Rework code using list_delete_cell() in MergeAttributes

When merging two attributes, we are sure that at least one remains.
However, when deleting one element in the attribute list we may finish
with an empty list returned as NIL by list_delete_cell(), but the code
failed to track that, which is not project-like.  Adjust the call so as
we check for an empty list, and make use of it in an assertion.

This has been introduced by e7b3349, when adding support for CREATE
TABLE OF.

Author: Mark Dilger
Reviewed-by: Álvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/CAE-h2TpPDqSWgOvfvSziOaMngMPwW+QZcmPpY8hQ_KOJ2+3hXQ@mail.gmail.com

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 95af5ec2dc709e05297eb2832d5f4e65dcc7255b..e13b96d252260fcee652fdcfd17b7105b8fcdebe 100644 (file)
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -2088,7 +2088,13 @@ MergeAttributes(List *schema, List *supers, char relpersistence,
                                        coldef->cooked_default = restdef->cooked_default;
                                        coldef->constraints = restdef->constraints;
                                        coldef->is_from_type = false;
-                                       list_delete_cell(schema, rest, prev);
+                                       schema = list_delete_cell(schema, rest, prev);
+
+                                       /*
+                                        * As two elements are merged and one is removed, we
+                                        * should never finish with an empty list.
+                                        */
+                                       Assert(schema != NIL);
                                }
                                else
                                        ereport(ERROR,