Re: Truncate in synchronous logical replication failed - Mailing list pgsql-hackers

On Fri, 16 Apr 2021 at 16:52, Amit Kapila <amit.kapila16@gmail.com> wrote:
> On Fri, Apr 16, 2021 at 12:55 PM Japin Li <japinli@hotmail.com> wrote:
>>
>> On Thu, 15 Apr 2021 at 19:25, Amit Kapila <amit.kapila16@gmail.com> wrote:
>> > On Thu, Apr 15, 2021 at 4:30 PM Japin Li <japinli@hotmail.com> wrote:
>> >>
>> >>
>> >> On Thu, 15 Apr 2021 at 18:22, Amit Kapila <amit.kapila16@gmail.com> wrote:
>> >> > On Wed, Apr 14, 2021 at 3:31 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>> >> >>
>> >> >> On Tue, Apr 13, 2021 at 8:07 PM Petr Jelinek
>> >> >> <petr.jelinek@enterprisedb.com> wrote:
>> >> >> >
>> >> >> > > On 12 Apr 2021, at 08:58, Amit Kapila <amit.kapila16@gmail.com> wrote:
>> >> >> > >
>> >> >> > > The problem happens only when we try to fetch IDENTITY_KEY attributes
>> >> >> > > because pgoutput uses RelationGetIndexAttrBitmap() to get that
>> >> >> > > information which locks the required indexes. Now, because TRUNCATE
>> >> >> > > has already acquired an exclusive lock on the index, it seems to
>> >> >> > > create a sort of deadlock where the actual Truncate operation waits
>> >> >> > > for logical replication of operation to complete and logical
>> >> >> > > replication waits for actual Truncate operation to finish.
>> >> >> > >
>> >> >> > > Do we really need to use RelationGetIndexAttrBitmap() to build
>> >> >> > > IDENTITY_KEY attributes? During decoding, we don't even lock the main
>> >> >> > > relation, we just scan the system table and build that information
>> >> >> > > using a historic snapshot. Can't we do something similar here?
>> >> >> > >
>> >> >> > > Adding Petr J. and Peter E. to know their views as this seems to be an
>> >> >> > > old problem (since the decoding of Truncate operation is introduced).
>> >> >> >
>> >> >> > We used RelationGetIndexAttrBitmap because it already existed, no other reason.
>> >> >> >
>> >> >>
>> >> >> Fair enough. But I think we should do something about it because using
>> >> >> the same (RelationGetIndexAttrBitmap) just breaks the synchronous
>> >> >> logical replication. I think this is broken since the logical
>> >> >> replication of Truncate is supported.
>> >> >>
>> >> >> > I am not sure what exact locking we need but I would have guessed at least AccessShareLock would be needed.
>> >> >> >
>> >> >>
>> >> >> Are you telling that we need AccessShareLock on the index? If so, why
>> >> >> is it different from how we access the relation during decoding
>> >> >> (basically in ReorderBufferProcessTXN, we directly use
>> >> >> RelationIdGetRelation() without any lock on the relation)? I think we
>> >> >> do it that way because we need it to process WAL entries and we need
>> >> >> the relation state based on the historic snapshot, so, even if the
>> >> >> relation is later changed/dropped, we are fine with the old state we
>> >> >> got. Isn't the same thing applies here in logicalrep_write_attrs? If
>> >> >> that is true then some equivalent of RelationGetIndexAttrBitmap where
>> >> >> we use RelationIdGetRelation instead of index_open should work?
>> >> >>
>> >> >
>> >> > Today, again I have thought about this and don't see a problem with
>> >> > the above idea. If the above understanding is correct, then I think
>> >> > for our purpose in pgoutput, we just need to call RelationGetIndexList
>> >> > and then build the idattr list for relation->rd_replidindex.
>> >>
>> >> Sorry, I don't know how can we build the idattr without open the index.
>> >> If we should open the index, then we should use NoLock, since the TRUNCATE
>> >> side hold AccessExclusiveLock. As Osumi points out in [1], The NoLock mode
>> >> assumes that the  appropriate lock on the index is already taken.
>> >>
>> >
>> > Why can't we use RelationIdGetRelation() by passing the required
>> > indexOid to it?
>>
>> Hi Amit, as your suggested, I try to use RelationIdGetRelation() replace
>> the index_open() to avoid specify the AccessSharedLock, then the TRUNCATE
>> will not be blocked.
>>
>
> It is okay as POC but we can't change the existing function
> RelationGetIndexAttrBitmap. It is used from other places as well. It
> might be better to write a separate function for this, something like
> what Osumi-San's patch is trying to do.

Agreed!

Hi Osumi-San, can you merge the test case in your next version?

-- 
Regrads,
Japin Li.
ChengDu WenWu Information Technology Co.,Ltd.