If some parent process has disabled SIGCHLD, that signal is inherited and stops that signal being delivered, which means that asyncio.Process.wait will never complete.

As an example, the plan9port terminal window, 9term, (https://9fans.github.io/plan9port/man/man1/9term.html) does this, and hence causes async Python processes to hang forever.

Python should probably code defensively against this and ensure that SIGCHLD is enabled regardless. Attached is some C code that demonstrates the issue when the following python code is saved to "tst.py".

    #!/usr/bin/env python3
    import asyncio
    
    async def do_exec():
        cmd = ['echo', 'hello, world']
        process = await asyncio.create_subprocess_exec(*cmd, env={})
        await process.wait()
    
    loop=asyncio.get_event_loop()
    try:
        loop.run_until_complete(do_exec())
    finally:
        loop.close()



demonstration of the issue.

Thank for bug report.
Do you know other widespread tools with this problem? 
plan9port sounds too esoteric to me.

#35621 fixes the problem for default ThreadedChildWatcher