fix(crashlytics): serialize INT64_MIN without overflow

[dajiaohuang]

Summary

• Avoid signed overflow in both int64_t writer paths using unsigned magnitude arithmetic, including INT64_MIN. The normal printed value was already correct; this change removes undefined behavior.
• Keep buffered/unbuffered JSON regression coverage with the corrected hash-start fixture.
• Record overflow avoidance under Unreleased with fix(crashlytics): serialize INT64_MIN without overflow #16773.

Validation

• clang-format 23.1.2 and git diff --check pass for the changed files.
• Actual pre-PR signed writer bodies compiled with Linux UBSan reproduce INT64_MIN overflow; both updated bodies pass 10 boundary cases. Storage and unsigned-output helpers were stubbed, so this is focused sanitizer evidence.
• At source 63a828c, the upstream Catalyst FirebaseCrashlytics-Unit-unit run executed 218 tests with zero failures, including testMinInt64: actual XCTest log.
• The ordinary XCTest pass verifies serialization and the corrected fixture; the before/after UBSan result establishes the overflow distinction.

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[paulb777]

Thanks for the contribution! A few housekeeping items:

• CHANGELOG: The entry in Crashlytics/CHANGELOG.md is under # 13.0.0, which has already shipped. Please move it under a # Unreleased heading at the top of the file (add the heading if it isn't there yet).
• PR number: end the entry with this PR's number, e.g. - [fixed] Description. (#16773).
• Formatting: after updating, please run scripts/style.sh so the infra.check job passes. It needs clang-format 23 and swiftformat (see scripts/setup_check.sh). If you can't run it locally, the infra.check log lists the files that need formatting.

[paulb777]

Thanks. Negating INT64_MIN is undefined behavior, so this is a fair cleanup. In practice, though, the written output was already -9223372036854775808 (only UBSan would flag it). Following up on my earlier comment: please reword the CHANGELOG entry to match (e.g. "Avoid signed integer overflow when serializing INT64_MIN"), or drop it.

CI shows testMinInt64 failing; the cause is inline. The other catalyst failure is the FIRCLSContextManagerTests crash fixed in #16776. Since this and #16772 both touch FIRCLSFile.m, feel free to fold this into #16772.

[dajiaohuang]

The JSON regression already has the missing FIRCLSFileWriteHashStart call from the prior follow-up. Both signed writer paths now use the suggested unsigned subtraction, and the changelog is under Unreleased with #16773 and describes avoiding signed overflow rather than changing the already-correct printed value. The changed source/test files pass clang-format 23.1.2 and git diff --check.

For the requested sanitizer distinction, I compiled the actual pre-PR and updated signed writer function bodies on Linux with UBSan and no recovery. The pre-PR INT64_MIN path reports signed overflow; both updated writer functions pass INT64_MIN, INT64_MAX, -1, 0 and 1 (10 cases). The storage and unsigned-output helpers were stubbed for this focused test; it is not a full Crashlytics/XCTest execution. Native Apple XCTest remains for CI. I kept this existing PR separate from #16772 so the overflow change stays independently reviewable.

[paulb777]

/gemini review

[gemini-code-assist]

Code Review

This pull request fixes a signed integer overflow issue when serializing INT64_MIN by casting the value to uint64_t before computing its magnitude, and adds corresponding unit tests. Feedback suggests refactoring the magnitude assignment in FIRCLSFileWriteInt64 and FIRCLSFileFDWriteInt64 to use an if-else block with idiomatic unary negation instead of redundant initialization and subtraction from zero. Additionally, it is recommended to use INT64_MIN directly in the unit test assertion for better readability.