A no-flow task should not merge and retrigger incomplete children

CHANGES.md

@@ -35,6 +35,9 @@ Maintenance release.
 
 ### Fixes
 
+[#5146](https://github.com/cylc/cylc-flow/pull/5146) - no-flow tasks should not
+retrigger incomplete children.
+
 [#5104](https://github.com/cylc/cylc-flow/pull/5104) - Fix retriggering of
 failed tasks after a reload.
 

cylc/flow/task_pool.py

@@ -1910,9 +1910,11 @@ def merge_flows(self, itask: TaskProxy, flow_nums: 'FlowNums') -> None:
 
         This also performs required spawning / state changing for edge cases.
         """
-        if flow_nums == itask.flow_nums:
-            # Don't do anything if trying to spawn the same task in the same
-            # flow. This arises downstream of an AND trigger (if "A & B => C"
+        if not flow_nums or (flow_nums == itask.flow_nums):
+            # Don't do anything if:
+            # 1. merging from a no-flow task, or
+            # 2. trying to spawn the same task in the same flow. This arises
+            # downstream of an AND trigger (if "A & B => C"
             # and A spawns C first, B will find C is already in the pool),
             # and via suicide triggers ("A =>!A": A tries to spawn itself).
             return

tests/functional/flow-triggers/13-noflow-nomerge.t

@@ -0,0 +1,47 @@
+#!/usr/bin/env bash
+# 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/>.
+#-------------------------------------------------------------------------------
+
+. "$(dirname "$0")/test_header"
+set_test_number 7
+
+install_workflow "${TEST_NAME_BASE}" "${TEST_NAME_BASE}"
+DB="${WORKFLOW_RUN_DIR}/log/db"
+
+run_ok "${TEST_NAME_BASE}-validate" cylc validate "${WORKFLOW_NAME}"
+run_ok "${TEST_NAME_BASE}-run" cylc play "${WORKFLOW_NAME}"
+poll_grep_workflow_log "Workflow stalled"
+
+run_ok "${TEST_NAME_BASE}-trigger" cylc trigger --flow=none "${WORKFLOW_NAME}//1/a"
+poll_grep_workflow_log -E "1/a running job:02 flows:none.*=> succeeded"
+
+cylc stop --now --now --max-polls=5 --interval=2 "$WORKFLOW_NAME"
+
+TEST_NAME="${TEST_NAME_BASE}-count"
+QUERY="SELECT COUNT(*) FROM task_states WHERE name=='a'"
+run_ok "${TEST_NAME}" sqlite3 "${DB}" "$QUERY"
+cmp_ok "${TEST_NAME}.stdout" <<__END__
+2
+__END__
+
+QUERY="SELECT COUNT(*) FROM task_states WHERE name=='b'"
+run_ok "${TEST_NAME}" sqlite3 "${DB}" "$QUERY"
+cmp_ok "${TEST_NAME}.stdout" <<__END__
+1
+__END__
+
+purge

tests/functional/flow-triggers/13-noflow-nomerge/flow.cylc

@@ -0,0 +1,7 @@
+[scheduling]
+   [[graph]]
+       R1 = "a => b"
+[runtime]
+   [[a]]
+   [[b]]
+       script = false  # die as incomplete