Re: (ITS#6716) syncprov sessionlog 'sl_mincsn' not assigned a value.

[hyc@symas.com]

cmikk@qwest.net wrote:
> On Sun, Dec 12, 2010 at 06:22:38PM +0100, Rein Tollevik wrote:
>> On 11/19/10 23:55 , cmikk@qwest.net wrote:
>>
>>> will evaluate to true in cases where it should not. A quick fix would be to
>>> compare directly against the CSN of the sessionlog's head:
>>
>> An extended version of this fix, which also ensures that the
>> sessionlog is kept in csn order, is now in head.  Please test,
>> syncprov.c revision 1.320.
>
> Looks good to me. Thank you.
>
> I discovered this bug while trying to reproduce ITS#6717.
>
> I've attached a patch to that ITS which appears to fix
> that problem. If you have the time, I'd appreciate some
> feedback on that patch. The issue makes it very inefficient
> to run a slave in refreshOnly mode with a multimaster /
> mirrormode master.
>
> Best Regards,

The current code is broken.

> Other than the patch I suggested (using the list head's
> CSN value directly), I can think of two other approaches:
>
> 	1) When adding an entry to the sessionlog, check
> 	if sl_mincsn is empty. If it is, update sl_mincsn
> 	to the new entry's CSN.
>
> 	2) When initializing the sessionlog, set sl_mincsn
> 	to the maximum contextCSN value of the underlying
> 	database.
>
> #2 seems ideal from an efficiency standpoint, although it
> differs from the algorithm the current code appears to be
> intended to implement.

We should have gone with #2. The problem scenario:

1: Provider and consumer in sync, both started fresh, sessionlog is empty.
2: stop consumer
3: write on provider
4: start consumer

Result: the sessionlog is ignored, because the sl_head is newer than the 
consumer's cookie.

After the consumer syncs, you can repeat steps 2-4 and see that the sessionlog 
is used from then on.

-- 
   -- Howard Chu
   CTO, Symas Corp.           http://www.symas.com
   Director, Highland Sun     http://highlandsun.com/hyc/
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