[PLAT-16992] Incorrect-stack-trace-on-iOS

[SB-meiyalaganr]

Goal

Fix incorrect stack traces for C++ exceptions on iOS — ensuring the original throw site is preserved across concurrent throws, rethrows, and exception inspection.

Design

Make stack trace state thread_local and gate capture on an atomic install flag, eliminating the process-global race condition that caused wrong traces.

Changeset

Converted bsg_g_stackTrace, bsg_g_stackTraceCount, and bsg_g_captureNextStackTrace to thread_local; switched bsg_g_installed to std::atomic; extracted thread state into a reusable private header; added new concurrent/rethrow/inspect scenarios.

Testing

New KSCrashSentry_CPPException_Tests unit tests verify thread isolation and frame-skipping logic; new Maze Runner scenarios cover concurrent, background-thread, rethrow, bare-throw-after-catch, and exception-inspecting cases.

[github-actions]

Bugsnag.framework binary size increased by 184 bytes from 920,592 to 920,776

    FILE SIZE        VM SIZE    
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  [ = ]       0  [NEW]    +248    __DATA,__thread_bss
  +4.7%    +160  +4.7%    +160    [Mach-O Headers]
  +0.1%    +104  +0.1%    +104    String Table
  +0.0%     +48  +0.0%     +48    Symbol Table
  +1.0%     +32  +1.0%     +32    Binding Info
  [NEW]     +24  [NEW]     +24    __DATA,__thread_vars
  +0.0%     +16  +0.0%     +16    __TEXT,__text
  +0.2%      +8  +0.2%      +8    __TEXT,__unwind_info
  -2.5%     -24  -0.3%     -24    [__DATA]
  [ = ]       0  -1.4%    -184    [__LINKEDIT]
  -0.6%    -184  -0.6%    -184    [__TEXT]
  [ = ]       0  -2.5%    -248    __DATA,__bss
  +0.0%    +184  [ = ]       0    TOTAL

Generated by 🚫 Danger

[Copilot]

Pull request overview

This PR addresses incorrect stack traces for C++ exceptions on iOS by eliminating process-global exception backtrace state and improving frame skipping and fallback capture behavior, including new automated coverage for concurrent/rethrow/inspection cases.

Changes:

• Made C++ exception capture state thread-local and gated capture via an atomic “installed” flag to avoid cross-thread overwrites.
• Adjusted stack trace frame skipping and added a safer fallback backtrace capture path (e.g., bare terminate / uncaptured throws).
• Expanded automated validation via new unit tests and additional Maze Runner scenarios/features for concurrency/rethrow/inspection behaviors.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Tests/KSCrashTests/KSCrashSentry_CPPException_Tests.mm	Adds unit tests for thread-local state isolation and stack-trace “view” frame skipping.
features/release/unhandled_cpp_exception.feature	Extends acceptance checks to assert correct top frame and adds scenario outlines for concurrency/rethrow/inspection/bare-throw cases.
features/fixtures/shared/scenarios/CxxExceptionScenario.mm	Adds new fixture scenarios to deterministically reproduce concurrent throw/reraise/inspection behaviors.
features/fixtures/shared/scenarios/CxxBareThrowScenario.mm	Adds a scenario to ensure a previously caught exception doesn’t supply a later bare-throw stack trace.
Bugsnag/KSCrash/Source/KSCrash/Recording/Sentry/BSG_KSCrashSentry_CPPException.mm	Converts backtrace state to thread_local, gates capture on an atomic install flag, and updates frame skipping + fallback capture logic.
Bugsnag/KSCrash/Source/KSCrash/Recording/Sentry/BSG_KSCrashSentry_CPPException_Private.h	Introduces a private header encapsulating a thread-local state struct and stack-trace view helper.
Bugsnag.xcodeproj/project.pbxproj	Wires the new unit test file and private header into the Xcode project.

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