`asyncio.create_subprocess_exec()` accepts a `loop` parameter, but doesn't use it to watch the child process. Instead uses `get_event_loop_policy().get_child_watcher()`, which doesn't doesn't know about `loop` but tries to use the current default event loop.

This fails if there is no current event loop or if that loop isn't running:

```
import asyncio

async def action(loop):
    proc = await asyncio.create_subprocess_exec('echo', loop=loop)
    await proc.wait()

loop = asyncio.new_event_loop()
loop.run_until_complete(action(loop))
loop.close()
```

This crashes because the main event loop never was created:

Traceback (most recent call last):
  File "sample.py", line 8, in <module>
    loop.run_until_complete(action(loop))
  File "/home/sth/devel/cpython.vanilla/Lib/asyncio/base_events.py", line 589, in run_until_complete
    return future.result()
  File "sample.py", line 4, in action
    proc = await asyncio.create_subprocess_exec('echo', loop=loop)
  File "/home/sth/devel/cpython.vanilla/Lib/asyncio/subprocess.py", line 213, in create_subprocess_exec
    transport, protocol = await loop.subprocess_exec(
  File "/home/sth/devel/cpython.vanilla/Lib/asyncio/base_events.py", line 1542, in subprocess_exec
    transport = await self._make_subprocess_transport(
  File "/home/sth/devel/cpython.vanilla/Lib/asyncio/unix_events.py", line 193, in _make_subprocess_transport
    watcher.add_child_handler(transp.get_pid(),
  File "/home/sth/devel/cpython.vanilla/Lib/asyncio/unix_events.py", line 924, in add_child_handler
    raise RuntimeError(
RuntimeError: Cannot add child handler, the child watcher does not have a loop attached


If we do have a current event loop, for example by calling `asyncio.get_event_loop()` before creating out own loop, then we don't get an error, but the program hangs indefinitely since that loop isn't running.

Expected behavior would be that the loop given to create_subprocess_exec() is used to watch the child process.

Also impacted.

A fix I found is to add `watcher.attach_loop(self)` just after `with events.get_child_watcher() as watcher:` in `_make_subprocess_transport` in `asyncio/unix_events.py`.

New changeset 13ed07998ad93dbdd94991ba0451b9b559f07972 by Andrew Svetlov in branch 'master':
bpo-35621: Support running subprocesses in asyncio when loop is executed in non-main thread (#13630)
https://github.com/python/cpython/commit/13ed07998ad93dbdd94991ba0451b9b559f07972

New buildbot failure on koobs-freebsd10 that appears related to (includes) this changeset

Full log attached

Also seems to occur on Ubuntu : https://buildbot.python.org/all/#/builders/141/builds/1912/steps/5/logs/stdio

karthi@ubuntu-s-1vcpu-1gb-blr1-01:~/cpython$ ./python -m unittest test.test_asyncio.test_unix_events
...................Exception in thread Thread-1:
Traceback (most recent call last):
  File "/home/karthi/cpython/Lib/threading.py", line 923, in _bootstrap_inner
    self.run()
  File "/home/karthi/cpython/Lib/threading.py", line 865, in run
    self._target(*self._args, **self._kwargs)
  File "/home/karthi/cpython/Lib/test/test_asyncio/test_unix_events.py", line 1833, in f
    self.assertIsInstance(watcher, asyncio.SafeChildWatcher)
  File "/home/karthi/cpython/Lib/unittest/case.py", line 1330, in assertIsInstance
    self.fail(self._formatMessage(msg, standardMsg))
  File "/home/karthi/cpython/Lib/unittest/case.py", line 748, in fail
    raise self.failureException(msg)
AssertionError: <asyncio.unix_events.ThreadedChildWatcher object at 0x7f00b357d640> is not an instance of <class 'asyncio.unix_events.SafeChildWatcher'>
../home/karthi/cpython/Lib/asyncio/base_events.py:646: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=False>
  _warn(f"unclosed event loop {self!r}", ResourceWarning, source=self)
ResourceWarning: Enable tracemalloc to get the object allocation traceback
...............................................................................................
----------------------------------------------------------------------
Ran 116 tests in 1.391s

OK

It seems the assertion has to use ThreadedChildWatcher instead of SafeChildWatcher as the default seems to be changed at [0] . If changing assertion fixes the test I am curious why it didn't fail in the primary CI.


[0] https://github.com/python/cpython/commit/13ed07998ad93dbdd94991ba0451b9b559f07972#diff-8441007d39090aec5e09dbfcfa9973b7R1299

I cannot reproduce this issue locally :(

When I ran the tests, the watcher I get is a SafeChildWatcher

Thanks for the report!

The failure depends on tests order execution (maybe you use -jN flag?).
Before default child watcher was SafeChildWatcher, not it is ThreaderChildWather.
The watcher is a global variable (a property of global event loop policy).

The failed test doesn't make sense for default ThreadChildWatcher but still valuable for old default SafeChildWatcher.
I've changed the test to use SafeChildWatcher explicitly.

See https://github.com/python/cpython/pull/13754 for the fix

> The failure depends on tests order execution (maybe you use -jN flag?).

Oh, I understand now!

> See https://github.com/python/cpython/pull/13754 for the fix

Thanks for the fix, Andrew! :)

New changeset c6789d6c85a290a35f3839efb52a3d34536dcebe by Miss Islington (bot) (Andrew Svetlov) in branch 'master':
bpo-35621: Fix tests when SafeChildWatcher is expected instead of ThreadedChildWatcher (GH-13754)
https://github.com/python/cpython/commit/c6789d6c85a290a35f3839efb52a3d34536dcebe

New changeset 9535aff9421f0a5639f6e4c4bb0f07a743ea8dba by Miss Islington (bot) (Andrew Svetlov) in branch 'master':
Revert "bpo-35621: Support running subprocesses in asyncio when loop is executed in non-main thread (#13630)" (GH-13793)
https://github.com/python/cpython/commit/9535aff9421f0a5639f6e4c4bb0f07a743ea8dba

New changeset 0d671c04c39b52e44597491b893eb0b6c86b3d45 by Andrew Svetlov in branch 'master':
bpo-35621: Support running subprocesses in asyncio when loop is executed in non-main thread  (GH-14344)
https://github.com/python/cpython/commit/0d671c04c39b52e44597491b893eb0b6c86b3d45

New changeset bf8cb31803558f1105efb15b0ee4bd184f3218c8 by Miss Islington (bot) in branch '3.8':
bpo-35621: Support running subprocesses in asyncio when loop is executed in non-main thread  (GH-14344)
https://github.com/python/cpython/commit/bf8cb31803558f1105efb15b0ee4bd184f3218c8

Is it possible to amend the documentation (https://docs.python.org/3.5/library/asyncio-subprocess.html, etc) to include a note that the loop parameter doesn't work? I'm impacted by this bug and lost a few hours to it.

is there a workaround for earlier Python versions that does not involve patching the standard library?

> is there a workaround for earlier Python versions that does not involve patching the standard library?

You could potentially try using the new default watcher, `ThreadedChildWatcher`, by implementing it locally and setting it as the child watcher to use (instead of `SafeChildWatcher`) with `set_child_watcher()`. AFAICT, the current implementation should work well for earlier versions of Python that don't have it, we just can't include it earlier than 3.8 since it's a new feature.

See https://github.com/python/cpython/blob/4649202ea75d48e1496e99911709824ca2d3170e/Lib/asyncio/unix_events.py#L1326 for reference.