Python asyncio: Running subprocess_exec on a worker thread

Question

So I'm using the Python asyncio module (on Linux) to launch a child process and then asynchronously monitor it. My code works fine... when run on the main thread. But when I run it on a worker thread, it hangs, and the process_exited callback is never invoked.

I suspect this may actually be some kind of undocumented defect or issue with running subprocess_exec on a worker thread, likely having to do with how the implementation handles signals in a background thread. But it could also just be me screwing things up.

A simple, reproducible example is as follows:

class MyProtocol(asyncio.SubprocessProtocol):
    def __init__(self, done_future):
        super().__init__()
        self._done_future = done_future

    def pipe_data_received(self, fd, data):
        print("Received:", len(data))

    def process_exited(self):
        print("PROCESS EXITED!")
        self._done_future.set_result(None)

def run(loop):
    done_future = asyncio.Future(loop = loop)
    transport = None
    try:
        transport, protocol = yield from loop.subprocess_exec(
            lambda : MyProtocol(done_future),
            "ls",
            "-lh",
            stdin = None
        )
        yield from done_future
    finally:
        if transport: transport.close()

    return done_future.result()

def run_loop():
    loop = asyncio.new_event_loop()
    asyncio.set_event_loop(loop) # bind event loop to current thread

    try:
        return loop.run_until_complete(run(loop))
    finally:
        loop.close()

So here, I setup an asyncio event loop to execute the shell command ls -lh, and then trigger a callback for when data is received from the subprocess, and another callback for when the subprocess exits.

If I just call run_loop() directly in the main thread of a Python program, everything goes fine. But if I say:

t = threading.Thread(target = run_loop)
t.start()
t.join()

Then what happens is that the pipe_data_received() callback is invoked successfully, but process_exited() is never invoked, and the program just hangs.

After Googling around and looking at the asyncio source code for the implementation of unix_events.py, I discovered it might be necessary to manually attach my event loop to the global "child watcher" object, as follows:

loop = asyncio.new_event_loop()
asyncio.set_event_loop(loop) # bind event loop to current thread
asyncio.get_child_watcher().attach_loop(loop)

Apparently, the child watcher is an (undocumented) object that is responsible for calling waitpid under the hood (or something like that). But when I tried this, and ran run_event_loop() in a background thread, I got the error:

  File "/usr/lib/python3.4/asyncio/unix_events.py", line 77, in add_signal_handler
    raise RuntimeError(str(exc))
RuntimeError: set_wakeup_fd only works in main thread

So here it looks like the implementation actually does a check to make sure that signal handlers can only be used on the main thread, leading me to believe that in the current implementation, using subprocess_exec on a background thread is in fact, simply impossible without changing the Python source code itself.

Am I correct about this? Sadly, the asyncio module is very under-documented, so it's hard for me to be confident about my conclusion here. I may simply be doing something wrong.

Answer 1

Handling subprocesses in a worker thread is fine as long as an asyncio loop is running in the main thread with its child watcher instanciated:

asyncio.get_child_watcher()
loop = asyncio.get_event_loop()
coro = loop.run_in_executor(None, run_loop)
loop.run_until_complete(coro)

See this post and the documentation.