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 7cf5944ab377392dfb45072ee7cb516f925898d8..616de81570fafc5e686fcc95359d1c0117dfe567 100644 (file)
--- a/src/backend/parser/parse_utilcmd.c
+++ b/src/backend/parser/parse_utilcmd.c
@@ -3042,7 +3042,7 @@ transformAlterTableStmt(Oid relid, AlterTableStmt *stmt,
                                         * if attribute not found, something will error about it
                                         * later
                                         */
-                                       if (attnum != InvalidAttrNumber && get_attidentity(relid, attnum))
+                                       if (attnum > 0 && get_attidentity(relid, attnum))
                                        {
                                                Oid                     seq_relid = getOwnedSequence(relid, attnum);
                                                Oid                     typeOid = typenameTypeId(pstate, def->typeName);