sync_to_async does not find root thread inside a task

When you use create_task (or any way of making a coroutine not through asgiref), and the root thread is synchronous, then sync_to_async in thread-sensitive mode will use the root thread outside of tasks and a single, specific new thread inside of tasks, ruining the thread-sensitive guarantee.

Instead, we should make sync_to_async always first look for an executor on the root thread and, if it finds it, use that to run code.

[andersk]

Thanks. I came up with a cleaner solution that also makes your test case pass: associate a CurrentThreadExecutor with each event loop we create, and have sync_to_async look up that CurrentThreadExecutor from the current event loop if it can’t find one from the context.

Updated #320.

[yourcelf]

@andersk here is a repo with as minimal a test case as I can manage with grpc’s boilerplate: https://github.com/yourcelf/grpc-asgiref

Test file is here: https://github.com/yourcelf/grpc-asgiref/blob/main/test_grpc_sync_to_async.py, the rest of the repo is boilerplate to set up the protobuf definition and compiled files for the grpc server to use.

Sadly, https://github.com/django/asgiref/pull/320 does not seem to address the problem – the @sync_to_async decorated method executed by the grpc server still runs on ThreadPoolExecutor-0_0 instead of MainThread with that branch with patched create_task.