refactor: Replace strerror with strerror_r

[philprime]

📜 Description

Replaces all occurrences of strerror with strerror_r using the thread-safe SENTRY_STRERROR_R macro across the SentryCrash codebase.

💡 Motivation and Context

Closes #4190

The strerror function is not thread-safe and can cause issues in multi-threaded environments. This PR replaces all uses of strerror with the thread-safe strerror_r function via the SENTRY_STRERROR_R macro.

📋 Changes Summary

Core Changes

• Added SENTRY_STRERROR_R macro in SentryAsyncSafeLog.h:

  • Thread-safe wrapper around strerror_r using XSI-compliant version (returns int on macOS/iOS)
  • Uses 1024-byte buffer matching glibc's implementation
  • Handles strerror_r failures gracefully

• Replaced strerror calls in 11 SentryCrash source files:

  • SentryCrashMonitor_AppState.c (2 replacements)
  • SentryCrashMonitor_MachException.c (4 replacements)
  • SentryCrashMonitor_Signal.c (4 replacements)
  • SentryCrashCachedData.c (1 replacement)
  • SentryCrashReport.c (6 replacements)
  • SentryCrashReportStore.c (2 replacements)
  • SentryCrashCxaThrowSwapper.c (4 replacements)
  • SentryCrashDebug.c (2 replacements)
  • SentryCrashFileUtils.c (20 replacements)
  • SentryCrashJSONCodec.c (2 replacements)
  • SentryCrashSysCtl.c (9 replacements)

Total: 56 strerror calls replaced with SENTRY_STRERROR_R

Test Coverage

• SentryCrashDebug_Tests.m: Added ests for sentrycrashdebug_isBeingTraced error handling
• SentryCrashFileUtils_Tests.m: Added tests covering:
  • File operation failures (open, read, write, close, stat, mkdir, rmdir, rename, remove)
  • Directory operation failures
  • Error message formatting with SENTRY_STRERROR_R
• SentryCrashSysCtl_Tests.m: Expanded tests covering:
  • sysctl failure scenarios
  • Various system call error conditions
  • Thread-safe error message retrieval

Documentation

• AGENTS.md: Added comprehensive guidelines for testing error handling paths:
  • Testable vs untestable error paths
  • Best practices for documenting untestable paths
  • Example test patterns following Arrange-Act-Assert
  • Guidelines for when to document rather than test

Other Changes

• Updated SentryLogC.h to use SENTRY_STRERROR_R macro
• Updated SentryViewHierarchyProviderHelper.m and SentrySessionReplaySyncC.c to use thread-safe error handling

💚 How did you test it?

Tested Error Paths

Added unit tests for all testable error handling paths:

• File operations (SentryCrashFileUtils.c):

  • open() failures - tested with invalid paths and closed file descriptors
  • read() failures - tested with closed file descriptors
  • write() failures - tested with closed file descriptors
  • close() failures - tested with invalid file descriptors
  • stat() failures - tested with invalid paths
  • mkdir() failures - tested with invalid parent directories
  • rmdir() failures - tested with non-empty directories
  • rename() failures - tested with invalid source/target paths
  • remove() failures - tested with invalid paths

• System calls (SentryCrashSysCtl.c):

  • sysctl() failures - tested with invalid parameters and system limits

• Debug operations (SentryCrashDebug.c):

  • sysctl() failures in sentrycrashdebug_isBeingTraced

All tests verify that error messages are retrieved using SENTRY_STRERROR_R(errno) for thread-safe error handling.

Untestable Error Paths

Some error handling paths cannot be reliably tested in a test environment. All of these paths have been verified through code review to correctly use SENTRY_STRERROR_R(errno) for thread-safe error message retrieval.

System Call Failures

The following functions handle system call failures but cannot be reliably tested:

SentryCrashReport.c:

• addTextFileElement() - Error handling when open() fails
• sentrycrashreport_writeRecrashReport() - Error handling when rename() or remove() fails

Why untestable:

• System call failures (open(), rename(), remove()) are difficult to force reliably
• File permissions and file system state may not consistently trigger failures
• System calls cannot be easily mocked in C without function interposition, which has limitations for statically linked symbols

Resource Exhaustion Failures

SentryCrashCachedData.c:

• sentrycrashccd_init - Error handling when pthread_create() fails

Why untestable:

• Tried setrlimit(RLIMIT_NPROC), thread exhaustion, and DYLD_INTERPOSE - none worked reliably
• On macOS/iOS, RLIMIT_NPROC limits processes, not threads directly
• Modern systems allow a very large number of threads
• DYLD_INTERPOSE only works for dynamically linked symbols; pthread_create calls may be statically linked

SentryCrashCxaThrowSwapper.c:

• addPair (used by perform_rebinding_with_section) - Error handling when malloc() fails

Why untestable:

• Tried memory exhaustion and setrlimit(RLIMIT_AS) - both failed due to system restrictions or virtual memory overcommit

SentryCrashDebug.c:

• sentrycrashdebug_isBeingTraced - Error handling when sysctl() fails (hardcoded valid parameters)

Why untestable:

• Function uses hardcoded valid parameters and sysctl calls may be statically linked
• Cannot reliably force failures without function interposition limitations

File I/O Failures

SentryCrashJSONCodec.c:

• updateDecoder_readFile() - Error handling when read() fails

Why untestable:

• sentrycrashjson_addJSONFromFile manages the file descriptor internally
• JSONFromFileContext (which contains the fd) is not exposed
• The function uses static callbacks we can't easily replace
• Testing would require duplicating significant code or exposing internal structures

Note: In contrast, SentryCrashFileUtils_Tests.m tests work because those functions take the fd as a parameter, allowing tests to open, close, and pass a closed fd.

Verification

All untestable error paths have been:

• Verified through code review to exist and correctly use SENTRY_STRERROR_R(errno)
• Documented with comments in the source code explaining why they cannot be tested
• Followed the guidelines documented in AGENTS.md for documenting untestable error paths

📝 Checklist

• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed (AGENTS.md).
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

[codecov]

Codecov Report

❌ Patch coverage is 42.02899% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.877%. Comparing base (70ac6c6) to head (ac098f5).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...SentryCrash/Recording/Tools/SentryCrashFileUtils.c	50.000%	12 Missing ⚠️
Sources/SentryCrash/Recording/SentryCrashReport.c	20.000%	4 Missing ⚠️
...Crash/Recording/Tools/SentryCrashCxaThrowSwapper.c	0.000%	4 Missing ⚠️
...es/SentryCrash/Recording/Tools/SentryCrashSysCtl.c	63.636%	4 Missing ⚠️
...ces/SentryCrash/Recording/SentryCrashReportStore.c	0.000%	3 Missing ⚠️
Sources/Sentry/SentryAsyncSafeLog.h	75.000%	2 Missing ⚠️
Sources/Sentry/SentrySessionReplaySyncC.c	33.333%	2 Missing ⚠️
...ording/Monitors/SentryCrashMonitor_MachException.c	0.000%	2 Missing ⚠️
...ash/Recording/Monitors/SentryCrashMonitor_Signal.c	0.000%	2 Missing ⚠️
...SentryCrash/Recording/Tools/SentryCrashJSONCodec.c	0.000%	2 Missing ⚠️
... and 3 more

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #6817       +/-   ##
=============================================
- Coverage   85.894%   85.877%   -0.017%     
=============================================
  Files          453       453               
  Lines        37531     37615       +84     
  Branches     17430     17497       +67     
=============================================
+ Hits         32237     32303       +66     
+ Misses        5251      5054      -197     
- Partials        43       258      +215     

Files with missing lines	Coverage Δ
Sources/Sentry/SentryViewHierarchyProviderHelper.m	100.000% <100.000%> (ø)
...h/Recording/Monitors/SentryCrashMonitor_AppState.c	91.946% <100.000%> (ø)
Sources/Sentry/include/SentryLogC.h	90.000% <0.000%> (ø)
...rces/SentryCrash/Recording/SentryCrashCachedData.c	92.907% <0.000%> (-2.837%)	⬇️
...ces/SentryCrash/Recording/Tools/SentryCrashDebug.c	81.818% <0.000%> (ø)
Sources/Sentry/SentryAsyncSafeLog.h	85.000% <75.000%> (+0.384%)	⬆️
Sources/Sentry/SentrySessionReplaySyncC.c	75.324% <33.333%> (ø)
...ording/Monitors/SentryCrashMonitor_MachException.c	37.671% <0.000%> (ø)
...ash/Recording/Monitors/SentryCrashMonitor_Signal.c	62.758% <0.000%> (ø)
...SentryCrash/Recording/Tools/SentryCrashJSONCodec.c	86.674% <0.000%> (-0.303%)	⬇️
... and 5 more

... and 56 files with indirect coverage changes

---

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[philprime]

Attempted Approaches for Testing pthread_create Failure in sentrycrashccd_init

We attempted to create a test for testInit_UsesSENTRY_STRERROR_R_ForPthreadCreateFailure to verify that sentrycrashccd_init uses SENTRY_STRERROR_R when pthread_create fails. However, we were unable to find a reliable way to force pthread_create to fail in a test environment.

Approaches Tried:

1. setrlimit(RLIMIT_NPROC) - Attempted to set the process thread limit to 1 to force pthread_create to fail with EAGAIN. This approach doesn't work reliably because:

   • On macOS/iOS, RLIMIT_NPROC limits processes, not threads directly
   • The test process already has multiple threads, so setting the limit to 1 doesn't prevent thread creation
   • On iOS Simulator, we may not have permission to change resource limits

2. Thread Exhaustion - Attempted to create many threads to exhaust system resources. This approach doesn't work because:

   • Modern systems allow a very large number of threads
   • Creating thousands of threads is slow and doesn't guarantee failure
   • The system may create more threads than we can exhaust in a reasonable test time

3. DYLD_INTERPOSE - Attempted to use function interposition to mock pthread_create and force it to return an error. This approach doesn't work because:

   • DYLD_INTERPOSE only works for dynamically linked symbols
   • pthread_create calls from the Sentry framework may be statically linked or resolved before interposition
   • The mock function was never called, indicating interposition didn't work

4. Fishhook-style Rebinding - Considered implementing fishhook-style symbol rebinding (similar to __cxa_throw swapping), but this would require:

   • Significant code duplication from SentryCrashCxaThrowSwapper.c
   • Complex mach-o header parsing and symbol table manipulation
   • More complexity than warranted for a single test

Conclusion:

It's not viable to continue finding a way to force pthread_create to fail in a test environment. The error handling code path exists in the source code (line 166 in SentryCrashCachedData.c) and correctly uses SENTRY_STRERROR_R(error) when pthread_create fails. The code change itself is correct and verified through code review.

[philprime]

@sentry review

[philprime]

@sentry review

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1217.04 ms	1244.16 ms	27.12 ms
Size	24.14 KiB	1.01 MiB	1015.53 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
26f7b17	1218.47 ms	1253.82 ms	35.35 ms
139db8b	1231.50 ms	1258.19 ms	26.69 ms
134fbdf	1219.71 ms	1240.35 ms	20.64 ms
83bb978	1238.33 ms	1260.04 ms	21.71 ms
331dad6	1210.40 ms	1242.06 ms	31.67 ms
85a741b	1217.02 ms	1239.27 ms	22.25 ms
083e8c5	1227.74 ms	1262.37 ms	34.62 ms
d7461dc	1233.69 ms	1255.29 ms	21.60 ms
939d583	1209.96 ms	1251.09 ms	41.13 ms
5fcb6a1	1198.86 ms	1226.89 ms	28.03 ms

App size

Revision	Plain	With Sentry	Diff
26f7b17	23.75 KiB	960.93 KiB	937.19 KiB
139db8b	23.75 KiB	920.64 KiB	896.89 KiB
134fbdf	23.75 KiB	875.25 KiB	851.50 KiB
83bb978	23.75 KiB	920.64 KiB	896.89 KiB
331dad6	23.75 KiB	928.12 KiB	904.37 KiB
85a741b	23.75 KiB	959.44 KiB	935.69 KiB
083e8c5	23.75 KiB	981.75 KiB	958.00 KiB
d7461dc	23.75 KiB	874.45 KiB	850.70 KiB
939d583	23.75 KiB	1023.82 KiB	1000.07 KiB
5fcb6a1	24.14 KiB	1.01 MiB	1014.60 KiB

Previous results on branch: philprime/sterror-replacement

Startup times

Revision	Plain	With Sentry	Diff
6e4179a	1221.29 ms	1261.98 ms	40.69 ms

App size

Revision	Plain	With Sentry	Diff
6e4179a	24.14 KiB	1.01 MiB	1015.51 KiB

[philprime]

@JoshuaMoelans @mujacica as this issue is quite hard to test with automated testing and is almost completely in the native layer of this SDK, I would appreciate your wisdom and kindly ask if you could please take a look if you spot any obvious issues with it.