Forbid DROP SCHEMA on temporary namespaces

This operation was possible for the owner of the schema or a superuser.
Down to 9.4, doing this operation would cause inconsistencies in a
session whose temporary schema was dropped, particularly if trying to
create new temporary objects after the drop.  A more annoying
consequence is a crash of autovacuum on an assertion failure when
logging information about an orphaned temp table dropped.  Note that
because of 246a6c8 (present in v11~), which has made the removal of
orphaned temporary tables more aggressive, the failure could be
triggered more easily, but it is possible to reproduce down to 9.4.

Reported-by: Mahendra Singh, Prabhat Sahu
Author: Michael Paquier
Reviewed-by: Kyotaro Horiguchi, Mahendra Singh
Discussion: https://postgr.es/m/CAKYtNAr9Zq=1-ww4etHo-VCC-k120YxZy5OS01VkaLPaDbv2tg@mail.gmail.com
Backpatch-through: 9.4

src/backend/commands/dropcmds.c

@@ -101,6 +101,21 @@ RemoveObjects(DropStmt *stmt)
 						 errhint("Use DROP AGGREGATE to drop aggregate functions.")));
 		}
 
+		/*
+		 * Prevent the drop of a temporary schema, be it owned by the current
+		 * session or another backend as this would mess up with the callback
+		 * registered to clean up temporary objects at the end of a session.
+		 * Note also that the creation of any follow-up temporary object would
+		 * result in inconsistencies within the session whose temporary schema
+		 * has been dropped.
+		 */
+		if (stmt->removeType == OBJECT_SCHEMA &&
+			isAnyTempNamespace(address.objectId))
+			ereport(ERROR,
+					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+					 errmsg("cannot drop temporary schema \"%s\"",
+							get_namespace_name(address.objectId))));
+
 		/* Check permissions. */
 		namespaceId = get_object_namespace(&address);
 		if (!OidIsValid(namespaceId) ||