[#504] Guard asyncValidator setting error with _statusChanges stream status

[LahaLuhem]

Connection with issue(s)

Close #504

Solution description

The StateError: Cannot add new events after calling close from #504 is the visible symptom of an orphan-subscription race in _cancelExistingSubscription(). The same race can also cause stale validator results to overwrite newer ones, even without dispose().

Root cause

_cancelExistingSubscription() was async and called without await from updateValueAndValidity():

Future<void> _cancelExistingSubscription() async {
  await _asyncValidationSubscription?.cancel();
  _asyncValidationSubscription = null;
}

1. The await suspends — even when _asyncValidationSubscription is null, await null still yields via a microtask.
2. Control returns to updateValueAndValidity(), which synchronously runs _runAsyncValidators() and stores a fresh subscription in _asyncValidationSubscription.
3. The deferred microtask resumes and runs _asyncValidationSubscription = null, orphaning the just-assigned subscription.
4. The orphan has no reference, so neither subsequent value changes nor dispose() can cancel it. When it eventually completes, its onDone:
   • crashes if _statusChanges has been closed (the original Use-after-free bug possible for AsyncValidators #504 symptom), or
   • re-injects a stale error via setErrors(...).addAll(errors).addAll(asyncValidationErrors), overwriting whatever the latest validator settled on.

Fix

1. Make _cancelExistingSubscription synchronous. StreamSubscription.cancel() stops event delivery synchronously; the returned future only signals cleanup completion, so awaiting it serves no purpose here and is what introduces the suspension point.
2. Cancel _debounceTimer in dispose(). Without this, disposing a control while a debounce window is open leaks the timer (and the closure holding this).

With both changes the symptom-level if (_statusChanges.isClosed) return; guard is unnecessary — the orphan that produced the bad onDone can no longer be created.

Tests

test/src/validators/async_validator_test.dart:

• Dispose during in-flight async validation — six tests covering dispose at various points during validation for FormControl / FormGroup, single and multiple validators, including the debounce window. Also asserts dispose() cancels the pending debounce timer via FakeAsync.pendingTimers.
• Async validator subscription cancellation — sets value = 'bad' (slow validator, returns an error), then value = 'good' (fast, returns no error); asserts the stale 'bad' error does not survive into the 'good' state. Fails on the original code because the orphaned 'bad' subscription is never cancelled and re-injects its error after 'good' has already settled.

Each test was verified to fail when the corresponding part of the fix is reverted.

To Do

• Check the original issue to confirm it is fully satisfied
• Add solution description to help guide reviewers
• Add unit test to verify new or fixed behaviour
• If apply, add documentation to code properties and package readme

[gemini-code-assist]

Code Review

This pull request adds a check to prevent a crash when _statusChanges is closed during the onDone callback in AbstractControl. However, the review feedback points out that this only addresses the symptom of a larger issue. The root cause is a subscription leak caused by a race condition in the asynchronous _cancelExistingSubscription() method, which gets overwritten with null and orphaned. The reviewer recommends making _cancelExistingSubscription() synchronous and ensuring the _debounceTimer is cancelled in dispose() to properly resolve the leak.