os.fork() potentially has some problematic behavior when called from a subinterpreter.  In additional to the normal fork+threads madness, there's the question of what to do with existing subinterpreters.  The simplest solution is to simply disallow fork in a subinterpreter and then wipe out all subinterpreters in the child process post-fork (if os.fork() called in the main interpreter).

+1 agreed.  this is the simplest approach to start with.

Code to restrict: os.fork itself and disallowing the use of preexec_fn on subprocess within subinterpreters.

feel free to ignore preexec_fn in subprocess for the time being if desired, we already promise people that it is a bad idea legacy API that we don't want, guaranteed to cause undiagnosable problems and deadlocks at times. :)

New changeset 5903296045b586b9cd1fce0b1e02caf896028d1d by Eric Snow in branch 'master':
bpo-34651: Only allow the main interpreter to fork. (gh-9279)
https://github.com/python/cpython/commit/5903296045b586b9cd1fce0b1e02caf896028d1d

It appears that as a result of this, subprocess.Popen cannot be called from within a subinterpreter either. Is that an obvious and desired limitation?

I reopen the issue to let Eric answer ;-)

If the behavior is deliberate, maybe it should just be documented somewhere?

I'd start by documenting the limitation and keeping a future feature request of "Allow subprocess to work from subinterpreters".  That is doable.  Supporting os.fork() is not.

> I'd start by documenting the limitation and keeping a future feature request of "Allow subprocess to work from subinterpreters".  That is doable.  Supporting os.fork() is not.

subprocess is able to use os.posix_spawn() in Python 3.8. In that case, it's not limited by os.fork() check ;-) Windows isn't limited neither, CreateProcess() isn't limited in subinterpreters.

FYI, I plan on looking into this either today or next Friday.

See also bpo-37266: "Daemon threads must be forbidden in subinterpreters".

The problem with subprocess.Popen has been fixed in https://bugs.python.org/issue37951

New changeset b22030073b9327a3aeccb69507694bce078192aa by Miss Islington (bot) (Phil Connell) in branch 'master':
bpo-38778: Document that os.fork is not allowed in subinterpreters (GH-17123)
https://github.com/python/cpython/commit/b22030073b9327a3aeccb69507694bce078192aa

New changeset a4be5aae6e587f5310f1fc0d66d37e032621ce39 by Miss Islington (bot) in branch '3.8':
bpo-38778: Document that os.fork is not allowed in subinterpreters (GH-17123)
https://github.com/python/cpython/commit/a4be5aae6e587f5310f1fc0d66d37e032621ce39