Prevent adding relations to a concurrently dropped schema.

In the previous coding, it was possible for a relation to be created
via CREATE TABLE, CREATE VIEW, CREATE SEQUENCE, CREATE FOREIGN TABLE,
etc.  in a schema while that schema was meanwhile being concurrently
dropped.  This led to a pg_class entry with an invalid relnamespace
value.  The same problem could occur if a relation was moved using
ALTER .. SET SCHEMA while the target schema was being concurrently
dropped.  This patch prevents both of those scenarios by locking the
schema to which the relation is being added using AccessShareLock,
which conflicts with the AccessExclusiveLock taken by DROP.

As a desirable side effect, this also prevents the use of CREATE OR
REPLACE VIEW to queue for an AccessExclusiveLock on a relation on which
you have no rights: that will now fail immediately with a permissions
error, before trying to obtain a lock.

We need similar protection for all other object types, but as everything
other than relations uses a slightly different set of code paths, I'm
leaving that for a separate commit.

Original complaint (as far as I could find) about CREATE by Nikhil
Sontakke; risk for ALTER .. SET SCHEMA pointed out by Tom Lane;
further details by Dan Farina; patch by me; review by Hitoshi Harada.

1 files changed, 9 insertions, 5 deletions

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index c37301671e4..d0843b2f588 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -451,10 +451,12 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId)
 
 	/*
 	 * Look up the namespace in which we are supposed to create the relation,
-	 * and check we have permission to create there.
+	 * check we have permission to create there, lock it against concurrent
+	 * drop, and mark stmt->relation as RELPERSISTENCE_TEMP if a temporary
+	 * namespace is selected.
 	 */
-	namespaceId = RangeVarGetAndCheckCreationNamespace(stmt->relation);
-	RangeVarAdjustRelationPersistence(stmt->relation, namespaceId);
+	namespaceId =
+		RangeVarGetAndCheckCreationNamespace(stmt->relation, NoLock, NULL);
 
 	/*
 	 * Security check: disallow creating temp tables from security-restricted
@@ -9417,6 +9419,7 @@ AlterTableNamespace(AlterObjectSchemaStmt *stmt)
 	Oid			oldNspOid;
 	Oid			nspOid;
 	Relation	classRel;
+	RangeVar   *newrv;
 
 	relid = RangeVarGetRelidExtended(stmt->relation, AccessExclusiveLock,
 									 false, false,
@@ -9441,8 +9444,9 @@ AlterTableNamespace(AlterObjectSchemaStmt *stmt)
 						get_rel_name(tableId))));
 	}
 
-	/* get schema OID and check its permissions */
-	nspOid = LookupCreationNamespace(stmt->newschema);
+	/* Get and lock schema OID and check its permissions. */
+	newrv = makeRangeVar(stmt->newschema, RelationGetRelationName(rel), -1);
+	nspOid = RangeVarGetAndCheckCreationNamespace(newrv, NoLock, NULL);
 
 	/* common checks on switching namespaces */
 	CheckSetNamespace(oldNspOid, nspOid, RelationRelationId, relid);