Fix ALTER TABLE .. ADD COLUMN with complex inheritance trees

This command, when used to add a column on a parent table with a complex
inheritance tree, tried to update multiple times the same tuple in
pg_attribute for a child table when incrementing attinhcount, causing
failures with "tuple already updated by self" because of a missing
CommandCounterIncrement() between two updates.

This exists for a rather long time, so backpatch all the way down.

Reported-by: Alexander Lakhin
Author: Tender Wang
Reviewed-by: Richard Guo
Discussion: https://postgr.es/m/18297-b04cd83a55b51e35@postgresql.org
Backpatch-through: 12

src/backend/commands/tablecmds.c

@@ -6291,6 +6291,10 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 							colDef->colname, RelationGetRelationName(rel))));
 
 			table_close(attrdesc, RowExclusiveLock);
+
+			/* Make the child column change visible */
+			CommandCounterIncrement();
+
 			return InvalidObjectAddress;
 		}
 	}

src/test/regress/expected/inherit.out

@@ -1063,6 +1063,23 @@ Inherits: inht1,
           inhs1
 
 DROP TABLE inhts;
+-- Test for adding a column to a parent table with complex inheritance
+CREATE TABLE inhta ();
+CREATE TABLE inhtb () INHERITS (inhta);
+CREATE TABLE inhtc () INHERITS (inhtb);
+CREATE TABLE inhtd () INHERITS (inhta, inhtb, inhtc);
+ALTER TABLE inhta ADD COLUMN i int;
+NOTICE:  merging definition of column "i" for child "inhtd"
+NOTICE:  merging definition of column "i" for child "inhtd"
+\d+ inhta
+                                   Table "public.inhta"
+ Column |  Type   | Collation | Nullable | Default | Storage | Stats target | Description 
+--------+---------+-----------+----------+---------+---------+--------------+-------------
+ i      | integer |           |          |         | plain   |              | 
+Child tables: inhtb,
+              inhtd
+
+DROP TABLE inhta, inhtb, inhtc, inhtd;
 -- Test for renaming in diamond inheritance
 CREATE TABLE inht2 (x int) INHERITS (inht1);
 CREATE TABLE inht3 (y int) INHERITS (inht1);

src/test/regress/sql/inherit.sql

@@ -353,6 +353,15 @@ ALTER TABLE inhts RENAME d TO dd;
 
 DROP TABLE inhts;
 
+-- Test for adding a column to a parent table with complex inheritance
+CREATE TABLE inhta ();
+CREATE TABLE inhtb () INHERITS (inhta);
+CREATE TABLE inhtc () INHERITS (inhtb);
+CREATE TABLE inhtd () INHERITS (inhta, inhtb, inhtc);
+ALTER TABLE inhta ADD COLUMN i int;
+\d+ inhta
+DROP TABLE inhta, inhtb, inhtc, inhtd;
+
 -- Test for renaming in diamond inheritance
 CREATE TABLE inht2 (x int) INHERITS (inht1);
 CREATE TABLE inht3 (y int) INHERITS (inht1);