In Modules/posixmodule.c, the function os__isdir_impl calls GetFileAttributesA/GetFileAttributesW without releasing the GIL. This is problematic since it can result in disk or network I/O, depending on the filesystem.

LGTM.

This is the kind of patch I can't wait to have a one-click merge button... right now it'll have to wait until I have a clean repo. But if anyone else wants to get it, go ahead.

Should this be backported to 2.7 posix__isdir()?

No, it's not a security fix, so it doesn't go in 2.7.

OK. I thought fixing bugs in 2.7 was at the discretion of core developers, including small tweaks for performance -- just not enhancements with significant amounts of new code and features.

Maybe so. If you find a core developer who cares about performance in 2.7, maybe they'll merge it :)

Correct, 2.7 is not in security fix only mode (yet), but new features (anything that changes an API) or bug fixes that we wouldn't put in a maintenance release (things that would break existing programs) are not allowed.  A special exception has been made for 2.7 for performance enhancements; we wouldn't normally do those in a maintenance release unless they were significant.

But as Steve says, it depends on a core developer being interested in applying the fix/backport.  Which we may have, we'll wait and see :)

New changeset ebf9a4c933e9 by Steve Dower in branch '3.5':
Issue #27533: Release GIL in nt._isdir
https://hg.python.org/cpython/rev/ebf9a4c933e9

New changeset 8823c660820e by Steve Dower in branch 'default':
Issue #27533: Release GIL in nt._isdir
https://hg.python.org/cpython/rev/8823c660820e