feat: add in-memory token caching

[andrewheard]

Added an in-memory token cache and made Keychain token caching failures non-fatal in GACAppCheck.

Previously, GACAppCheck.refreshToken failed the entire token request with GACAppCheckErrorCodeKeychain if writing to Keychain storage failed, discarding the valid fetched token. This meant that workflows in environments without Keychain entitlements, such as swift test on macOS and CI, were unsupported. Now, tokens are added to an in-memory cache that functions even if writes to Keychain can't be performed and offers faster read times in environments where the Keychain is available.

• Non-Fatal Storage: Caught Keychain write errors during refreshToken, logged a warning (GACLoggerAppCheckMessageCodeTokenStorageFailed), and returned the valid token. Keychain writes are still attempted on every refresh to preserve persistence across cold launches.
• In-Memory Cache: Added inMemoryToken to GACAppCheck. Unexpired tokens are now served directly from memory, eliminating redundant Keychain IPC queries.
• Forced Refresh Invalidation: Cleared inMemoryToken when forcingRefresh: YES was requested so revoked tokens are not retained if a refresh fails.
• Tests: Updated and expanded GACAppCheckTests.m to cover Keychain write error resilience, in-memory cache hits, forced refresh invalidation, and expiration fallback.

Walkthrough - In-Memory Fallback & Non-Fatal Keychain Storage

We investigated the token caching and Keychain error handling behavior in
AppCheckCore and resolved the issue where Keychain write failures (such as
those encountered in swift test or un-entitled environments) discarded
valid tokens and failed operations with GACAppCheckErrorCodeKeychain.

Summary of Changes

GACAppCheckErrors.h

• Added documented message code:

  /// Failed to cache the App Check token in persistent storage.
  GACLoggerAppCheckMessageCodeTokenStorageFailed = 2004,

GACAppCheck.m

• In-Memory Cache & Concurrency:
  • Added @property(nonatomic, strong, nullable) GACAppCheckToken *inMemoryToken;.
  • Removed anti-pattern @synchronized(self), relying on queue confinement
    consistent with ongoingRetrieveOrRefreshTokenPromise on
    FBLPromise.defaultDispatchQueue.
• Forced Refresh Invalidation:
  • getCachedValidTokenForcingRefresh: explicitly sets self.inMemoryToken = nil;
    when forcingRefresh: YES is passed. If a token was revoked and a refresh
    fails, the revoked token is no longer retained in memory.
• Cache Hierarchy (Memory -> Keychain -> Network):
  • Checks inMemoryToken first via extracted helper - (BOOL)isTokenExpiredOrExpiresSoon:(GACAppCheckToken *)token.
  • If absent or expiring within kTokenExpirationThreshold (5 min), checks
    Keychain storage.
  • On cache miss, expiration, or Keychain read error, refreshes from the
    provider.
• Resilient Token Refresh:
  • In refreshToken, [self.storage setToken:token] catches Keychain write
    failures with .recover(^id _Nullable(NSError *_Nonnull error) { ... }).
  • On failure, logs a warning with message code 2004 and returns the valid
    token, allowing the caller to receive the fetched token without failing.
• Cold Launch Persistence:
  • Every token refresh continues to attempt persisting to the Keychain,
    preserving persistence across app cold launches in typical environments.
• Performance Improvement:
  • Bypassing redundant Keychain IPC (securityd) when a valid token is
    already held in memory significantly reduces token retrieval overhead.

GACAppCheckTests.m

• Operation Merging Tests:
  • Updated testGetToken_WhenCalledSeveralTimesError_ThenThereIsOnlyOneOperation
    to test operation merging on provider failures now that storage failures
    are non-fatal.
  • Updated testGetToken_WhenCalledSeveralTimesSuccess_ThenThereIsOnlyOneOperation
    to verify subsequent calls reuse the in-memory cached token.
• Storage Failure Resilience:
  • Added testGetToken_WhenStorageFails_ThenTokenReturnedAndCachedInMemory to
    verify that Keychain write errors log a warning, succeed with the valid
    token, and cache it in memory.
• Cache Invalidation & Expiration:
  • Enhanced testGetToken_WhenForcingRefresh_ThenInMemoryCacheIsBypassed to
    verify the refreshed token is served on subsequent calls.
  • Added testGetToken_WhenForcingRefresh_ThenInMemoryTokenIsInvalidated to
    verify that forcingRefresh: YES invalidates the in-memory token even if
    the refresh fails.
  • Added testGetToken_WhenInMemoryTokenExpires_ThenRefreshesWithProvider to
    verify that an expiring in-memory token triggers a refresh.

Verification Results

Automated Tests

Ran the full test suite using swift test and formatted with xcsift:

swift test 2>&1 | tee /tmp/swift-test.log 2>&1 | /opt/homebrew/bin/xcsift -w -f toon

Output:

status: success
summary:
  errors: 0
  warnings: 0
  failed_tests: 0
  linker_errors: 0
  passed_tests: 136
  build_time: 3.73 secs.
  test_time: 134.286s

All 136 unit tests passed with zero errors and zero warnings.