Fix logical decoding error when system table w/ toast is repeatedly rewritten.

Repeatedly rewriting a mapped catalog table with VACUUM FULL or
CLUSTER could cause logical decoding to fail with:
ERROR, "could not map filenode \"%s\" to relation OID"

To trigger the problem the rewritten catalog had to have live tuples
with toasted columns.

The problem was triggered as during catalog table rewrites the
heap_insert() check that prevents logical decoding information to be
emitted for system catalogs, failed to treat the new heap's toast table
as a system catalog (because the new heap is not recognized as a
catalog table via RelationIsLogicallyLogged()). The relmapper, in
contrast to the normal catalog contents, does not contain historical
information. After a single rewrite of a mapped table the new relation
is known to the relmapper, but if the table is rewritten twice before
logical decoding occurs, the relfilenode cannot be mapped to a
relation anymore.  Which then leads us to error out.   This only
happens for toast tables, because the main table contents aren't
re-inserted with heap_insert().

The fix is simple, add a new heap_insert() flag that prevents logical
decoding information from being emitted, and accept during decoding
that there might not be tuple data for toast tables.

Unfortunately that does not fix pre-existing logical decoding
errors. Doing so would require not throwing an error when a filenode
cannot be mapped to a relation during decoding, and that seems too
likely to hide bugs.  If it's crucial to fix decoding for an existing
slot, temporarily changing the ERROR in ReorderBufferCommit() to a
WARNING appears to be the best fix.

Author: Andres Freund
Discussion: https://postgr.es/m/20180914021046.oi7dm4ra3ot2g2kt@alap3.anarazel.de
Backpatch: 9.4-, where logical decoding was introduced

1 files changed, 16 insertions, 3 deletions

diff --git a/src/backend/access/heap/rewriteheap.c b/src/backend/access/heap/rewriteheap.c
index 85f92973c95..71277889649 100644
--- a/src/backend/access/heap/rewriteheap.c
+++ b/src/backend/access/heap/rewriteheap.c
@@ -652,10 +652,23 @@ raw_heap_insert(RewriteState state, HeapTuple tup)
 		heaptup = tup;
 	}
 	else if (HeapTupleHasExternal(tup) || tup->t_len > TOAST_TUPLE_THRESHOLD)
+	{
+		int options = HEAP_INSERT_SKIP_FSM;
+
+		if (!state->rs_use_wal)
+			options |= HEAP_INSERT_SKIP_WAL;
+
+		/*
+		 * The new relfilenode's relcache entrye doesn't have the necessary
+		 * information to determine whether a relation should emit data for
+		 * logical decoding.  Force it to off if necessary.
+		 */
+		if (!RelationIsLogicallyLogged(state->rs_old_rel))
+			options |= HEAP_INSERT_NO_LOGICAL;
+
 		heaptup = toast_insert_or_update(state->rs_new_rel, tup, NULL,
-										 HEAP_INSERT_SKIP_FSM |
-										 (state->rs_use_wal ?
-										  0 : HEAP_INSERT_SKIP_WAL));
+										 options);
+	}
 	else
 		heaptup = tup;