[Python-Dev] Inherance of file descriptor and handles on Windows (PEP 446) (original) (raw)

Guido van Rossum guido at python.org
Thu Jul 25 00:25:50 CEST 2013

On Wed, Jul 24, 2013 at 2:57 PM, Antoine Pitrou <solipsis at pitrou.net> wrote:

On Wed, 24 Jul 2013 10:56:05 -0700 Guido van Rossum <guido at python.org> wrote:

But I'm also ready to propose that all this is such a mess that we should change the default fd/handle inheritance to False, *across platforms*, and damn the torpedoes -- i.e. accept breaking all existing 3rd party UNIX code for subprocess creation that bypasses the subprocess module, as well as breaking uses of os.spawn*() on both platforms that depend on FD inheritance beyond stdin/stdout/stderr). So I suppose you mean "change it to False except for stdin/stdout/stderr"?

Hm, that's a tricky detail. I expect that on UNIX those are pre-opened and inherited from the shell, and we should not change their cloexec status; IIRC on Windows inheritance of the stdin/out/err handles is guided by a separate flag to CreateProcess().

With the new, sane default, all we need instead of PEP 446 is a way to make an FD inheritable after it's been created, which can be a single os.makeinheritable(fd) call that you must apply to the fileno() of the stream or socket object you want inherited (or directly to a FD you created otherwise, e.g. with os.pipe()). On Windows, this should probably only work with os.spawn*(), since otherwise you need handle inheritance, not FD inheritance, and that's a non-portable concept anyway. I'm not sure how fd inheritance could work without handle inheritance. (since a fd seems to just be a proxy for a handle)

The MS spawn() implementation takes care of this -- I presume they make the handles inheritable (I think some of the bugs probably refer to this fact) and then use the "internal" interface to pass the mapping from FDs to handles to the subprocess.

To reduce the need for 3rd party subprocess creation code, we should have better daemon creation code in the stdlib -- I wrote some damn robust code for this purpose in my previous job, but it never saw the light of day. What do you call "daemon"? An actual Unix-like daemon?

Yeah, a background process with parent PID 1 and not associated with any terminal group.

-- --Guido van Rossum (python.org/~guido)

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