allow workflow config to unset global events config (#6518)

changes.d/6518.fix.md

@@ -0,0 +1 @@
+Allow setting empty values in `flow.cylc[scheduler][events]` to override the global configuration.

cylc/flow/parsec/config.py

@@ -35,6 +35,10 @@
     from optparse import Values
 
 
+class DefaultList(list):
+    """List subclass to indicate unassigned list values in expanded config."""
+
+
 class ParsecConfig:
     """Object wrapper for parsec functions."""
 
@@ -112,7 +116,7 @@ def expand(self) -> None:
                     else:
                         if node.default == ConfigNode.UNSET:
                             if node.vdr and node.vdr.endswith('_LIST'):
-                                defs[node.name] = []
+                                defs[node.name] = DefaultList()
                             else:
                                 defs[node.name] = None
                         else:

cylc/flow/workflow_events.py

@@ -24,6 +24,7 @@
 from cylc.flow.cfgspec.glbl_cfg import glbl_cfg
 from cylc.flow.hostuserutil import get_host, get_user
 from cylc.flow.log_diagnosis import run_reftest
+from cylc.flow.parsec.config import DefaultList
 from cylc.flow.subprocctx import SubProcContext
 
 if TYPE_CHECKING:
@@ -198,7 +199,7 @@ def process_mail_footer(
     """
     try:
         return (mail_footer_tmpl + '\n') % template_vars
-    except (KeyError, ValueError):
+    except (KeyError, TypeError, ValueError):
         LOG.warning(
             f'Ignoring bad mail footer template: {mail_footer_tmpl}'
         )
@@ -235,7 +236,7 @@ def get_events_conf(
             glbl_cfg().get(['scheduler', 'mail'])
         ):
             value = getter.get(key)
-            if value is not None and value != []:
+            if value is not None and not isinstance(value, DefaultList):
                 return value
         return default
 
@@ -328,7 +329,7 @@ def _run_event_custom_handlers(self, schd, template_variables, event):
             cmd_key = ('%s-%02d' % (self.WORKFLOW_EVENT_HANDLER, i), event)
             try:
                 cmd = handler % (template_variables)
-            except KeyError as exc:
+            except (KeyError, TypeError, ValueError) as exc:
                 message = f'{cmd_key} bad template: {handler}\n{exc}'
                 LOG.error(message)
                 continue

tests/unit/test_workflow_events.py

@@ -34,6 +34,9 @@
         ('handlers', True, True, ['stall']),
         ('handlers', False, True, None),
         ('handlers', False, False, None),
+        ('mail events', True, True, []),
+        ('mail events', False, True, ['abort']),
+        ('mail events', False, False, None),
         ('from', True, True, 'docklands@railway'),
         ('from', False, True, 'highway@mixture'),
         ('from', False, False, None),
@@ -55,13 +58,14 @@ def test_get_events_handler(
                     from = highway@mixture
                 [[events]]
                     abort on workflow timeout = True
+                    mail events = abort
             '''
         )
 
     config = SimpleNamespace()
     config.cfg = {
         'scheduler': {
-            'events': {'handlers': ['stall']},
+            'events': {'handlers': ['stall'], 'mail events': []},
             'mail': {'from': 'docklands@railway'},
         } if workflow_cfg else {'events': {}}
     }