[mythtv] mytharchive lock file issues
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
> 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
> 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
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.
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