Skip deleting options if columnar.options is already dropped (#5458)

Drop extension might cascade to columnar.options before dropping a
columnar table. In that case, we were getting below error when opening
columnar.options to delete records for the columnar table that we are
about to drop.: "ERROR:  could not open relation with OID 0".

I somehow reproduced this bug easily when upgrading pg, that is why
adding added the test to after_pg_upgrade_schedule.

(cherry picked from commit 25024b776e)

 Conflicts:
	src/test/regress/after_pg_upgrade_schedule

src/backend/columnar/columnar_metadata.c

@@ -335,8 +335,13 @@ DeleteColumnarTableOptions(Oid regclass, bool missingOk)
 	 */
 	Assert(!IsBinaryUpgrade);
 
-	Relation columnarOptions = relation_open(ColumnarOptionsRelationId(),
-											 RowExclusiveLock);
+	Relation columnarOptions = try_relation_open(ColumnarOptionsRelationId(),
+												 RowExclusiveLock);
+	if (columnarOptions == NULL)
+	{
+		/* extension has been dropped */
+		return false;
+	}
 
 	/* find existing item to remove */
 	ScanKeyData scanKey[1] = { 0 };

src/test/regress/after_pg_upgrade_schedule

@@ -1 +1,5 @@
-test: upgrade_basic_after upgrade_columnar_after upgrade_type_after upgrade_ref2ref_after upgrade_distributed_function_after upgrade_rebalance_strategy_after upgrade_list_citus_objects
+test: upgrade_basic_after upgrade_type_after upgrade_ref2ref_after upgrade_distributed_function_after upgrade_rebalance_strategy_after upgrade_list_citus_objects
+
+# This attempts dropping citus extension (and rollbacks), so please do
+# not run in parallel with any other tests.
+test: upgrade_columnar_after

src/test/regress/expected/upgrade_columnar_after.out

@@ -247,3 +247,15 @@ BEGIN;
 	-- since we run "after schedule" twice, rollback the transaction
 	-- to avoid getting "table already exists" errors
 ROLLBACK;
+BEGIN;
+  -- Show that we can still drop the extension after upgrading
+  SET client_min_messages TO WARNING;
+  -- Drop extension migth cascade to columnar.options before dropping a
+  -- columnar table. In that case, we were getting below error when opening
+  -- columnar.options to delete records for the columnar table that we are
+  -- about to drop.: "ERROR:  could not open relation with OID 0".
+  --
+  -- I somehow reproduced this bug easily when upgrading pg, that is why
+  -- adding the test to this file.
+  DROP EXTENSION citus CASCADE;
+ROLLBACK;

src/test/regress/sql/upgrade_columnar_after.sql

@@ -117,3 +117,17 @@ BEGIN;
 	-- since we run "after schedule" twice, rollback the transaction
 	-- to avoid getting "table already exists" errors
 ROLLBACK;
+
+BEGIN;
+  -- Show that we can still drop the extension after upgrading
+  SET client_min_messages TO WARNING;
+
+  -- Drop extension migth cascade to columnar.options before dropping a
+  -- columnar table. In that case, we were getting below error when opening
+  -- columnar.options to delete records for the columnar table that we are
+  -- about to drop.: "ERROR:  could not open relation with OID 0".
+  --
+  -- I somehow reproduced this bug easily when upgrading pg, that is why
+  -- adding the test to this file.
+  DROP EXTENSION citus CASCADE;
+ROLLBACK;