[mythtv] mytharchive lock file issues

Paul Harrison mythtv at dsl.pipex.com
Mon Mar 5 23:01:51 UTC 2007


D. Hugh Redelmeier wrote:
> [Perhaps this should be a trac ticket, but I cannot seem to create one.]
>
> mytharchive has been acting up on me, so I did some investigation.
>
> Sometimes a mythburn step fails (I'm vague on how or what), leaving the 
> mythburn.lck lock file but no mythburn.py process.  When this happens,
> cancel requests are ineffective, and any attempt to archive something
> just gets you into the logviewer program, whatching the log make no
> further process.  Nothing will ever delete the lock.
>
> For example, we tried to put too much on a DVD and got into that
> state.
>
> So there are two problems:
> (1) a failure with no proper diagnostic or termination
> (2) and then no way to archive after this.
>
> This message was prompted by investigation of problem 2.
>
> I'm not the only one to have experienced this.  See, for example
> http://threebit.net/mail-archive/mythtv-users/msg35922.html
>
>
> The mythburn.lck lockfile logic in mythburn.py has a few problems.
>
> - The checking for and creation of a lock ought to be "atomic".
>   Otherwise there can be a race condition.  The conventional way of
>   doing this on UNIX-like systems is to try to creat a lockfile
>   using, in Python terms,
>   	os.open(lockfilepath, O_WRONLY | O_CREAT | O_EXCL)
>   If there is already a lockfile, this fails
>   Otherwise, it creates one.
>   This is atomic.
>
> - the logic of mythburn.py's try ... finally block at lines 3571
>   through 3611 seems wrong.  The finally part removes the lock file
>   but it may not belong to this process.  Luckilly, the program that
>   invokes mythburn.py will not do so if the lock is present.  So this
>   case will not come up.  Still, this seems quite wrong.
>
> - the lock file, when present, simply contains the string "lock".  It
>   is better to follow the convention that the lock file contains the
>   process ID of the owning process.  This allows people and processes
>   to determine if a lock is stale: held by a now-dead process.
>
> - It seems to me that each place where the lock is checked should then
>   check to see if the lock is stale and act accordingly (break the
>   lock if stale, perhaps with a warning).
>
> Once this last step is implemented, my problem 2 will go away.
>
> I started to code changes to mythburn.py but before getting serious,
> I'd like to know if the relevant authorities think I'm on the right
> track.
>
>   

I can't say it's ever been a problem for me but if you want to code 
these changes then go ahead. You'll also need to look at 
mytharchivehelper which also sometimes creates the lock file.

Paul H.



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