BUG #17465: Wrong error message while trying to change system column type.

The following bug has been logged on the website:

Bug reference: 17465
Logged by: Roman Zharkov
Email address: r(dot)zharkov(at)postgrespro(dot)ru
PostgreSQL version: 13.4
Operating system: Windows
Description:

Hello,
accidentally I tried to change a system column data type on Windows and got
a strange error message "no owned sequence found":

psql (15devel)
postgres=# alter table t1 alter column cmin type text;
ERROR: no owned sequence found
postgres=# alter table t1 alter column cmax type text;
ERROR: cannot alter system column "cmax"

This happens due incorrect checking for negative attnum:
if (TupleDescAttr(tupdesc, attnum - 1)->attidentity)
( line
https://github.com/postgres/postgres/blob/1a8b110539efe18803c1fa8aa452a2178dbad9a9/src/backend/parser/parse_utilcmd.c#L3427
)

I think the ATParseTransformCmd() function needs the same check as the check
in the ATPrepAlterColumnType() function.

diff --git a/src/backend/commands/tablecmds.c
b/src/backend/commands/tablecmds.c
index 2cd8546d471..0034a06ea71 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -4630,10 +4630,11 @@ ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd
*cmd,
case AT_AlterColumnType: /* ALTER COLUMN TYPE */
ATSimplePermissions(cmd->subtype, rel,
ATT_TABLE | ATT_COMPOSITE_TYPE | ATT_FOREIGN_TABLE);
+
+ Assert(cmd != NULL);
/* See comments for ATPrepAlterColumnType */
cmd = ATParseTransformCmd(wqueue, tab, rel, cmd, recurse, lockmode,
AT_PASS_UNSET, context);
- Assert(cmd != NULL);
/* Performs own recursion */
ATPrepAlterColumnType(wqueue, tab, rel, recurse, recursing, cmd,
lockmode, context);
@@ -5250,6 +5251,26 @@ ATParseTransformCmd(List **wqueue, AlteredTableInfo
*tab, Relation rel,
List *beforeStmts;
List *afterStmts;
ListCell *lc;
+ char *colName = cmd->name;
+ HeapTuple tuple;
+ Form_pg_attribute attTup;
+ AttrNumber attnum;
+
+ tuple = SearchSysCacheAttName(RelationGetRelid(rel), colName);
+ if (!HeapTupleIsValid(tuple))
+ ereport(ERROR,
+ (errcode(ERRCODE_UNDEFINED_COLUMN),
+ errmsg("column \"%s\" of relation \"%s\" does not exist",
+ colName, RelationGetRelationName(rel))));
+ attTup = (Form_pg_attribute)GETSTRUCT(tuple);
+ attnum = attTup->attnum;
+
+ /* Can't alter a system attribute */
+ if (attnum <= 0)
+ ereport(ERROR,
+ (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+ errmsg("cannot alter system column \"%s\"",
+ colName)));

/* Gin up an AlterTableStmt with just this subcommand and this table */
atstmt->relation =