fix(database): serialize push ID generation

[dajiaohuang]

Summary

• Protect the shared Realtime Database push ID timestamp and random suffix while generating IDs.
• Cover concurrent same-timestamp uniqueness and monitor release after an exception; document the user-visible duplicate-ID race under Unreleased with fix(database): serialize push ID generation #16768.

childByAutoId invokes the generator synchronously on the caller's thread. Concurrent calls could interleave reads and updates of process-wide state, so the whole state transition and ID construction now run under one lock. The exception test accepts Foundation's NSException hierarchy rather than assuming one specific exception name.

Validation

• Affected files pass clang-format 23.1.2 and git diff --check.
• At source 30deadd, the actual upstream Catalyst Database unit suite executes 431 tests with zero failures, including concurrent push ID uniqueness and exception handling: native XCTest log.
• The broader workflow still fails its separate quickstart dependency resolution because FirebaseUI 16 and the required gtm-session-fetcher range conflict. The unit pass above does not claim the full workflow is green.

[gemini-code-assist]

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[paulb777]

Thanks for the contribution! A few housekeeping items:

• CHANGELOG: The entry in FirebaseDatabase/CHANGELOG.md was added above # 12.19.0 without a heading, so it reads as part of a release that 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. (#16768).
• 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]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces thread safety to the generation of Realtime Database push IDs in FNextPushId.m by utilizing an NSLock, and adds a concurrent unit test to verify that push IDs generated concurrently for the same timestamp remain unique. The review feedback suggests a minor improvement to use [id copy] instead of [NSString stringWithString:id] for better idiomatic efficiency in Objective-C.

[paulb777]

Thanks, this is a real race: childByAutoId calls +[FNextPushId get:] synchronously on the caller's thread, and the static state isn't synchronized, so concurrent calls can produce duplicate IDs (silently overwriting data) or even an out-of-range characterAtIndex:. Android already makes this method synchronized.

Optional: the CHANGELOG entry could describe the user-visible effect, e.g. - [fixed] Fixed a race where calling childByAutoId() concurrently from multiple threads could generate duplicate push IDs. (#16768)

[dajiaohuang]

Replaced the manual NSLock/dispatch_once setup with @synchronized([FNextPushId class]) around the whole shared-state update and ID construction, so an Objective-C exception releases the monitor. The fixed class monitor also covers subclasses sharing this static state. Removed the redundant dispatch header and added a bounded regression that verifies a generation exception does not block a later valid call; the 1000-call same-timestamp uniqueness test remains.

The CHANGELOG now uses Unreleased and the suggested user-visible description with #16768. Both changed Objective-C files pass clang-format 23.1.2 validation and git diff --check. Apple tests have not run locally; current-head native CI must validate the regression.

[paulb777]

/gemini review

[gemini-code-assist]

Code Review

This pull request fixes a race condition in childByAutoId() by wrapping the push ID generation logic in FNextPushId.m within a @synchronized block, and adds corresponding unit tests to verify thread safety. Feedback on the changes suggests optimizing the ID generation by avoiding the reserved keyword id as a variable name and replacing the inefficient NSMutableString loop with a stack-allocated unichar array.

[dajiaohuang]

Current-head macOS CI exposed an overly specific assumption in the new exception-release test: constant NSString raises NSInvalidArgumentException for the negative index, while the test named NSRangeException. The regression now asserts NSException and retains the background generation completion timeout and 20-character output check. This tests monitor release without depending on Foundation's exception-name choice. Concurrent uniqueness passed in the captured macOS run; the whole DatabaseUnit run was 431 tests with this one failure. Formatter/diff checks pass for the one-line correction; new-head native CI still needs to finish.

[paulb777]

/gemini review

[gemini-code-assist]

Code Review

This pull request addresses a race condition in childByAutoId() by wrapping the push ID generation logic in FNextPushId.m within a @synchronized block, and adds corresponding unit tests to verify thread safety and exception handling. Feedback on the changes highlights a critical buffer underflow risk when incrementing the random character array, the discouraged use of the reserved keyword 'id' as a variable name in Objective-C, and redundant floor function calls on integer values.

[ncooke3]

Thanks @dajiaohuang!