Fix a bug where suicide triggers could prevent initial cycle point ta…

…sks spawning.

changes.d/6602.fix.md

@@ -0,0 +1 @@
+Fix a bug where suicide triggers could prevent initial cycle point tasks spawning.

cylc/flow/taskdef.py

@@ -99,9 +99,10 @@ def generate_graph_parents(
 ) -> Dict['SequenceBase', List[TaskTuple]]:
     """Determine concrete graph parents of task tdef at point.
 
-    Infer parents be reversing upstream triggers that lead to point/task.
+    Infer parents by reversing upstream triggers that lead to point/task.
     """
     graph_parents: Dict['SequenceBase', List[TaskTuple]] = {}
+
     for seq, triggers in tdef.graph_parents.items():
         if not seq.is_valid(point):
             # Don't infer parents if the trigger belongs to a sequence that
@@ -339,8 +340,8 @@ def has_only_abs_triggers(self, point):
                     if (
                         trig.offset_is_absolute or
                         trig.offset_is_from_icp or
-                        # Don't count self-suicide as a normal trigger.
-                        dep.suicide and trig.task_name == self.name
+                        # Don't count suicide as a normal trigger:
+                        dep.suicide
                     ):
                         has_abs = True
                     else:

tests/integration/test_taskdef.py

@@ -0,0 +1,44 @@
+# THIS FILE IS PART OF THE CYLC WORKFLOW ENGINE.
+# Copyright (C) NIWA & British Crown (Met Office) & Contributors.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+
+async def test_almost_self_suicide(flow, scheduler, start):
+    """Suicide triggers should not count as upstream tasks when looking
+    to spawn parentless tasks.
+
+    https://github.com/cylc/cylc-flow/issues/6594
+
+    For the example under test, pre-requisites for ``!a`` should not be
+    considered the same as pre-requisites for ``a``. If the are then then
+    is parentless return false for all cases of ``a`` not in the inital cycle
+    and subsequent cycles never run.
+    """
+    wid = flow({
+        'scheduler': {'cycle point format': '%Y'},
+        'scheduling': {
+            'initial cycle point': 1990,
+            'final cycle point': 1992,
+            'graph': {
+                'R1': 'install_cold',
+                'P1Y': 'install_cold[^] => a? => b?\nb:fail? => !a?'
+            }
+        }
+    })
+    schd = scheduler(wid)
+    async with start(schd):
+        tasks = [str(t) for t in schd.pool.get_tasks()]
+        for task in ['1990/a:waiting', '1991/a:waiting', '1992/a:waiting']:
+            assert task in tasks