This panic could happen in the following cases:
- A non-fatal error being thrown from a worker, that doesn't terminate
the worker's execution, but propagates to the main thread without
being handled, and makes the main thread terminate.
- A nested worker being alive while its parent worker gets terminated.
- A race condition if the main event loop terminates the worker as part
of its last task, but the worker doesn't fully terminate before the
main event loop stops running.
This panic happens because a worker's event loop should have pending ops
as long as the worker isn't closed or terminated – but if an event loop
finishes running while it has living workers, its associated
`WorkerThread` structs will be dropped, closing the channels that keep
those ops pending.
This change adds a `Drop` implementation to `WorkerThread`, which
terminates the worker without waiting for a response. This fixes the
panic, and makes it so nested workers are automatically terminated once
any of their ancestors is closed or terminated.
This change also refactors a worker's termination code into a
`WorkerThread::terminate()` method.
Closes #11342.
Co-authored-by: Bartek Iwańczuk <biwanczuk@gmail.com>
