_Py_set_inheritable() raises a Python-level exception on error and thus is  not async-signal-safe, but child_exec() must use only async-signal-safe functions because it's executed between fork() and exec().

Since a non-raising version is already implemented in Python/fileutils.c for internal use (set_inheritable), I suggest to simply expose it via another public function (similar to _Py_open_noraise(), etc.).

out of curiosity, did you actually diagnose a process deadlocked due to this or was it noted via code inspection?

this issue has been around since 3.4 and the file descriptor inheritance changes from the looks of things.

New changeset c1e46e94de38a92f98736af9a42d89c3975a9919 by Gregory P. Smith (Alexey Izbyshev) in branch 'master':
bpo-32777: Fix _Py_set_inheritable async-safety in subprocess (GH-5560)
https://github.com/python/cpython/commit/c1e46e94de38a92f98736af9a42d89c3975a9919

New changeset 2bb0bfafb0beb819b7c60d9004382ce6867847c0 by Gregory P. Smith (Miss Islington (bot)) in branch '3.7':
bpo-32777: Fix _Py_set_inheritable async-safety in subprocess (GH-5560) (GH-5562)
https://github.com/python/cpython/commit/2bb0bfafb0beb819b7c60d9004382ce6867847c0

New changeset b90c68586e1f2c45c736dd38880f182be267e2ef by Gregory P. Smith (Miss Islington (bot)) in branch '3.6':
bpo-32777: Fix _Py_set_inheritable async-safety in subprocess (GH-5560) (GH-5563)
https://github.com/python/cpython/commit/b90c68586e1f2c45c736dd38880f182be267e2ef

> out of curiosity, did you actually diagnose a process deadlocked due to this or was it noted via code inspection?

The latter. I noted it while working on another issue. While it was easy to trigger a malloc() in child_exec() by e.g. specifying an invalid fd in pass_fds for Popen, I haven't tried to arrange a deadlock.