fix(storage): serialize auth and app check token updates

[dajiaohuang]

Summary

• Serialize Auth/App Check header updates on the existing serialAuthArgsQueue, preserving the public completion queue.
• The concurrent mutation is reachable with custom interop providers or when an application changes the Promises default callback queue. Shipping Auth and App Check providers otherwise deliver callbacks on main.
• Add parallel-provider regression coverage with bounded semaphore waits; update Unreleased changelog with fix(storage): serialize auth and app check token updates #16766.

Validation

• Changed Swift files pass pinned SwiftFormat 0.55.5 lint and git diff --check.
• Actual FirebaseStorageUnit/StorageAuthorizerTests on Xcode 26.4.1 and an iOS simulator: 17 tests passed, zero failures, including testAuthAndAppCheckTokenHeadersAreMutatedSerially, at source 76fbd7f.
• Native XCTest run uses the repository Swift Package test setup. This is focused class coverage; the full platform matrix remains separate.

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[Lzybear22]

I'm new to Swift. What was causing the issue with the request headers?

[paulb777]

Thanks for the contribution! A few housekeeping items:

• CHANGELOG: The entry in FirebaseStorage/CHANGELOG.md is under # 12.19.0, 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. (#16766).
• 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. One note on reachability: with the shipping providers, both callbacks already arrive on the main queue. Auth delivers getToken on main, and App Check's callback runs on the Promises default queue, which is main. So this race can only happen if an app changes DispatchQueue.promises (FBLPromise.defaultDispatchQueue) or uses a custom interop provider. Could you mention that in the PR description?

The change itself is safe, and it finally puts the unused serialAuthArgsQueue to work.

[dajiaohuang]

Both semaphore waits now use two-second timeouts and XCTFail, while resumeBoth still releases both callback permits even after a timeout so the test cannot leave providers blocked indefinitely. Updated the PR description and Unreleased/#16766 changelog to state that concurrent callbacks require custom interop providers or a changed Promises default callback queue; shipping providers normally deliver on main.

Kept the existing serialAuthArgsQueue implementation, which you confirmed is safe, rather than switching to the optional notify-block alternative. Pinned SwiftFormat 0.55.5 lint and git diff --check pass. Native Apple XCTest is unrun locally and remains for CI.

[paulb777]

/gemini review

[gemini-code-assist]

Code Review

This pull request serializes Auth and App Check token header updates in StorageTokenAuthorizer using a serial queue to prevent concurrent mutation of requests, and adds corresponding unit tests. The review feedback recommends increasing the timeout values for semaphores and expectations in the new tests to 5 seconds to avoid potential flakiness on heavily loaded CI environments.

[dajiaohuang]

The revised bounded-wait regression now runs in the repository's actual Swift Package test target. At source 76fbd7f, StorageAuthorizerTests executed 17 tests with zero failures, including the parallel Auth/App Check header mutation regression: native XCTest results. The PR description retains the custom-provider/default-callback-queue reachability boundary.