In case close_fds=True is passed to subprocess.Popen()
or its users (subprocess.call() etc), it might spend
some considerable time closing non-opened file descriptors,
as demonstrated by the following snippet from strace:

	close(3)                    = -1 EBADF (Bad file descriptor)
	close(5)                    = -1 EBADF (Bad file descriptor)
	close(6)                    = -1 EBADF (Bad file descriptor)
	close(7)                    = -1 EBADF (Bad file descriptor)
	...
	close(1021)                 = -1 EBADF (Bad file descriptor)
	close(1022)                 = -1 EBADF (Bad file descriptor)
	close(1023)                 = -1 EBADF (Bad file descriptor)

This happens because the code in _close_fds() iterates from 3 up to
MAX_FDS = os.sysconf("SC_OPEN_MAX").

Now, syscalls are cheap, but SC_OPEN_MAX (also known as RLIMIT_NOFILE
or ulimit -n) can be quite high, for example:

	$ docker run --rm python python3 -c \
		$'import os\nprint(os.sysconf("SC_OPEN_MAX"))'
	1048576

This means a million syscalls before spawning a child process, which
can result in major delay, like 0.1s as measured on my fast and mostly
idling laptop. Here is the comparison with python3 (which apparently
as well as according to strace do not have this problem):

	$ docker run --rm python python3 -c $'import subprocess\nimport time\ns = time.time()\nsubprocess.check_call([\'/bin/true\'], close_fds=True)\nprint(time.time() - s)\n'
	0.0009245872497558594

	$ docker run --rm python python2 -c $'import subprocess\nimport time\ns = time.time()\nsubprocess.check_call([\'/bin/true\'], close_fds=True)\nprint(time.time() - s)\n'
	0.0964419841766

The solution (for Linux, at least) is easy: instead of iterating through
all the _possibly_ opened files, get the list of _actually_ opened
files (from /proc/self/fd/) and iterate through these. In case it
doesn't work (for example when /proc not available or some draconian
selinux/apparmor restrictions), fall back to older method.

[v2: do not use bare exceptions]

Signed-off-by: Kir Kolyshkin <[email protected]>