Re: logical decoding bug when mapped relation with toast contents isrewritten repeatedly - Mailing list pgsql-hackers
From | Andres Freund |
---|---|
Subject | Re: logical decoding bug when mapped relation with toast contents isrewritten repeatedly |
Date | |
Msg-id | 20181010205719.hs3cvfakipmfmkvv@alap3.anarazel.de Whole thread Raw |
In response to | logical decoding bug when mapped relation with toast contents isrewritten repeatedly (Andres Freund <andres@anarazel.de>) |
List | pgsql-hackers |
Hi, On 2018-09-13 19:10:46 -0700, Andres Freund wrote: > (Tomas, CCing you because you IIRC mentioned encountered an issue like > this) > > I just spent quite a while debugging an issue where running logical > decoding yielded a: > ERROR: could not map filenode "base/$X/$Y" to relation OID > error. > > After discarding like 30 different theories, I have found the cause: > > During rewrites (i.e. VACUUM FULL / CLUSTER) of a mapped relation with a > toast table with actual live toasted tuples (pg_proc in my case and > henceforth) heap inserts with the toast data happen into the new toast > relation, triggered by: > At that point the new toast relation does *NOT* appear to be a system >OA catalog, it's just appears as an "independent" table. Therefore we do > not trigger, in heap_insert(): > > /* > * RelationIsLogicallyLogged > * True if we need to log enough information to extract the data from the > * WAL stream. > * > * We don't log information for unlogged tables (since they don't WAL log > * anyway) and for system tables (their content is hard to make sense of, and > * it would complicate decoding slightly for little gain). Note that we *do* > * log information for user defined catalog tables since they presumably are > * interesting to the user... > */ > #define RelationIsLogicallyLogged(relation) \ > (XLogLogicalInfoActive() && \ > RelationNeedsWAL(relation) && \ > !IsCatalogRelation(relation)) > > /* > * For logical decoding, we need the tuple even if we're doing a full > * page write, so make sure it's included even if we take a full-page > * image. (XXX We could alternatively store a pointer into the FPW). > */ > if (RelationIsLogicallyLogged(relation)) > { > xlrec.flags |= XLH_INSERT_CONTAINS_NEW_TUPLE; > bufflags |= REGBUF_KEEP_DATA; > } > > i.e. the inserted toast tuple will be marked as > XLH_INSERT_CONTAINS_NEW_TUPLE - which it shouldn't, because it's a > system table. Which we currently do not allow do be logically decoded. > > That normally ends up being harmless, because ReorderBufferCommit() has the > following check: > if (!RelationIsLogicallyLogged(relation)) > goto change_done; > > but to reach that check, we first have to map the relfilenode from the > WAL to the corresponding OID: > reloid = RelidByRelfilenode(change->data.tp.relnode.spcNode, > change->data.tp.relnode.relNode); > > That works correctly if there's only one rewrite - the relmapper > contains the data for the new toast table. But if there's been *two* > consecutive rewrites, the relmapper *does not* contain the intermediary > relfilenode of pg_proc. There's no such problem for non-mapped tables, > because historic snapshots allow us to access the relevant data, but the > relmapper isn't mvcc. > > Therefore the catalog-rewrite escape hatch of: > /* > * Catalog tuple without data, emitted while catalog was > * in the process of being rewritten. > */ > if (reloid == InvalidOid && > change->data.tp.newtuple == NULL && > change->data.tp.oldtuple == NULL) > goto change_done; > does not trigger and we run into: > else if (reloid == InvalidOid) > elog(ERROR, "could not map filenode \"%s\" to relation OID", > relpathperm(change->data.tp.relnode, > MAIN_FORKNUM)); > > > commenting out this error / converting it into a warning makes this case > harmless, but could obviously be problematic in other scenarios. > > > I suspect the proper fix would be to have a new HEAP_INSERT_NO_LOGICAL > option, and specify that in raw_heap_insert() iff > RelationIsLogicallyLogged(state->rs_old_rel) or something like that. > > Attached is a *prototype* patch of that approach. Without the code > level changes the addition to test_decoding's rewrite.sql trigger the > bug, after it they're fixed. > > > The only reason the scenario I was debugging hit this was that there was > a cluster wide VACUUM FULL a couple times a day, and replication was > several hours behind due to slow network / receiving side. I've pushed this now. I added more tests,which found an issue around with the change around rewrites of non-mapped catalog tables. Greetings, Andres Freund
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