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bug#24659: 'mkdir' procedure is neither thread-safe nor safe (2.0.12)

From: Ludovic Courtès
Subject: bug#24659: 'mkdir' procedure is neither thread-safe nor safe (2.0.12)
Date: Mon, 10 Oct 2016 18:39:47 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux)


When the optional ‘mode’ argument to the ‘mkdir’ procedure is omitted,
umask(2) is used to find out what the current mask is and to compute the
mode argument to mkdir(2):

--8<---------------cut here---------------start------------->8---
  if (SCM_UNBNDP (mode))
      mask = umask (0);
      umask (mask);
      STRING_SYSCALL (path, c_path, rv = mkdir (c_path, 0777 ^ mask));
--8<---------------cut here---------------end--------------->8---

The problem is that this changes the process’ umask globally for a short
duration, during which other threads will get 0 as the mask…

Furthermore, AFAICS, the above logic is redundant with what the kernel
does anyway.  That is, in a single-threaded program,

  mask = umask (0);
  umask (mask);
  mkdir (file, 0777 ^ mask);

is equivalent to:

  mkdir (file, 0777);

Am I right that we should just remove these two ‘umask’ calls?


PS: This code has been there since 1996 or earlier.

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