Re: BUG #18055: logical decoding core on AllocateSnapshotBuilder()

On Thu, Aug 17, 2023 at 9:10 PM Zhijie Hou (Fujitsu)
<houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>
> On Thursday, August 17, 2023 6:07 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Thu, Aug 17, 2023 at 3:33 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> > wrote:
> > >
> > > On Thu, Aug 17, 2023 at 12:21 PM Zhijie Hou (Fujitsu)
> > > <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> > > >
> > > > On Tuesday, August 15, 2023 12:05 AM PG Bug reporting form
> > <noreply(at)postgresql(dot)org> wrote:
> > > > >
> > > > > The following bug has been logged on the website:
> > > > >
> > > > > Bug reference: 18055
> > > > > Logged by: ocean li
> > > > > Email address: ocean_li_996(at)163(dot)com
> > > > > PostgreSQL version: 11.9
> > > > > Operating system: centos7 5.10.84 x86_64
> > > > > Description:
> > > > >
> > > > > For testing logical decoding module, *pg_logical_slot_get_changes*
> > > > > function is used. Sometimes i got an core whose stack was like that:
> > > > > ...
> > > > > And in level #3 of stack above, NInitialRunningXacts is 2 and
> > > > > InitialRunningXacts is not NULL observed in one of cores.
> > > > >
> > > > > Using of NInitialRunningXacts and InitialRunningXacts are clear.
> > > > > Currently, the core, as far as i know, maybe caused by this way:
> > > > > an ERROR raised when calling
> > > > > *pg_logical_slot_get_changes_guts* function. The code part of
> > > > > PG_CATCH() doses not reset NInitialRunningXacts and
> > InitialRunningXacts.
> > > > > Then, calling pg_logical_slot_get_changes_guts again, the core may
> > occur.
> > > > > Unfortunately, I couldn't find a minimal reproduction case.
> > > > > However, I observed an *ERROR: canceling statement due to
> > > > > statement timeout* logged before each core occurred. (For some
> > > > > reason, I can't provide the information of
> > > > > log)
> > > >
> > > > Thanks for the report and the fix!
> > > >
> > > > I can also reproduce by the steps[1] in PG15~11(Note that we need to
> > > > change LOG_SNAPSHOT_INTERVAL_MS to a bigger number to avoid extra
> > > > running xacts wal records).
> > > >
> > > > About the patch:
> > > >
> > > > ---
> > > > /* free context, call shutdown callback */
> > > > FreeDecodingContext(ctx);
> > > >
> > > > ReplicationSlotRelease();
> > > > InvalidateSystemCaches();
> > > > }
> > > > PG_CATCH();
> > > > {
> > > > +
> > > > + /* free context, call shutdown callback */
> > > > + FreeDecodingContext(ctx);
> > > > +
> > > > + ReplicationSlotRelease();
> > > >
> > > > I think we could not directly call the cleanup functions here. There
> > > > could be two risks:
> > > > 1) it's possible either 'ctx' hasn't been initialized before the
> > > > error happen, we don't need to clean it up in this case
> > > > 2) It's possible 'ctx' or 'MyReplicationSlot' been be cleaned up
> > > > normally before entering the catch() block which means we will double
> > cleanup(free) it.
> > > >
> > > > To improve this, I think we can use PG_FINALLY() here and move all
> > > > these cleanup functions in it and do the null check for 'ctx' before cleaning
> > up.
> > > >
> > > > Just share one small patch based on your fix for reference, feel free to
> > update it.
> > > >
> > >
> > > - LogicalDecodingContext *ctx;
> > > + LogicalDecodingContext *ctx = NULL;
> > >
> > > Don't we need to use volatile for ctx? See the following comment in
> > > elog.h: "Note: if a local variable of the function containing PG_TRY
> > > is modified in the PG_TRY section and used in the PG_CATCH section,
> > > that variable must be declared "volatile" for POSIX compliance."?
> > >
>
> Thanks for pointing it out, I think we need to use volatile for it.
>
> >
> > BTW, don't we need a similar change in pg_logical_replication_slot_advance()?
>
> You are right, I can reproduce the same following the steps in [1] and
> replace the last two pg_logical_slot_get_changes function calls with two
> "SELECT pg_replication_slot_advance('isolation_slot', (select
> pg_current_wal_lsn()));"

I've not tested but the same is true in a case where
pg_create_logical_replication_slot() raises an error (e.g., cancelled
while DecodingContextFindStartpoint()) after creating the decoding
context?

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com