pool: update DB after removing a task

* Closes #6315
* If the DB is not updated after a task is removed, then it can be
  respawned in its previous state as the result of upstream output
  completion.

cylc/flow/task_pool.py

@@ -870,6 +870,11 @@ def remove(self, itask, reason=None):
                 msg += " - active job orphaned"
 
             LOG.log(level, f"[{itask}] {msg}")
+
+            # ensure this task is written to the DB before moving on
+            # https://github.com/cylc/cylc-flow/issues/6315
+            self.workflow_db_mgr.process_queued_ops()
+
             del itask
 
     def get_tasks(self) -> List[TaskProxy]:

tests/integration/test_task_pool.py

@@ -2188,3 +2188,58 @@ async def test_expire_dequeue_with_retries(flow, scheduler, start, expire_type):
 
         # the task should also have been removed from the queue
         assert not schd.pool.task_queue_mgr.remove_task(itask)
+
+
+async def test_downstream_complete_before_upstream(
+    flow, scheduler, start, db_select
+):
+    """It should handle an upstream task completing before a downstream task.
+
+    See https://github.com/cylc/cylc-flow/issues/6315
+    """
+    id_ = flow(
+        {
+            'scheduling': {
+                'graph': {
+                    'R1': 'a => b',
+                },
+            },
+        }
+    )
+    schd = scheduler(id_)
+    async with start(schd):
+        # 1/a should be pre-spawned (parentless)
+        a_1 = schd.pool.get_task(IntegerPoint('1'), 'a')
+        assert a_1
+
+        # spawn 1/b (this can happens as the result of request e.g. trigger)
+        b_1 = schd.pool.spawn_task('b', IntegerPoint('1'), {1})
+        schd.pool.add_to_pool(b_1)
+        assert b_1
+
+        # mark 1/b as succeeded
+        schd.task_events_mgr.process_message(b_1, 'INFO', 'succeeded')
+
+        # 1/b should be removed from the pool (completed)
+        assert schd.pool.get_tasks() == [a_1]
+
+        # as a side effect the DB should have been updated
+        assert (
+            TASK_OUTPUT_SUCCEEDED
+            in db_select(
+                schd,
+                # "False" means "do not run the DB update before checking it"
+                False,  # do not change this to "True"
+                'task_outputs',
+                'outputs',
+                name='b',
+                cycle='1',
+            )[0][0]
+        )
+
+        # mark 1/a as succeeded
+        schd.task_events_mgr.process_message(a_1, 'INFO', 'succeeded')
+
+        # 1/a should be removed from the pool (completed)
+        # 1/b should not be re-spawned by the success of 1/a
+        assert schd.pool.get_tasks() == []