Re: Hot standby, prepared xacts, locks

Simon Riggs wrote:
> On Wed, 2009-10-21 at 19:37 +0300, Heikki Linnakangas wrote:
>
>> So, I'm quite eager to just revert all those lock_twophase_recover()
>> changes, and always rely on the "grant lock to dummy proc, then
>> release
>> it in startup process" method. If we don't want to rely on that,
>> PostPrepare_Locks is an example of how to transfer lock ownership from
>> one process to another correctly.
>
> Yes, I realised after I wrote it that PostPrepare already does that
> switch, just been busy with other stuff to switch over the code.
>
> I think we do need some special code because of handling whole lock
> queues. i.e. if there is a backend requesting an AEL but a prepared xact
> has it, the lock queue will initially be Backend->Startup and needs to
> end up looking like Backend->DummyProc.

Hmm, dunno about that, but there is one problem with the "grant to dummy
proc, then release in startup process" approach. What if there isn't
enough shared memory available to re-acquire the lock for the dummy
proc? It would be rather unfortunate to throw an error and shut down the
standby, instead of promoting it to a new master.

In fact, what happens if you ran out of shared memory when replaying a
relation_redo_lock record? Panic?

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com