runahead: handle no sequences within start-stop cycle interval

changes.d/6638.fix.md

@@ -0,0 +1 @@
+Fixed possible crash when restarting a workflow after changing the graph.

cylc/flow/task_pool.py

@@ -408,8 +408,11 @@ def compute_runahead(self, force=False) -> bool:
             self._prev_runahead_base_point = base_point
 
         if count_cycles:
-            # (len(list) may be less than ilimit due to sequence end)
-            limit_point = sorted(sequence_points)[:ilimit + 1][-1]
+            if not sequence_points:
+                limit_point = base_point
+            else:
+                # (len(list) may be less than ilimit due to sequence end)
+                limit_point = sorted(sequence_points)[:ilimit + 1][-1]
         else:
             limit_point = max(sequence_points)
 

tests/integration/test_task_pool.py

@@ -1800,6 +1800,35 @@ async def test_compute_runahead_with_no_tasks(flow, scheduler, run):
         assert schd.pool.get_tasks() == []
 
 
+async def test_compute_runahead_with_no_sequences(flow, scheduler, start, run, complete):
+    """It should handle no sequences within the start-stop cycle range.
+
+    See https://github.com/cylc/cylc-flow/issues/6154
+    """
+    cfg = {
+        'scheduling': {
+            'cycling mode': 'integer',
+            'initial cycle point': '1',
+            'graph': {
+               'P1': 'foo[-P1] => foo'
+            }
+        }
+    }
+    id_ = flow(cfg)
+    schd = scheduler(id_, paused_start=False)
+    async with run(schd):
+        await complete(schd, '2/foo')
+
+    cfg['scheduling']['graph']['R1'] = cfg['scheduling']['graph']['P1']
+    cfg['scheduling']['graph'].pop('P1')
+    flow(cfg, workflow_id=id_)
+
+    schd = scheduler(id_, paused_start=False)
+    async with start(schd):
+        schd.pool.compute_runahead()
+        assert schd.pool.runahead_limit_point == IntegerPoint('3')
+
+
 @pytest.mark.parametrize('rhlimit', ['P2D', 'P2'])
 @pytest.mark.parametrize('compat_mode', ['compat-mode', 'normal-mode'])
 async def test_runahead_future_trigger(