An exception can be raised in SubprocessTransport.__init__() from SubprocessTransport._start() - for instance because an exception is raised in the preexec_fn callback.

In this case, the calling function never gets a reference to the transport object, and cannot close the transport. Hence, when the object is deleted, an "unclosed transport" ResourceWarning is always triggered.

Here is a test case showing this behavior:
import asyncio

loop = asyncio.get_event_loop()
try:
    loop.run_until_complete(asyncio.create_subprocess_exec('/doesntexist'))
except FileNotFoundError:
    pass
finally:
    loop.close()


I propose the attached patch as a solution, which call SubprocessTransport.close() when an exception is raised in SubprocessTransport._start() in the constructor.

popen_error_close.patch: I put more code in the try block to ensure that the transport is close on any kind of error, and I added an unit test.

New changeset f6b8a0c6b8c9 by Victor Stinner in branch '3.4':
Fix ResourceWarning in asyncio.BaseSubprocessTransport
https://hg.python.org/cpython/rev/f6b8a0c6b8c9

I applied your patch + my patch to Python 3.4, 3.5 and 3.6 and asyncio at Github. Thanks for the report and the patch Martin!

New changeset 6703ac68bf49 by Victor Stinner in branch '3.4':
Issue #24763: Fix asyncio test on Windows
https://hg.python.org/cpython/rev/6703ac68bf49

New changeset 2648dddd3248 by Yury Selivanov in branch '3.4':
Issue #24763: Fix asyncio test on Windows (fix reverted change)
https://hg.python.org/cpython/rev/2648dddd3248

New changeset aa527ee0d27f by Yury Selivanov in branch '3.5':
Merge 3.4 (issues #24835, #24763)
https://hg.python.org/cpython/rev/aa527ee0d27f

New changeset f304ba9425a3 by Yury Selivanov in branch 'default':
Merge 3.5 (issues #24835, #24763)
https://hg.python.org/cpython/rev/f304ba9425a3