use Perl

• Easy (Score:1)

  by Aristotle (5147)

  Operator precedence.
  • Re:Easy (Score:1)

    by Abigail (26)

    No, operator precedence isn't the problem. The problem hasn't anything to do with syntax.
    • Re:Easy (Score:1)

      by Aristotle (5147)

      Then I guess I just found a bonus bug for you.

      $ perl -MO=Deparse,-p -e'! -f $lock_file || unlink $lock_file || warn "unlink: $!"'
      ((not -f($lock_file)) or unlink(($lock_file || warn("unlink: $!"))));
      -e syntax OK

      Admittedly, it’s not a fatal issue.
• flock(2) (Score:1)

  by knew (5814)

  "On success, zero is returned. On error, -1 is returned, and errno is set appropriately."
  
  whoops.
  • Re:flock(2) (Score:1)

    by Aristotle (5147)

    No. Second sentence in perldoc -f flock:

    Returns true for success, false on failure.
    • Re:flock(2) (Score:1)

      by knew (5814)

      Bah, so it is indeed. Back to looking at it then :)
• Ah, got it. (Score:1)

  by Aristotle (5147)

  A third process will erroneously be allowed to launch after the first of two concurrently launched processes has already exited, despite the second is still hanging around.

  But how did you fix it? I can’t see a correct solution other than just not cleaning up the lockfile.
  • Re:Ah, got it. (Score:1)

    by Abigail (26)

    Indeed.

    It's easily fixed - set a variable right after getting the lock, and don't delete the file unless the variable is set.
• END{} (Score:1)

  by knew (5814)

  oh hehe.
  
  END {! -f $lock_file || unlink $lock_file || warn "unlink: $!"}
  
  or in other words...
  
  "Let's delete the lock file if it exists when we're ENDing, even if we're doing so because we've detected a previous instance of the program"
  
• Lock DATA? (Score:1)

  by nik (3476)

  If the processes are running on the same host, why not lock the DATA filehandle?

  flock(DATA, LOCK_EX|LOCK_NB) or die "Already running.";

  should do it. No need for messy temp files to remember to cleanup.
  • Re:Lock DATA? (Score:2)

    by Dom2 (2981)

    Heh, locking the DATA filehandle is a neat idea.

    But more to the point, who would consider using flock() over NFS? That's scary bad stuff. You're now relying on correct implementation of lockd everywhere, which is not guaranteed. Better to use some form of lockfile, the way that the MTAs & procmail do...

    -Dom
    • Re:Lock DATA? (Score:2)

      by jhi (318)

      Slightly more reliable than creating a lock FILE is creating a lock DIRECTORY. I have no doubt some NFS implementation gets this wrong too, but it's quite hard to create a directory non-atomically.