StreamingResponse with async generator get stuck indefinitely · encode/starlette...

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• Initially raised as discussion in FastAPI StreamingResponse with async generator sometimes causes socket.send() exception indefinitely tiangolo/fastapi#5183

import asyncio

from starlette.applications import Starlette
from starlette.responses import StreamingResponse
from starlette.routing import Route


def sync_streamer():
    try:
        while True:
            yield b"--boundary\r\nContent-Type: text/plain\r\nContent-Length: 1\r\n\r\n1\r\n"
    except asyncio.CancelledError:
        print("caught cancelled error")
        raise GeneratorExit


async def async_streamer():
    try:
        while True:
            yield b"--boundary\r\nContent-Type: text/plain\r\nContent-Length: 1\r\n\r\n1\r\n"
    except asyncio.CancelledError:
        print("caught cancelled error")
        raise GeneratorExit


async def sync_endpoint(request):
    try:
        return StreamingResponse(
            sync_streamer(),
            media_type="multipart/x-mixed-replace; boundary=boundary",
        )
    except asyncio.CancelledError:
        print("caught cancelled error")
        raise GeneratorExit


async def async_endpoint(request):
    try:
        return StreamingResponse(
            async_streamer(),
            media_type="multipart/x-mixed-replace; boundary=boundary",
        )
    except KeyboardInterrupt:
        exit()


routes = [
    Route('/sync', sync_endpoint),
    Route('/async', async_endpoint),
]

app = Starlette(routes=routes)

CASE 1: When /async (Asynchronous Generator) is hit

>uvicorn stream:app
INFO:     Started server process [23288]
INFO:     Waiting for application startup.                               
INFO:     Application startup complete.                                  
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     127.0.0.1:65107 - "GET /async HTTP/1.1" 200 OK

# stuck indefinitely after client cancels the request.

CASE 2: When /sync (Synchronous Generator) is hit

>uvicorn stream:app
INFO:     Started server process [24968]
INFO:     Waiting for application startup.                               
INFO:     Application startup complete.                                  
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     127.0.0.1:55582 - "GET /sync HTTP/1.1" 200 OK
Exception in callback _ProactorBasePipeTransport._call_connection_lost(None)
handle: <Handle _ProactorBasePipeTransport._call_connection_lost(None)>     
Traceback (most recent call last):                                          
  File "...\lib\asyncio\events.py", line 80, in _run
    self._context.run(self._callback, *self._args)
  File "...\lib\asyncio\proactor_events.py", line 162, in _call_connection_lost
    self._sock.shutdown(socket.SHUT_RDWR)
ConnectionResetError: [WinError 10054] An existing connection was forcibly closed by the remote host

When CASE 1, even `Keyboard Interrupts are ignored. I had to kill Python process forcefully.