Summary

• Collection lock contention during PING no longer triggers a destructive device state reset or an HTTP 500.
• Lock contention in the Sql and Mongo state drivers now throws Horde_ActiveSync_Exception_TemporaryFailure instead of a generic Horde_ActiveSync_Exception.
• loadState() no longer emits PHP 8 "Undefined array key" warnings when invoked with an empty collection array during error recovery.

Motivation

Production logs showed this failure chain when a PING polled a collection whose lock was held by a parallel SYNC:

1. initCollectionState() threw the generic Horde_ActiveSync_Exception('Collection lock held.').
2. The generic catch in pollForChanges() treated this as corrupt state and started a recovery reset: loadState([], null, SYNC, $id).
3. The empty collection array caused Undefined array key "class" / "serverid" warnings in State/Base.php.
4. The reset itself tried to acquire the same held lock, threw again, and escaped uncaught — the client received HTTP 500.

Lock contention is a normal, transient condition. Reacting with a state reset risks wiping healthy sync state whenever two requests overlap.

Changes

• State/Sql.php, State/Mongo.php: throw Horde_ActiveSync_Exception_TemporaryFailure when the collection lock cannot be acquired. As a subclass of Horde_ActiveSync_Exception, all existing generic catch sites keep working.
• Collections.php (pollForChanges()): catch TemporaryFailure before the generic handler and skip the collection for this iteration (retried on the next poll) instead of resetting state. The two StaleState recovery resets are wrapped the same way: if the reset is blocked by a held lock, it is deferred to the next iteration instead of aborting the request.
• State/Base.php (loadState()): read class/serverid with null coalescing so an error-recovery reset with an empty collection array creates a plain Horde_ActiveSync_Folder_Collection without PHP 8 warnings (matches pre-PHP-8 behavior).

Test plan

• New: held lock throws TemporaryFailure and rolls back the lock transaction (CollectionLockTest::testHeldLockThrowsTemporaryFailure)
• New: loadState([], null, SYNC, $id) emits no warnings and creates a folder object (CollectionLockTest::testLoadStateResetWithEmptyCollectionEmitsNoWarnings)
• New: pollForChanges() skips the collection on lock contention without calling loadState() reset (PingCollectionLockContentionTest)
• Existing state/lock/ping unit tests pass