Gracefully handle race condition of 'makedirs'

[jquast]

Problem

In the imperative sequence:

   if not os.path.isdir(folder_name):
       os.makedirs(folder_name)

There exists the possibility that another process may create folder_name between the system path test and the system folder construction. This is especially true when launching both the salt-minion and salt-master simultaneously:

# systemctl status -l salt-minion salt-master
* salt-minion.service - The Salt Minion
   Loaded: loaded (/usr/lib/systemd/system/salt-minion.service; enabled)
   Active: inactive (dead) since Sat 2015-03-07 21:38:53 UTC; 7h ago
  Process: 1168 ExecStart=/usr/bin/salt-minion (code=exited, status=0/SUCCESS)
 Main PID: 1168 (code=exited, status=0/SUCCESS)

Mar 07 21:36:29 local.dev systemd[1]: Started The Salt Minion.
Mar 07 21:38:53 local.dev salt-minion[1168]: [ERROR   ] Attempt to authenticate with the salt master failed

* salt-master.service - The Salt Master Server
   Loaded: loaded (/usr/lib/systemd/system/salt-master.service; enabled)
   Active: failed (Result: exit-code) since Sat 2015-03-07 21:37:53 UTC; 7h ago
  Process: 1166 ExecStart=/usr/bin/salt-master (code=exited, status=17)
 Main PID: 1166 (code=exited, status=17)

Mar 07 21:37:52 local.dev salt-master[1166]: Failed to create path "/var/log/salt/master" - [Errno 17] File exists: '/var/log/salt'
Mar 07 21:37:53 local.dev systemd[1]: salt-master.service: main process exited, code=exited, status=17/n/a
Mar 07 21:37:53 local.dev systemd[1]: Failed to start The Salt Master Server.
Mar 07 21:37:53 local.dev systemd[1]: Unit salt-master.service entered failed state.

In such cases, the salt-minion tested for the non-existence (true), salt-master also tested (true), salt-minion created the folder, and then so salt-master failed with Errno 17: File exists.

Solution

Do not test for path existence at all, always create, expecting errno.EEXISTS as an acceptable condition. This delegates responsibility of handling such "race conditions" to the kernel where locks exist to prevent it.

[jfindlay]

@jquast, excellent, thank you.

[jfindlay]

@jquast, in the future, feel free to submit fixes directly to the oldest release branch, http://docs.saltstack.com/en/latest/topics/development/contributing.html#id1