[issue29214] Standard open() does not allow to specify file permissions.

[STINNER Victor]

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

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue29214>
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