[tor-bugs] #18959 [Core Tor/Stem]: Poor error when unable to read the auth cookie

[Tor Bug Tracker & Wiki]

#18959: Poor error when unable to read the auth cookie
-------------------------------+--------------------
     Reporter:  atagar         |      Owner:  atagar
         Type:  defect         |     Status:  new
     Priority:  Low            |  Milestone:
    Component:  Core Tor/Stem  |    Version:
     Severity:  Minor          |   Keywords:
Actual Points:                 |  Parent ID:
       Points:                 |   Reviewer:
      Sponsor:                 |
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 umoukun on irc points out that when your user can't read tor's auth cookie
 stem fails with...

 {{{
 stem.connection.UnreadableCookieFile: Authentication failed:
 '/var/run/tor/control.authcookie' doesn't exist
 }}}

 Trouble looks to be that
 [https://gitweb.torproject.org/stem.git/tree/stem/connection.py#n1080
 _read_cookie()] uses
 [https://docs.python.org/2/library/os.path.html#os.path.exists
 os.path.exists()] which returns False if the file's not readable. We
 should perform a second permission check.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/18959>
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