Check criticalSharedRelcachesBuilt in GetSharedSecurityLabel().

An extension may want to call GetSecurityLabel() on a shared object before the shared relcaches are fully initialized. For instance, a ClientAuthentication_hook might want to retrieve the security label on a role. Discussion: https://postgr.es/m/ecb7af0b26e3be1d96d291c8453a86f1f82d9061.camel@j-davis.com Backpatch-through: 9.6

Author: jeff-davis

src/backend/commands/seclabel.c

@@ -244,8 +244,8 @@ GetSharedSecurityLabel(const ObjectAddress *object, const char *provider)
 
 pg_shseclabel = table_open(SharedSecLabelRelationId, AccessShareLock);
 
-scan = systable_beginscan(pg_shseclabel, SharedSecLabelObjectIndexId, true,
- NULL, 3, keys);
+scan = systable_beginscan(pg_shseclabel, SharedSecLabelObjectIndexId,
+ criticalSharedRelcachesBuilt, NULL, 3, keys);
 
 tuple = systable_getnext(scan);
 if (HeapTupleIsValid(tuple))