Disable threading in WASM environments

[ryanking13]

Hi, I am the maintainer of Pyodide, which runs Python in the browser based on WebAssembly. First of all, thank you for maintaining starlette and uvicorn.

While I was experimenting with starlette through fastAPI, I noticed that when I tried to upload a large file, it delegates the process to thread (run_in_threadpool), which makes it fail in WASM environment becuase of missing threading support.

starlette/starlette/datastructures.py

Lines 453 to 461 in fd53a8f

	async def write(self, data: bytes) -> None:
	new_data_len = len(data)
	if self.size is not None:
	self.size += new_data_len
	if self._will_roll(new_data_len):
	await run_in_threadpool(self.file.write, data)
	else:
	self.file.write(data)

So I was wondering if it would make sense to

1. disable threading in WASM environment entirely
2. or if there is a way to tell starlette (or fastAPI) to not use threading.

[Kludex]

Hey @ryanking13 , sorry the delay here.

That's not the only place that we have threads involved. I'm not sure how easy it would be to fully disable threading, but I'm open to review a PR if you want to give it a try. Maybe we do it via env var to see how it would look like first? 🤔

[ryanking13]

Sure I can give it a try to see if I can disable threading globally in some way

[Kludex]

It didn't look that easy. There were a lot of changes to actually make it work.