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> Is sys.platform equal to 'linux' on WSL? Sorry, I don't know WSL. If it's equal, is it possible to explicitly exclude WSL in the subprocess test, _use_posix_spawn()?
I don't have immediate access to WSL right now, but I'll try to get one and investigate what we have there wrt. posix_spawn() and vfork().
> So the bug only affects "QEMU User Space". I never used that before. 
Yes, I was specifically referring to QEMU user-space. One use case that heavily relies on it is Tizen. Its packages are built in a chroot jail containing the build environment with binaries native to the target architecture, making an illusion that packages are built on the target system and are not cross-compiled. So the binaries are run under QEMU user-space emulation. But in reality, because of unacceptable performance of binary translation, many frequently-used binaries, like coreutils and compilers, are replaced with host-native binaries in a way transparent to the build system (so it has no idea whether it runs host-native or target-native binaries).
> Does the difference matters? The bug only occurs in some very specific cases:
> * WSL
> * QEMU User Emulation
> Are these use cases common enough to block the whole idea of using posix_spawn() on Linux. I don't think so. I really want to use posix_spawn() for best performances! Moreover, I expect that glibc implementation of posix_spawn() is safer than Python _posixsubprocess. The glibc has access to low-level stuff like it's internal signals, cancellation points, etc. _posixsubprocess is more generic and doesn't worry about such low-level stuff, whereas they might cause bad surprised.
It's true that a C library is in a better position to implement something like posix_spawn(), but I still think that vfork()/exec() is worth to consider at least on Linux. See bpo-35823, which should also work under QEMU user-mode and WSL (but needs testing).