`umask`ed `os.makedirs()` sometimes fails on Emscripten

[freakboy3742]

Bug report

Bug description:

When os.umask is used on a os.makedirs() call, the permissions aren't always honoured:

os.umask(0o002)
os.makedirs("./dir1/dir2")
print(os.stat("./dir1").st_mode & 0o777)

Should return 0o775; but returns 0o755 on some installs.

This can be observed as a failure of test_os.MakedirTests.test_mode. The Emscripten buildbot fails consistently on this; it also fails on my macOS machine. However, @hoodmane has been unable to reproduce it. The test has been skipped as part of #127146 to get a passing buildbot; more investigation is required.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Other

[freakboy3742]

Pairing with @hoodmane at Europython - we've found the cause. Emscripten's umask function appears to be a no-op; on @hoodmane machine, the default umask is the same as the one used in the test, so the test passes. On the test machine and my machine, the default umask is 0o022, so the no-op umask is a problem.

This strictly means the test is a little fragile, as it will always pass if the host machine where the test is executing has a umask of 0o002.