[Zope] Re: database conflicts and the _p_oid missing attribute bug

[Dennis Allison]

Thank you for the pointers.  I'll modify the logging code and see what 
turns up.  Unfortunately this is a problem we only see when the system 
is live with real users.

On Thu, 22 Jan 2004, Jeremy Hylton wrote:

> I'll be at LinuxWorld tomorrow and mostly aware from my email, but
> here's a brief response that might offer some clues.
> 
> On Thu, 2004-01-22 at 21:45, Dennis Allison wrote:
> > I have been having problems with Zope locking up (deadlock?) when 
> > ZODB conflicts occur.  I am running Zope 2.6.2b3 with the 3.1.2b2
> > version of the ZODB (that is, a vanilla install).  Python is 2.1.3.
> 
> It's possible that upgrading to a non-beta release will fix, although
> most of the issues resolved in the 3.1.x maintenance releases were ZEO
> issues.
> 
> > The diagnostic is always:
> > 
> > 2004-01-22T12:25:15 ERROR(200) ZODB A storage error occured during 
> > transaction abort.  This shouldn't happen.
> > Traceback (innermost last):
> >   File /home/zopesys/lib/python/ZODB/Transaction.py, line 262, in commit
> >   File /home/zopesys/lib/python/ZODB/Transaction.py, line 393, in 
> > _commit_error
> > AttributeError: Surrogate instance has no attribute '_p_oid'
> 
> There is more than one error here, which makes the log message very
> confusing.  In particular, there is an exception that occurs during the
> second phase of the two-phase commit, a second error that occurs trying
> to recover from the first error, and a third error trying to log a
> message about the second error.  It's a real mess.  It might help to
> change the way logging is done so that you can see the first two error
> messages.
> 
> In _commit_error(), change the self.log() call in these lines
> 
>             except:
>                 # nothing to do but log the error
>                 self.log("Failed to abort object %s" % repr(o._p_oid),
>                          error=sys.exc_info())
> 
> to say this:
>   self.log("Failed to abort object %s" % repr(o), error=sys.exc_info())
> 
> 
> And change one line in _commit
> 
>                 # Ugh, we got an got an error during commit, so we
>                 # have to clean up.  First save the original exception
>                 # in case the cleanup process causes another
>                 # exception.
>                 error = sys.exc_info()
>                 try:
>                     self._commit_error(objects, ncommitted, jars,
> subjars)
>                 except:
>                     LOG('ZODB', ERROR,
>                         "A storage error occured during transaction "
>                         "abort.  This shouldn't happen.",
>                         error=sys.exc_info())
> 
> The LOG() call should be passing error=error, not error=sys.exc_info().
> 
> I don't think those changes will introduce any new problems and will
> tell us a lot about what is really going on.
> 
> > As I understand what's happening (after reading the logs carefully), I
> > have one or more folks trying to use the same object.  Zope's ZODB
> > serializes the access and aborts all but the oldest of the conflicting
> > transactions.  The diagnostic occurs because a registered object is 
> > missing the _p_oid attribute that's used to rollback the transaction.  The 
> > big question is which of the several sub-transactions is the culprit--or 
> > is it a bug as Dieter has suggested, soluable by providing a default 
> > value for _p_oid -- 
> 
> That right in a rough sense.  ZODB uses optimistic concurrency control,
> so later transactions are aborted if they conflict with
> already-committed ones.  But the transactions actually run concurrently.
> 
> The original error in the second-phase of two-phase commit is part of
> the problem.  Since it's in the second phase, it's not caused by a ZODB
> conflict but by some more serious failure of one of the transactional
> resource managers.  Unfortunately, we can't tell which one because the
> buggy logging code has swallowed the error.
> 
> Jeremy
> 
> 
> 
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