Getting `Failed assertion: line 2242 pos 12: ‘!timersPending’` in sqflite_ffi library when testing

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Summary

The application encounters a “Failed assertion: line 2242 pos 12: ‘!timersPending'” error during widget tests when using the sqflite_ffi library. The root cause is that the test environment (FakeAsync) terminates before sqflite‘s internal asynchronous operations—specifically database locking and transaction timeouts—can complete. These operations create internal timers (often defaulting to 10 seconds for busy loops) that the test framework detects as active leaks when the test finishes, causing the assertion to fail.

Root Cause

The failure stems from a mismatch between the persistence requirements of the database library and the instantaneous nature of the test lifecycle.

  • Internal Locking Timeouts: sqflite uses a 10-second timeout when attempting to acquire a database lock. In a transactional write, it spins up a timer to retry if the database is busy.
  • Unawaited Asynchronous Work: The user admits to firing off asynchronous database writes without awaiting them. While acceptable in a running app, this leaves “dangling” futures in the test environment.
  • Premature Test Teardown: The testWidgets function runs inside FakeAsync. When the test logic finishes, FakeAsync advances time to completion. However, if the database operations haven’t resolved (because they are waiting on locks or IO), the 10-second timer is still “ticking.”
  • Invariant Violation: The AutomatedTestWidgetsFlutterBinding checks !timersPending before cleaning up. Seeing the active timer, it throws the assertion error.

Why This Happens in Real Systems

This is a classic race condition specific to deterministic simulation environments.

In production (real device/emulator), the event loop keeps running after a function returns. The 10-second database lock timer eventually fires, the lock is acquired, and the write completes in the background. The user never notices the delay.

In testing (FakeAsync), we do not have the luxury of real time. The test runner forces time to be “fake.” When the test body returns, the framework expects zero active timers. The sqflite_ffi library, designed for the patience of real time, is too “slow” for the impatience of the test runner.

Real-World Impact

  • Flaky Tests: Tests that rely on database state may pass or fail depending on how fast the machine processes the lock.
  • Test Suite Blockers: Developers cannot run their test suites because the suite crashes on exit with assertion errors.
  • Poor Developer Experience: The error stack trace is buried inside the sqflite internals, making it look like a library bug rather than an architectural usage issue.
  • Noise: It floods the logs with stack traces that obscure actual logic failures in the code being tested.

Example or Code (if necessary and relevant)

To reproduce or fix this, you usually need to adjust how the database is initialized or how the test waits for completion. Here is an example of how the code might look conceptually to force synchronization:

// Example of how to force synchronous behavior or proper awaiting
// to prevent the timer from lingering.

Future saveData(SqfliteDatabase db, Map data) async {
  // If you fire and forget (bad for tests):
  // db.transaction((txn) async { ... }); 

  // To fix, ensure the awaited Future resolves before the test exits:
  await db.transaction((txn) async {
    await txn.insert('table', data);
  });
}

How Senior Engineers Fix It

Senior engineers recognize that tests must be deterministic. You cannot rely on background timers finishing “eventually.”

  1. Explicit Awaiting: The simplest fix is to stop “fire and forgetting.” Refactor the code to await the database operations. This ensures the test doesn’t finish until the database transaction and its internal lock timers have successfully resolved.
  2. Flush Asynchronous Queues: Use await Future.delayed(Duration.zero) or a specific “flush” mechanism if the architecture prevents awaiting directly.
  3. Mock the Database Layer: The most robust solution is to mock sqflite in widget tests. Widget tests should verify UI behavior, not database engine locking mechanisms. By mocking the database, you replace the real 10-second timer with an instant return, making the test fast and stable.
  4. Disable Assertions (Last Resort): If you must use the real database and cannot await, you can configure the test binding to tolerate pending timers, though this hides the underlying inefficiency.

Why Juniors Miss It

  • Mental Model of “Fire and Forget”: Junior developers often treat asynchronous methods as synchronous side effects. They don’t realize that db.insert() returns a Future that must be tracked.
  • Confusion of Environment: They see the code work in the app and assume the test environment is the same. They don’t understand that FakeAsync is a virtual environment that “fast-forwards” time and checks for leaks.
  • Reading Stack Traces: The error appears deep in sqflite_common code. A junior engineer might file a bug report against the library, thinking it is broken, rather than looking at their own test setup or unawaited calls in WorkspaceState.save.
  • Assumption of Irrelevance: The user stated “it doesn’t really matter if it succeeds or not.” In production, this is true. In testing, it matters immensely because the pending state itself is an error.