Mark http_exception as async to prevent thread creation. #2922

danlapid · 2025-04-13T22:22:01Z

Summary

In https://github.com/encode/starlette/blob/master/starlette/_exception_handler.py#L61 we check if the exception_handler is an async function and if not we execute it in a different thread.
This is meant to ensure that long-running sync IO operations don't block the main async eventloop.
However, in the case of http_exception, a simple function that does not do any io at all - the aforementioned logic spawns a thread to run the function.
Marking this function async is a simple way of making sure this function does not accidently cause a thread to be created.

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I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
I've updated the documentation accordingly.

…eation. In https://github.com/encode/s 8000 tarlette/blob/master/starlette/_exception_handler.py#L61 we check if the exception_handler is an async function and if not we execute it in a different thread. This is meant to ensure that long-running sync IO operations don't block the main async eventloop. However, in the case of http_exception, a simple function that does not do any io at all the aforementioned logic spawns a thread to run the function. Marking this function async is a simple way of making sure this function does not accidently cause a thread to be created.

Kludex

Thanks :)

I'm not sure how no one saw this...

danlapid · 2025-04-14T08:35:27Z

@Kludex no worries 😄
Do you mind hitting the merge button as well? I don't have permissions 🙏

danlapid · 2025-04-16T08:53:08Z

@Kludex bumping this, would appreciate some help hitting the merge button 😄

danlapid force-pushed the danlapid/mark_http_exception_handler_as_async branch 5 times, most recently from bdd5b6b to 117ec49 Compare April 13, 2025 22:31

danlapid force-pushed the danlapid/mark_http_exception_handler_as_async branch from 117ec49 to d17b116 Compare April 13, 2025 22:33

Kludex approved these changes Apr 14, 2025

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encode locked as spam and limited conversation to collaborators Apr 16, 2025

adriangb merged commit a766a58 into encode:master Apr 16, 2025
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Mark http_exception as async to prevent thread creation. #2922

Mark http_exception as async to prevent thread creation. #2922

Mark http_exception as async to prevent thread creation. #2922

Mark http_exception as async to prevent thread creation. #2922

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