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

[cmikk@qwest.net]

On Thu, Jan 13, 2011 at 12:30:32PM -0800, Howard Chu wrote:
> cmikk@qwest.net wrote:
> >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:

The current code (in HEAD) covers the vast majority of
cases, but still has a hole which can come up quite often
in MMR + refreshOnly scenarios. The scenario:

  1: provider 1, provider 2, and consumer all in sync. SID 1 CSN
     is a few minutes old, SID 2 is a few hours old. Both sessionlogs
     are empty with sl_mincsn set to the newest (provider 1) CSN value
  2: stop consumer
  3: write on provider 2
  4: start consumer

The provider will see that the consumer is out of date
wrt the SID 2 CSN and that the old value of the SID 2 CSN
predates sl_mincsn, so it will skip the session log.

I think the following approach will cover this case:

	* sl_mincsn holds a CSN value for each SID

	* in syncprov_op_search, mincsn is compared to
	the sl_mincsn value with the same SID

	* when expiring log entries in in syncprov_add_slog,
	the sl_mincsn value with the same SID as the expired
	entry's CSN is set to the maximum of the two values.

If there is no matching SID in sl_mincsn, then:

	* in syncprov_op_search, this implies that the
	mincsn's SID has been added to the provider's
	configuration since startup or that the first
	change recorded by that SID occurred since startup,
	and thus the sessionlog contains sufficient
	information

	* in syncprov_add_slog, sl_mincsn will need to
	be expanded to hold the new SID's CSN

Thoughts?

-- 
Chris Mikkelson  |  It was mentioned on CNN that the prime number
cmikk@qwest.net  |  discovered recently is four times bigger than
                    the previous record.  -- unknown