Avoid invalid array reference in transformAlterTableStmt().

Don't try to look at the attidentity field of system attributes,
because they're not there in the TupleDescAttr array.  Sometimes
this is harmless because we accidentally pick up a zero, but
otherwise we'll report "no owned sequence found" from an attempt
to alter a system attribute.  (It seems possible that a SIGSEGV
could occur, too, though I've not seen it in testing.)

It's not in this function's charter to complain that you can't
alter a system column, so instead just hard-wire an assumption
that system attributes aren't identities.  I didn't bother with
a regression test because the appearance of the bug is very
erratic.

Per bug #17465 from Roman Zharkov.  Back-patch to all supported
branches.  (There's not actually a live bug before v12, because
before that get_attidentity() did the right thing anyway.
But for consistency I changed the test in the older branches too.)

Discussion: https://postgr.es/m/17465-f2a554a6cb5740d3@postgresql.org

diff --git a/src/backend/parser/parse_utilcmd.c b/src/backend/parser/parse_utilcmd.c
index 2826559d09b746bd3d7c02dfc68d987d0137550d..1a64a522798919c7e1252867fd4446938740cbe8 100644 (file)
--- a/src/backend/parser/parse_utilcmd.c
+++ b/src/backend/parser/parse_utilcmd.c
@@ -3424,7 +3424,8 @@ transformAlterTableStmt(Oid relid, AlterTableStmt *stmt,
                                 errmsg("column \"%s\" of relation \"%s\" does not exist",
                                        cmd->name, RelationGetRelationName(rel))));
 
-                   if (TupleDescAttr(tupdesc, attnum - 1)->attidentity)
+                   if (attnum > 0 &&
+                       TupleDescAttr(tupdesc, attnum - 1)->attidentity)
                    {
                        Oid         seq_relid = getIdentitySequence(relid, attnum, false);
                        Oid         typeOid = typenameTypeId(pstate, def->typeName);