Handling and Propagating Exceptions from Trio Background Tasks in Unit Tests

[seetadev]

Hi Reza,

Great to see you're tackling this key aspect. You're absolutely right — handling exceptions from trio.nursery tasks in unit tests is crucial for ensuring reliable test outcomes, especially when it comes to things like SwarmException from SecurityUpgradeFailure and MuxerUpgradeFailure.

✅ Here's a quick Suggestion and an experiment we can try:

To ensure exceptions propagate correctly and cause the test to fail, there are a few steps we can take:

1. Catch Exceptions Inside the Task, but Propagate Them to the Parent:

• In trio, exceptions raised in child tasks should generally propagate to the parent (like the nursery) unless handled otherwise. If you're catching exceptions inside the task handler but not allowing them to propagate, that could be why they're not showing up in our tests.
• We can add proper exception handling inside the nursery tasks to ensure exceptions bubble up correctly. Here's an example:

async def handler():
    try:
        # Code that could raise exceptions
    except Exception as exc:
        raise SwarmException(f"Exception in handler: {str(exc)}") from exc

This ensures that the exception gets raised back up to the nursery, allowing the unit test to catch it.

2. Ensure That Exceptions Are Caught in Test Framework:

• If exceptions aren't propagating as expected, the test framework itself might need to be adjusted to catch these.
• In the test case, we could wrap the nursery or the specific background tasks inside trio.testing, using trio.testing.expect_timeout or trio.testing.raises to catch any unexpected errors during test execution.

Example:

with trio.testing.assertRaises(SwarmException):
    # Run the code that should trigger the exception
    await some_async_function()

3. Improving Test Coverage with PR #593:

• Since PR Fix exception handling in TCPListener.listen() #593 aims to catch these exceptions, testing this properly will involve ensuring that the task you are testing correctly handles the exception. If the exceptions are internal to the task and not passed back up, it’s important to mock or simulate the behavior of these tasks in your unit tests.
• Additionally, make sure your tests simulate the conditions where exceptions should arise (e.g., SecurityUpgradeFailure and MuxerUpgradeFailure) and ensure they are captured as part of the test assertions.

4. Ensure Your Tasks Are Running in the Right Context:

• Sometimes, exceptions don’t propagate if they are running in a context that’s disconnected from the main event loop. Make sure that your background tasks are executed in the correct nursery scope and are correctly linked to the event loop during tests.