crashlytics: Fix flaky tests caused by EXC_GUARD on stdin

[ncooke3]

Tip: Ignore whitespace when reviewing – https://github.com/firebase/firebase-ios-sdk/pull/15583/files?w=1

This PR addresses one source of flaky tests in the Crashlytics test suite.

Problem: A race condition occurred where FIRCLSBinaryImageInit zero-initialized the global context while a background thread from a previous test was still cleaning up. This caused a valid file descriptor to be overwritten with 0 (stdin). Attempting to close(0) triggers the operating system's EXC_GUARD protection, killing the process.

Solution: Patched FIRCLSFileClose to strictly ignore file descriptors <= STDERR_FILENO (0, 1, 2). This ensures the SDK never accidentally closes standard input/output streams, even if the file structure is improperly initialized to zero.

Process

1. Repro locally (may take a few attempts)

xcodebuild \                                                 
  -scheme FirebaseCrashlyticsUnit \
  -destination "platform=iOS Simulator,name=iPhone 16e" \
  ONLY_ACTIVE_ARCH=YES \
  CODE_SIGNING_REQUIRED=NO \
  CODE_SIGNING_ALLOWED=YES \
  COMPILER_INDEX_STORE_ENABLE=NO \
  CC=clang \
  CPLUSPLUS=clang++ \
  LD=clang \
  LDPLUSPLUS=clang++ \
  IPHONEOS_DEPLOYMENT_TARGET=13.0 \
  TVOS_DEPLOYMENT_TARGET=13.0 \
  clean test

2. I saw this suspicious message.

Test Case '-[FIRCLSSettingsTests testActivatedSettingsMissingCacheKey]' started.

Restarting after unexpected exit, crash, or test timeout; summary will include totals from previous launches.

3. At the bottom of the test session logs was:

Test session results, code coverage, and logs:

/Users/nickcooke/Library/Developer/Xcode/DerivedData/firebase-ios-sdk-anxqgakbhyqozfhbkutiwmcpphvm/Logs/Test/Test-FirebaseCrashlyticsUnit-2025.12.09_14-59-50--0500.xcresult

4. Open the xcresult in Xcode by running open <xcresult path> on command line.
5. The crash was due to the file close method in Crashlytics (right click to see full trace).

Manual Testing

I ran 10 invocations of the test command for each of the following cases.

• Without fix: 40% flake rate (each flake was a different test)
• With fix: 0% flake rate 🎉

#no-changelog

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[ncooke3]

re: CI– Unfortunately, it isn't a cure-all, but I think it's progress, considering the positive local testing.

[paulb777]

Nice find!

[ncooke3]

cc: @themiswang