add sequential arg option and to wall_clock as True

cylc/flow/cfgspec/workflow.py

@@ -791,8 +791,8 @@ def get_script_common_text(this: str, example: Optional[str] = None):
             Instead of out to the runahead limit (default: ``False``).
 
             This workflow wide default can be overridden by a reserved
-            keyword argument in the xtrigger function declaration
-            (``sequential=True/False``).
+            keyword argument in the xtrigger function declaration and/or
+            function (``sequential=True/False``).
 
             The presence of one sequential xtrigger on a parentless task with
             multiple xtriggers will cause sequential behavior.

cylc/flow/xtrigger_mgr.py

@@ -311,15 +311,27 @@ def validate_xtrigger(label: str, fctx: SubFuncContext, fdir: str) -> None:
                 fname,
                 f"'{fname}' not callable in xtrigger module '{fname}'",
             )
-        if 'sequential' in getfullargspec(func).args:
-            raise XtriggerConfigError(
-                label,
-                fname,
-                (
-                    f"xtrigger module '{fname}' contains reserved"
-                    " argument name 'sequential'"
-                ),
-            )
+        x_argspec = getfullargspec(func)
+        if 'sequential' in x_argspec.args:
+            if (
+                x_argspec.defaults is None
+                or not isinstance(
+                    x_argspec.defaults[x_argspec.args.index('sequential')],
+                    bool
+                )
+            ):
+                raise XtriggerConfigError(
+                    label,
+                    fname,
+                    (
+                        f"xtrigger module '{fname}' contains reserved argument"
+                        " name 'sequential' that has no boolean default"
+                    ),
+                )
+            elif 'sequential' not in fctx.func_kwargs:
+                fctx.func_kwargs['sequential'] = x_argspec.defaults[
+                    x_argspec.args.index('sequential')
+                ]
 
         # Check any string templates in the function arg values (note this
         # won't catch bad task-specific values - which are added dynamically).

cylc/flow/xtriggers/wall_clock.py

@@ -19,11 +19,13 @@
 from time import time
 
 
-def wall_clock(trigger_time=None):
+def wall_clock(trigger_time=None, sequential=True):
     """Return True after the desired wall clock time, False.
 
     Args:
         trigger_time (int):
             Trigger time as seconds since Unix epoch.
+        sequential (bool):
+            Used by the workflow to flag corresponding xtriggers as sequential.
     """
     return time() > trigger_time