STINNER Victor added the comment:
> @haypo, suppose, one thread wants file with permissions A, and other thread
> wants permissions B. If they will "set" them through umask(), race condition
> may occur.
Ok, let's say thta you want file "x" with permission A=0o777 and file "y" with
permission B=0b700.
You start with os.umask(0o077), as suggested by Christian.
>>> import os
>>> os.umask(0o077)
2
>>> x=open("x", "w")
>>> y=open("y", "w")
>>> os.system("ls -l x y")
-rw-------. 1 haypo haypo 0 11 janv. 08:48 x
-rw-------. 1 haypo haypo 0 11 janv. 08:48 y
0
>>> os.fchmod(x.fileno(), 0o777)
>>> os.system("ls -l x y")
-rwxrwxrwx. 1 haypo haypo 0 11 janv. 08:48 x
-rw-------. 1 haypo haypo 0 11 janv. 08:48 y
0
This is a time window where the file "x" has the permission -rw------- instead
of the expected -rwxrwxrwx. Ok, it's expected. But it's not an easy since
initial permissions are are more strict that expected permisions.
The race condition only occurs if you work the other way, start with
os.umask(0777) (default umask) and then change permissions of the file "x" to
stricter. In this specific case, you *already* have two ways to do that in
Python 3 (no change needed):
fp = open("x", opener=partial(os.open, mode=0o700))
or:
fd = os.open("x", os.O_WRONLY | os.O_CREAT, 0o700)
fp = open(fd, "wb")
--
About documenting the default mode: the problem is that I'm not sure that *all*
functions in Python, especially when using third party libraries, use the same
default mode. Morever, the mode on Windows has a very different meaning.
Permission per group is handled by ACLs, not with the integer "mode":
https://msdn.microsoft.com/en-us/library/z0kc8e3z.aspx
On Windows, pmode of _open() only contains read and write permissions of the
file, for everyone.
For all these reasons, I close the issue.
----------
resolution: -> fixed
status: open -> closed
_______________________________________
Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue29214>
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