Try to acquire relation locks in RangeVarGetRelid.

In the previous coding, we would look up a relation in RangeVarGetRelid,
lock the resulting OID, and then AcceptInvalidationMessages().  While
this was sufficient to ensure that we noticed any changes to the
relation definition before building the relcache entry, it didn't
handle the possibility that the name we looked up no longer referenced
the same OID.  This was particularly problematic in the case where a
table had been dropped and recreated: we'd latch on to the entry for
the old relation and fail later on.  Now, we acquire the relation lock
inside RangeVarGetRelid, and retry the name lookup if we notice that
invalidation messages have been processed meanwhile.  Many operations
that would previously have failed with an error in the presence of
concurrent DDL will now succeed.

There is a good deal of work remaining to be done here: many callers
of RangeVarGetRelid still pass NoLock for one reason or another.  In
addition, nothing in this patch guards against the possibility that
the meaning of an unqualified name might change due to the creation
of a relation in a schema earlier in the user's search path than the
one where it was previously found.  Furthermore, there's nothing at
all here to guard against similar race conditions for non-relations.
For all that, it's a start.

Noah Misch and Robert Haas

1 files changed, 12 insertions, 3 deletions

diff --git a/src/backend/commands/sequence.c b/src/backend/commands/sequence.c
index be04177a2ee..de6e2a3e33f 100644
--- a/src/backend/commands/sequence.c
+++ b/src/backend/commands/sequence.c
@@ -427,8 +427,8 @@ AlterSequence(AlterSeqStmt *stmt)
 	FormData_pg_sequence new;
 	List	   *owned_by;
 
-	/* open and AccessShareLock sequence */
-	relid = RangeVarGetRelid(stmt->sequence, false);
+	/* Open and lock sequence. */
+	relid = RangeVarGetRelid(stmt->sequence, AccessShareLock, false, false);
 	init_sequence(relid, &elm, &seqrel);
 
 	/* allow ALTER to sequence owner only */
@@ -507,7 +507,16 @@ nextval(PG_FUNCTION_ARGS)
 	Oid			relid;
 
 	sequence = makeRangeVarFromNameList(textToQualifiedNameList(seqin));
-	relid = RangeVarGetRelid(sequence, false);
+
+	/*
+	 * XXX: This is not safe in the presence of concurrent DDL, but
+	 * acquiring a lock here is more expensive than letting nextval_internal
+	 * do it, since the latter maintains a cache that keeps us from hitting
+	 * the lock manager more than once per transaction.  It's not clear
+	 * whether the performance penalty is material in practice, but for now,
+	 * we do it this way.
+	 */
+	relid = RangeVarGetRelid(sequence, NoLock, false, false);
 
 	PG_RETURN_INT64(nextval_internal(relid));
 }