Reading umask (thread-safe)

umask is inherited by child processes. You could create a pipe, fork a child process, get the umask there and write the result to the pipe so the parent can read it.

Quite expensive, but without any special requirements like /proc virtual filesystem. An example with just low-level OS calls (all async-safe) and no error checking below:

import os
import struct

def get_umask():
    pipe = os.pipe()
    pid = os.fork()
    if pid == 0:
        os.close(pipe[0])
        umask = os.umask(0)
        os.write(pipe[1], struct.pack('H', umask))
        os.close(pipe[1])
        os._exit(0)
    else:
        os.close(pipe[1])
        value = os.read(pipe[0], 2)
        os.close(pipe[0])
        os.waitpid(pid, 0)
        return struct.unpack('H', value)[0]

print("umask {:03o}".format(get_umask()))

The only truly, unambiguously thread-safe way I know is to call a new process.

import subprocess
umask_cmd = ('python', '-c', 'import os; print(os.umask(0777))')
umask = int(subprocess.check_output(umask_cmd))

Note that if you have bash or another shell, you could also call that. Since it might be on a weird system, I've chosen to use a python subprocess in umask_cmd, since you must have python. If you're on a non-weird *nix system, then you can use sh or bash instead.

if your system has Umask field in /proc/[pid]/status, you could read from on it:

import os

def getumask():
    pid = os.getpid()
    with open(f'/proc/{pid}/status') as f:
        for l in f:
            if l.startswith('Umask'):
                return int(l.split()[1], base=8)
        return None

tested under CentOS 7.5, Debian 9.6.

or, you could add a thread lock :)