Re: logical decoding of two-phase transactions

On 2017-03-28 03:30:28 +0100, Simon Riggs wrote:
> On 28 March 2017 at 02:25, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > On 2017-03-28 04:12:41 +0300, Stas Kelvich wrote:
> >>
> >> > On 28 Mar 2017, at 00:25, Andres Freund <andres(at)anarazel(dot)de> wrote:
> >> >
> >> > Hi,
> >> >
> >> > On 2017-03-28 00:19:29 +0300, Stas Kelvich wrote:
> >> >> Ok, here it is.
> >> >
> >> > On a very quick skim, this doesn't seem to solve the issues around
> >> > deadlocks of prepared transactions vs. catalog tables. What if the
> >> > prepared transaction contains something like LOCK pg_class; (there's a
> >> > lot more realistic examples)? Then decoding won't be able to continue,
> >> > until that transaction is committed / aborted?
> >>
> >> But why is that deadlock? Seems as just lock.
> >
> > If you actually need separate decoding of 2PC, then you want to wait for
> > the PREPARE to be replicated. If that replication has to wait for the
> > to-be-replicated prepared transaction to commit prepared, and commit
> > prepare will only happen once replication happened...
>
> Surely that's up to the decoding plugin?

It can't do much about it, so not really. A lot of the functions
dealing with datatypes (temporarily) lock relations. Both the actual
user tables, and system catalog tables (cache lookups...).

> If the plugin takes locks it had better make sure it can get the locks
> or timeout. But that's true of any resource the plugin needs access to
> and can't obtain when needed.

> This issue could occur now if the transaction tool a session lock on a
> catalog table.

That's not a self deadlock, and we don't don't do session locks outside
of operations like CIC?

Greetings,

Andres Freund