Test what happens to workflow run_mode upon restart.

Raise an exception if the user tries to change mode.

changes.d/5789.fix.md

@@ -0,0 +1 @@
+Stop users changing run modes on restart.

cylc/flow/rundb.py

@@ -606,6 +606,19 @@ def select_workflow_params_restart_count(self):
         result = self.connect().execute(stmt).fetchone()
         return int(result[0]) if result else 0
 
+    def select_workflow_params_run_mode(self):
+        """Return original run_mode for workflow_params."""
+        stmt = rf"""
+            SELECT
+                value
+            FROM
+                {self.TABLE_WORKFLOW_PARAMS}
+            WHERE
+                key == 'run_mode'
+        """  # nosec (table name is code constant)
+        result = self.connect().execute(stmt).fetchone()
+        return result[0] if result else None
+
     def select_workflow_template_vars(self, callback):
         """Select from workflow_template_vars.
 

cylc/flow/scheduler.py

@@ -434,6 +434,18 @@ async def configure(self, params):
             # Mark this exc as expected (see docstring for .schd_expected):
             exc.schd_expected = True
             raise exc
+
+        # Prevent changing mode on restart.
+        if self.is_restart:
+            # check run mode against db
+            og_run_mode = self.workflow_db_mgr.get_pri_dao(
+            ).select_workflow_params_run_mode()
+            run_mode = self.config.run_mode()
+            if run_mode != og_run_mode:
+                raise InputError(
+                    f'This workflow was originally run in {og_run_mode} mode:'
+                    f' Will not restart in {run_mode} mode.')
+
         self.profiler.log_memory("scheduler.py: after load_flow_file")
 
         self.workflow_db_mgr.on_workflow_start(self.is_restart)

tests/integration/test_mode_on_restart.py

@@ -14,34 +14,41 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 """What happens to the mode on restart?
-
-+--------------------+--------+-------+-------+
-| ↓restart \ start → | live   | sim   | dummy |
-+====================+========+=======+=======+
-| live               | live   | sim * | ???   |
-| sim                | live * | sim   | ???   |
-| dummy              | ???    | ???   | dummy |
-+--------------------+--------+-------+-------+
-
-* A warning ought to be emitted, since the user doesn't otherwise know
-  what's happening.
 """
 
 import pytest
 
+from cylc.flow.exceptions import InputError
+
+
+MODES = (('live'), ('simulation'), ('dummy'))
+
 
-@pytest.mark.parametrize('mode_before', (('live'), ('simulation')))
-@pytest.mark.parametrize('mode_after', (('live'), ('simulation')))
+@pytest.mark.parametrize('mode_before', MODES)
+@pytest.mark.parametrize('mode_after', MODES)
 async def test_restart_mode(
-    flow, scheduler, start, one_conf, mode_before, mode_after
+    flow, run, scheduler, start, one_conf,
+    mode_before, mode_after
 ):
-    """Restarting a workflow in live mode leads to workflow in live mode
+    """Restarting a workflow in live mode leads to workflow in live mode.
+
+    N.B - we need use run becuase the check in question only happens
+    on start.
     """
     id_ = flow(one_conf)
     schd = scheduler(id_, run_mode=mode_before)
     async with start(schd):
         assert schd.config.run_mode() == mode_before
 
-    schd.options.run_mode = mode_after
-    async with start(schd):
-        assert schd.config.run_mode() == mode_before
+    schd = scheduler(id_, run_mode=mode_after)
+
+    if mode_before == mode_after:
+        # Restarting in the same mode is fine.
+        async with run(schd):
+            assert schd.config.run_mode() == mode_before
+    else:
+        # Restarting in a new mode is not:
+        errormsg = f'^This.*{mode_before} mode: Will.*{mode_after} mode.$'
+        with pytest.raises(InputError, match=errormsg):
+            async with run(schd):
+                pass