Re: [BUG] Logical replication failure "ERROR: could not map filenode "base/13237/442428" to relation OID" with catalog modifying txns - Mailing list pgsql-hackers

On Wed, Jul 6, 2022 at 3:01 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Wed, Jul 6, 2022 at 7:38 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > I'll post a new version patch in the next email with replying to other comments.
> >
>
> Okay, thanks for working on this. Few comments/suggestions on
> poc_remember_last_running_xacts_v2 patch:
>
> 1.
> +ReorderBufferSetLastRunningXactsCatalogChanges(ReorderBuffer *rb,
> TransactionId xid,
> +    uint32 xinfo, int subxcnt,
> +    TransactionId *subxacts, XLogRecPtr lsn)
> +{
> ...
> ...
> +
> + test = bsearch(&xid, rb->last_running_xacts, rb->n_last_running_xacts,
> +    sizeof(TransactionId), xidComparator);
> +
> + if (test == NULL)
> + {
> + for (int i = 0; i < subxcnt; i++)
> + {
> + test = bsearch(&subxacts[i], rb->last_running_xacts, rb->n_last_running_xacts,
> +    sizeof(TransactionId), xidComparator);
> ...
>
> Is there ever a possibility that the top transaction id is not in the
> running_xacts list but one of its subxids is present? If yes, it is
> not very obvious at least to me so adding a comment here could be
> useful. If not, then why do we need this additional check for each of
> the sub-transaction ids?

I think there is no possibility. The check for subtransactions is not necessary.

>
> 2.
> @@ -627,6 +647,15 @@ DecodeCommit(LogicalDecodingContext *ctx,
> XLogRecordBuffer *buf,
>   commit_time = parsed->origin_timestamp;
>   }
>
> + /*
> + * Set the last running xacts as containing catalog change if necessary.
> + * This must be done before SnapBuildCommitTxn() so that we include catalog
> + * change transactions to the historic snapshot.
> + */
> + ReorderBufferSetLastRunningXactsCatalogChanges(ctx->reorder, xid,
> parsed->xinfo,
> +    parsed->nsubxacts, parsed->subxacts,
> +    buf->origptr);
> +
>   SnapBuildCommitTxn(ctx->snapshot_builder, buf->origptr, xid,
>      parsed->nsubxacts, parsed->subxacts);
>
> As mentioned previously as well, marking it before SnapBuildCommitTxn
> has one disadvantage, we sometimes do this work even if the snapshot
> state is SNAPBUILD_START or SNAPBUILD_BUILDING_SNAPSHOT in which case
> SnapBuildCommitTxn wouldn't do anything. Can we instead check whether
> the particular txn has invalidations and is present in the
> last_running_xacts list along with the check
> ReorderBufferXidHasCatalogChanges? I think that has the additional
> advantage that we don't need this additional marking if the xact is
> already marked as containing catalog changes.

Agreed.

>
> 3.
> 1.
> + /*
> + * We rely on HEAP2_NEW_CID records and XACT_INVALIDATIONS to know
> + * if the transaction has changed the catalog, and that information
> + * is not serialized to SnapBuilder.  Therefore, if the logical
> + * decoding decodes the commit record of the transaction that actually
> + * has done catalog changes without these records, we miss to add
> + * the xid to the snapshot so up creating the wrong snapshot.
>
> The part of the sentence "... snapshot so up creating the wrong
> snapshot." is not clear. In this comment, at one place you have used
> two spaces after a full stop, and at another place, there is one
> space. I think let's follow nearby code practice to use a single space
> before a new sentence.

Agreed.

>
> 4.
> +void
> +ReorderBufferProcessLastRunningXacts(ReorderBuffer *rb,
> xl_running_xacts *running)
> +{
> + /* Quick exit if there is no longer last running xacts */
> + if (likely(rb->n_last_running_xacts == 0))
> + return;
> +
> + /* First call, build the last running xact list */
> + if (rb->n_last_running_xacts == -1)
> + {
> + int nxacts = running->subxcnt + running->xcnt;
> + Size sz = sizeof(TransactionId) * nxacts;;
> +
> + rb->last_running_xacts = MemoryContextAlloc(rb->context, sz);
> + memcpy(rb->last_running_xacts, running->xids, sz);
> + qsort(rb->last_running_xacts, nxacts, sizeof(TransactionId), xidComparator);
> +
> + rb->n_last_running_xacts = nxacts;
> +
> + return;
> + }
>
> a. Can we add the function header comments for this function?

Updated.

> b. We seem to be tracking the running_xact information for the first
> running_xact record after start/restart. The name last_running_xacts
> doesn't sound appropriate for that, how about initial_running_xacts?

Sound good, updated.

>
> 5.
> + /*
> + * Purge xids in the last running xacts list if we can do that for at least
> + * one xid.
> + */
> + if (NormalTransactionIdPrecedes(rb->last_running_xacts[0],
> + running->oldestRunningXid))
>
> I think it would be a good idea to add a few lines here explaining why
> it is safe to purge. IIUC, it is because the commit for those xacts
> would have already been processed and we don't need such a xid
> anymore.

Right, updated.

>
> 6. As per the discussion above in this thread having
> XACT_XINFO_HAS_INVALS in the commit record doesn't indicate that the
> xact has catalog changes, so can we add somewhere in comments that for
> such a case we can't distinguish whether the txn has catalog change
> but we still mark the txn has catalog changes?

Agreed.

> Can you please share one example for this case?

I think it depends on what we did in the transaction but one example I
have is that a commit record for ALTER DATABASE has only a snapshot
invalidation message:

=# alter database postgrse set log_statement to 'all';
ALTER DATABASE

$ pg_waldump $PGDATA/pg_wal/000000010000000000000001 | tail -1
rmgr: Transaction len (rec/tot):     66/    66, tx:        821, lsn:
0/019B50A8, prev 0/019B5070, desc: COMMIT 2022-07-11 21:38:44.036513
JST; inval msgs: snapshot 2964

I've attached an updated patch, please review it.

Regards,

--
Masahiko Sawada
EDB:  https://www.enterprisedb.com/