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From Damien Katz <dam...@apache.org>
Subject Re: Data loss
Date Sat, 07 Aug 2010 23:51:34 GMT
Looks like all that's necessary is a single delayed conflict write attempt, and all subsequent
delayed commits won't be commit, the header never gets written.

1.0 loses data. This is ridiculously bad.

We need a test to reproduce this and fix.

-Damien

On Aug 7, 2010, at 4:35 PM, Adam Kocoloski wrote:

> Good sleuthing guys, and my apologies for letting this through.  Randall, your patch
in COUCHDB-794 was actually fine, it was my reworking of it that caused this serious bug.
> 
> With respect to that gist 513282, I think it would be better to return Db#db{waiting_delayed_commit=nil}
when the headers match instead of moving the cancel_timer() command as you did.  After all,
we did perform the check here -- it was just that nothing needed to be committed.
> 
> Adam
> 
> On Aug 7, 2010, at 6:55 PM, Damien Katz wrote:
> 
>> Yes, I think it requires 2 conflicting writes in row, because it needs to trigger
the delayed_commit timer without actually having anything to commit, so the header never changes.
>> 
>> Try to reproduce this and add a test case.
>> 
>> -Damien
>> 
>> 
>> On Aug 7, 2010, at 3:47 PM, Randall Leeds wrote:
>> 
>>> I think you may be right, Damien.
>>> If ever a write happens that only contains conflicts while waiting for
>>> a delayed commit message we might still be cancelling the timer. Is
>>> this what you're thinking? This would be the fix:
>>> http://gist.github.com/513282
>>> 
>>> On Sat, Aug 7, 2010 at 15:42, Damien Katz <damien@apache.org> wrote:
>>>> I think the problem might be that 2 conflicting write attempts in row can
leave the #db.waiting_delayed_commit set but the timer has been cancelled. One that happens,
the header may never be written, as it always thinks a delayed commit will fire soon.
>>>> 
>>>> -Damien
>>>> 
>>>> 
>>>> On Aug 7, 2010, at 12:08 PM, Randall Leeds wrote:
>>>> 
>>>>> On Sat, Aug 7, 2010 at 11:56, Randall Leeds <randall.leeds@gmail.com>
wrote:
>>>>>> I agree completely! I immediately thought of this because I wrote
that
>>>>>> change. I spent a while staring at it last night but still can't
>>>>>> imagine how it's a problem.
>>>>>> 
>>>>>> On Sat, Aug 7, 2010 at 11:12, Damien Katz <damien@apache.org>
wrote:
>>>>>>> SVN commit r954043 looks suspicious. Digging further.
>>>>>>> 
>>>>>>> -Damien
>>>>>> 
>>>>> 
>>>>> I still want to stare at r954043, but it looks to me like there's at
>>>>> least one situation where we do not commit data correctly during
>>>>> compaction. This has to do with the way we now use the path to sync
>>>>> outside the couch_file:process. Check this diff:
>>>>> http://gist.github.com/513081
>>>> 
>>>> 
>> 
> 


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