inconsistent file descriptor blocking state when running in asyncio vs uvloop #712
Description
When running under uvloop, registering stdin, stdout, or stderr with loop.add_reader() / loop.add_writer() implicitly changes their blocking mode from blocking to non-blocking. This behaviour does not occur under the standard asyncio selector event loop.
This is causing some issues when running with rich/promptkit in fast-agent
Under the following test code
import anyio
import sys
import os
import asyncio
async def main():
loop = asyncio.get_running_loop()
print(f"Using event loop: {type(loop)}")
stdin = sys.stdin.fileno()
stdout = sys.stdout.fileno()
stderr = sys.stderr.fileno()
print(f"initial:\tstdin fd: {stdin}, stdout fd: {stdout}, stderr fd: {stderr}")
print(f"blocking:\tstdin fd: {os.get_blocking(stdin)}, stdout fd: {os.get_blocking(stdout)}, stderr fd: {os.get_blocking(stderr)}")
loop.add_writer(stdin, lambda: None)
loop.add_writer(stdout, lambda: None)
loop.add_writer(stderr, lambda: None)
print(f"writer: \tstdin fd: {stdin}, stdout fd: {stdout}, stderr fd: {stderr}")
print(f"blocking:\tstdin fd: {os.get_blocking(stdin)}, stdout fd: {os.get_blocking(stdout)}, stderr fd: {os.get_blocking(stderr)}")
loop.add_reader(stdin, lambda: None)
loop.add_reader(stdout, lambda: None)
loop.add_reader(stderr, lambda: None)
print(f"reader: \tstdin fd: {stdin}, stdout fd: {stdout}, stderr fd: {stderr}")
print(f"blocking:\tstdin fd: {os.get_blocking(stdin)}, stdout fd: {os.get_blocking(stdout)}, stderr fd: {os.get_blocking(stderr)}")
anyio.run(main, backend="asyncio", backend_options={"use_uvloop": False})
print("-" * 40)
anyio.run(main, backend="asyncio", backend_options={"use_uvloop": True})I see the following output, where the blocking status of the stdio file descriptors are changed from blocking to non blocking when running under uvloop but not asyncio.
Using event loop: <class 'asyncio.unix_events._UnixSelectorEventLoop'>
initial: stdin fd: 0, stdout fd: 1, stderr fd: 2
blocking: stdin fd: True, stdout fd: True, stderr fd: True
writer: stdin fd: 0, stdout fd: 1, stderr fd: 2
blocking: stdin fd: True, stdout fd: True, stderr fd: True
reader: stdin fd: 0, stdout fd: 1, stderr fd: 2
blocking: stdin fd: True, stdout fd: True, stderr fd: True
----------------------------------------
Using event loop: <class 'uvloop.Loop'>
initial: stdin fd: 0, stdout fd: 1, stderr fd: 2
blocking: stdin fd: True, stdout fd: True, stderr fd: True
writer: stdin fd: 0, stdout fd: 1, stderr fd: 2
blocking: stdin fd: False, stdout fd: False, stderr fd: False
reader: stdin fd: 0, stdout fd: 1, stderr fd: 2
blocking: stdin fd: False, stdout fd: False, stderr fd: False