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I created another patch that attempts to create the pipes atomically.
On GNU/Linux, if pipe2 is available, it uses it to create the pipes, and there is no race. On other POSIX platforms, pipe and fcntl are called without releasing the GIL - relying on the fact that _posixsubprocess.fork_exec doesn't release the GIL either, so the two can't run at the same time (bonus: os.fork doesn't release the GIL either). I can't reproduce neither issue 7213 nor issue 2320 with either implementation, so the patch seems to fix them.
Issues:
1. If the _posixsubprocess module isn't compiled, the race still exists (well, without it subprocess isn't safe to use with threads anyway).
2. On GNU/Linux systems where glibc was compiled with kernel headers >=2.6.27, but the running kernel is <2.6.27, the subprocess module won't work. (There should be a fix for that?)
3. I have no way to tell that the non-Linux implementation works for sure. I've been running it in an endless loop, and so far there have been no hangs (*), but that doesn't mean that it doesn't have a rare race that's beyond my comprehension. With pipe2() you can be certain, but I have my doubts about the other implementation.
All unit tests seem to pass.
(*) Actually, I *thought* it hang on my first attempt, but I interrupted the process too soon to tell for sure. No hangs after that. :(