoid capturing self. The termination handler (handleApplicationWillTerminate:) remains synchronous because MPBackendController does not observe that notification (no race) and dispatch_async risks losing the write if the process is killed.
• UnitTests/ObjCTests/MPStateMachineTests.m — Added 3 regression tests:
  • testUpdateLastUseDateSerializedWithMessageQueueWork: 500-iteration stress test simulating the fixed pattern (both updateLastUseDate and backend controller work on messageQueue with concurrent reads from global queues).
  • testSubscriptAccessorThreadSafety: 1000-iteration concurrent read/write stress test on MPUserDefaults subscript accessors.
  • testUpdateLastUseDateWithNilDate: edge case verifying nil date produces @0 rather than crashing.

Additional Notes

Reported crash 1

Crashed: com.apple.main-thread
0  libobjc.A.dylib                0x144c objc_retain_x0 + 16
1  libobjc.A.dylib                0x144c objc_retain + 16
2  CoreFoundation                 0x19eefc -[__NSDictionaryM __setObject:forKey:] + 360
3  CoreFoundation                 0x12aed4 __72-[CFPrefsSource mergeIntoDictionary:sourceDictionary:cloudKeyEvaluator:]_block_invoke + 32
4  CoreFoundation                 0x1a0a18 -[__NSFrozenDictionaryM __apply:context:] + 128
5  CoreFoundation                 0x12ae74 -[CFPrefsSource mergeIntoDictionary:sourceDictionary:cloudKeyEvaluator:] + 160
6  CoreFoundation                 0x1b188c -[CFPrefsSearchListSource alreadylocked_getDictionary:] + 724
7  CoreFoundation                 0x170630 -[CFPrefsSearchListSource alreadylocked_copyValueForKey:] + 172
8  CoreFoundation                 0x12ada4 -[CFPrefsSource copyValueForKey:] + 44
9  CoreFoundation                 0x198c88 __76-[_CFXPreferences copyAppValueForKey:identifier:container:configurationURL:]_block_invoke + 32
10 CoreFoundation                 0x172e74 __108-[_CFXPreferences(SearchListAdditions) withSearchListForIdentifier:container:cloudConfigurationURL:perform:]_block_invoke + 396
11 CoreFoundation                 0x13524 normalizeQuintuplet + 404
12 CoreFoundation                 0x1b1f24 -[_CFXPreferences withSearchListForIdentifier:container:cloudConfigurationURL:perform:] + 164
13 CoreFoundation                 0x198c0c -[_CFXPreferences copyAppValueForKey:identifier:container:configurationURL:] + 156
14 CoreFoundation                 0x12a8c _CFPreferencesCopyAppValueWithContainerAndConfiguration + 112
15 Foundation                     0x9d6470 -[NSUserDefaults(NSUserDefaults) objectForKey:] + 60
16 mParticle_Apple_SDK            0xac424 __swift_project_boxed_opaque_existential_0Tm + 2848
17 mParticle_Apple_SDK            0xac9a4 __swift_project_boxed_opaque_existential_0Tm + 4256
18 mParticle_Apple_SDK            0x163d4 +[MPPersistenceController_PRIVATE mpId] + 240
19 mParticle_Apple_SDK            0xb0350 __swift_project_boxed_opaque_existential_0Tm + 19020
20 mParticle_Apple_SDK            0xacc10 __swift_project_boxed_opaque_existential_0Tm + 4876
21 mParticle_Apple_SDK            0x3f0bc +[MPApplication_PRIVATE updateLastUseDate:] + 132
22 mParticle_Apple_SDK            0x63a40 -[MPStateMachine_PRIVATE handleApplicationDidEnterBackground:] + 32
23 CoreFoundation                 0x50860 __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ + 148
24 CoreFoundation                 0x50910 ___CFXRegistrationPost_block_invoke + 92
25 CoreFoundation                 0x5076c _CFXRegistrationPost + 436
26 CoreFoundation                 0x50f0c _CFXNotificationPost + 736
27 Foundation                     0x9633cc -[NSNotificationCenter postNotificationName:object:userInfo:] + 92
28 UIKitCore                      0x3ae1b0 __47-[UIApplication _applicationDidEnterBackground]_block_invoke + 256
29 UIKitCore                      0xc98eb0 +[UIViewController _performWithoutDeferringTransitionsAllowingAnimation:actions:] + 140
30 UIKitCore                      0x3acaf4 -[UIApplication _applicationDidEnterBackground] + 136
31 UIKitCore                      0xa2e844 __101-[_UISceneLifecycleMultiplexer _evalTransitionToSettings:fromSettings:forceExit:withTransitionStore:]_block_invoke_2 + 512
32 UIKitCore                      0xd8eb0 _UIScenePerformActionsWithLifecycleActionMask + 112
33 UIKitCore                      0xa2e564 __101-[_UISceneLifecycleMultiplexer _evalTransitionToSettings:fromSettings:forceExit:withTransitionStore:]_block_invoke + 252
34 UIKitCore                      0xa2e070 -[_UISceneLifecycleMultiplexer _performBlock:withApplicationOfDeactivationReasons:fromReasons:] + 212
35 UIKitCore                      0xa2e37c -[_UISceneLifecycleMultiplexer _evalTransitionToSettings:fromSettings:forceExit:withTransitionStore:] + 608
36 UIKitCore                      0xa2de08 -[_UISceneLifecycleMultiplexer uiScene:transitionedFromState:withTransitionContext:] + 244
37 UIKitCore                      0xa39288 __186-[_UIWindowSceneFBSSceneTransitionContextDrivenLifecycleSettingsDiffAction _performActionsForUIScene:withUpdatedFBSScene:settingsDiff:fromSettings:transitionContext:lifecycleActionType:]_block_invoke + 148
38 UIKitCore                      0xeabec4 +[BSAnimationSettings(UIKit) tryAnimatingWithSettings:fromCurrentState:actions:completion:] + 736
39 UIKitCore                      0xd9cc8 _UISceneSettingsDiffActionPerformChangesWithTransitionContextAndCompletion + 224
40 UIKitCore                      0xa38f84 -[_UIWindowSceneFBSSceneTransitionContextDrivenLifecycleSettingsDiffAction _performActionsForUIScene:withUpdatedFBSScene:settingsDiff:fromSettings:transitionContext:lifecycleActionType:] + 316
41 UIKitCore                      0x8781e4 __64-[UIScene scene:didUpdateWithDiff:transitionContext:completion:]_block_invoke.218 + 616
42 UIKitCore                      0x877278 -[UIScene _emitSceneSettingsUpdateResponseForCompletion:afterSceneUpdateWork:] + 208
43 UIKitCore                      0x877e50 -[UIScene scene:didUpdateWithDiff:transitionContext:completion:] + 244
44 UIKitCore                      0xed502c -[UIApplicationSceneClientAgent scene:handleEvent:withCompletion:] + 336
45 FrontBoardServices             0x1fc2c __76-[FBSScene updater:didUpdateSettings:withDiff:transitionContext:completion:]_block_invoke.129 + 380
46 FrontBoardServices             0x1e8a4 -[FBSScene _callOutQueue_maybeCoalesceClientSettingsUpdates:] + 128
47 FrontBoardServices             0x1f8dc -[FBSScene updater:didUpdateSettings:withDiff:transitionContext:completion:] + 708
48 FrontBoardServices             0x8fbb0 __94-[FBSWorkspaceScenesClient _queue_updateScene:withSettings:diff:transitionContext:completion:]_block_invoke_2.cold.1 + 352
49 FrontBoardServices             0x4bb30 __94-[FBSWorkspaceScenesClient _queue_updateScene:withSettings:diff:transitionContext:completion:]_block_invoke_2 + 96
50 FrontBoardServices             0x2bf64 -[FBSWorkspace _calloutQueue_executeCalloutFromSource:withBlock:] + 168
51 libdispatch.dylib              0x1b7fc _dispatch_client_callout + 16
52 libdispatch.dylib              0x6bb4 _dispatch_block_invoke_direct + 284
53 BoardServices                  0x91d4 __BSSERVICEMAINRUNLOOPQUEUE_IS_CALLING_OUT_TO_A_BLOCK__ + 52
54 BoardServices                  0x9054 BSServiceMainRunLoopSourceHandler + 224
55 CoreFoundation                 0x68f10 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 28
56 CoreFoundation                 0x68e84 __CFRunLoopDoSource0 + 172
57 CoreFoundation                 0x46acc __CFRunLoopDoSources0 + 232
58 CoreFoundation                 0x1d6d8 __CFRunLoopRun + 820
59 CoreFoundation                 0x1ca6c _CFRunLoopRunSpecificWithOptions + 532
60 GraphicsServices               0x1498 GSEventRunModal + 120
61 UIKitCore                      0x9ddf8 -[UIApplication _run] + 792
62 UIKitCore                      0x46e54 UIApplicationMain + 336
63 order                      0x4821b8 (Missing UUID 4c4c44ec55553144a1d504140580fce9)
64 order                      0x80c0 main + 8 (main.swift:8)
65 ???                            0x1813bae28 (Missing)

Checklist

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have tested this locally.

Reference Issue (For employees only. Ignore if you are an outside contributor)

• Closes https://go/j/SDKE-1164

[cursor]

PR Summary

Medium Risk
Touches app lifecycle background handling and changes the threading model for updateLastUseDate, which could affect persistence timing or introduce ordering issues if misused. Mitigated by added regression/stress tests around concurrent MPUserDefaults access.

Overview
Prevents a background-entry crash caused by concurrent NSUserDefaults access by moving MPApplication_PRIVATE updateLastUseDate: in MPStateMachine’s handleApplicationDidEnterBackground: onto the SDK’s serial messageQueue (capturing _launchDate into a local before dispatch).

Adds new unit tests that stress concurrent reads/writes to MPUserDefaults during background-like workloads and verifies updateLastUseDate:nil persists @0 rather than crashing.

^{Written by Cursor Bugbot for commit 55c51fe. This will update automatically on new commits. Configure here.}

[github-actions]

📦 SDK Size Impact Report

Measures how much the SDK adds to an app's size (with-SDK minus without-SDK).

Metric	Target Branch	This PR	Change
App Bundle Impact	1.82 MB	1.82 MB	+N/A
Executable Impact	896 bytes	896 bytes	+N/A
XCFramework Size	9.49 MB	9.49 MB	-4 KB

➡️ SDK size impact change is minimal.

Raw measurements

Target branch (main):

{"baseline_app_size_kb":84,"baseline_executable_size_bytes":75464,"with_sdk_app_size_kb":1944,"with_sdk_executable_size_bytes":76360,"sdk_impact_kb":1860,"sdk_executable_impact_bytes":896,"xcframework_size_kb":9720}

This PR:

{"baseline_app_size_kb":84,"baseline_executable_size_bytes":75464,"with_sdk_app_size_kb":1944,"with_sdk_executable_size_bytes":76360,"sdk_impact_kb":1860,"sdk_executable_impact_bytes":896,"xcframework_size_kb":9716}