Add func test.

tests/functional/reload/27-stall-retrigger.t

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+#!/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/>.
+#-------------------------------------------------------------------------------
+
+# Test retriggering a failed task after fixing the bug and reloading.
+# It should run correctly with the updated settings.
+
+# https://github.com/cylc/cylc-flow/issues/5103
+
+. "$(dirname "$0")/test_header"
+set_test_number 2
+reftest

tests/functional/reload/27-stall-retrigger/bin/stall-handler.sh

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+#!/bin/bash
+
+# Change "script = false" -> "true" in 1/foo, then reload and retrigger it.
+
+if grep "\[command\] reload_workflow" "${CYLC_WORKFLOW_LOG_DIR}/log" >/dev/null; then
+    # Abort if not the first call (avoid an endless loop if the reload does not
+    # have the intended effect).
+    cylc stop --now --now "${CYLC_WORKFLOW_ID}"
+    exit 1
+fi
+sed -i "s/false/true/" "${CYLC_WORKFLOW_RUN_DIR}/flow.cylc"
+cylc reload "${CYLC_WORKFLOW_ID}"
+cylc trigger "${CYLC_WORKFLOW_ID}//1/foo"

tests/functional/reload/27-stall-retrigger/flow.cylc

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+# Use a stall handler to fix and reload the workflow config, then retrigger the
+# failed task, which should run successfully with the new settings.
+[scheduler]
+   [[events]]
+       stall handlers = stall-handler.sh
+       expected task failures = 1/foo
+[scheduling]
+    [[graph]]
+        R1 = "foo => bar"
+[runtime]
+    [[foo]]
+        script = false
+    [[bar]]

tests/functional/reload/27-stall-retrigger/reference.log

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+1/foo -triggered off [] in flow 1
+1/foo -triggered off [] in flow 1
+1/bar -triggered off ['1/foo'] in flow 1