Squashed commit of the following:

commit ad8231a8f22dd3cd8d887774474f97df2c83e429
Merge: acc0ca3 6fc3c58
Author: Hilary James Oliver <hilary.j.oliver@gmail.com>
Date:   Mon Mar 20 10:27:19 2023 +1300

    Merge branch 'master' into initial-final-graph

commit acc0ca3
Author: Hilary Oliver <hilary.j.oliver@gmail.com>
Date:   Wed Oct 26 12:00:00 2022 +1300

    Style fix.

commit a7170fb
Author: Hilary Oliver <hilary.j.oliver@gmail.com>
Date:   Wed Oct 26 11:53:31 2022 +1300

    Update change log.

commit 9024065
Merge: 858a8c1 b0ff549
Author: Hilary Oliver <hilary.j.oliver@gmail.com>
Date:   Wed Oct 26 11:32:26 2022 +1300

    Merge branch 'master' into initial-final-graph

commit 858a8c1
Author: Hilary James Oliver <hilary.j.oliver@gmail.com>
Date:   Mon Sep 26 23:27:01 2022 +1300

    Fix alpha/omega graph runahead release.

commit b581ab2
Author: Hilary James Oliver <hilary.j.oliver@gmail.com>
Date:   Mon Sep 26 23:26:33 2022 +1300

    Add new func tests.

commit 8d09900
Author: Hilary James Oliver <hilary.j.oliver@gmail.com>
Date:   Mon Sep 26 15:53:42 2022 +1300

    Revert graph sorting change.

commit 75c1763
Author: Hilary James Oliver <hilary.j.oliver@gmail.com>
Date:   Fri Sep 23 22:44:08 2022 +1200

    Adapt integration tests.

commit 1b685a0
Author: Hilary James Oliver <hilary.j.oliver@gmail.com>
Date:   Fri Sep 23 15:55:57 2022 +1200

    Tidy up.

commit 7a62450
Author: Hilary James Oliver <hilary.j.oliver@gmail.com>
Date:   Fri Sep 23 13:20:51 2022 +1200

    Switch to alpha and omega.

commit 129c2e3
Author: Hilary James Oliver <hilary.j.oliver@gmail.com>
Date:   Fri Sep 23 10:32:54 2022 +1200

    Implicit integer ICP if only nocycle graphs.

commit a53ac82
Author: Hilary James Oliver <hilary.j.oliver@gmail.com>
Date:   Thu Sep 22 16:23:16 2022 +1200

    startup and shutdown graphs; needs tidying

CHANGES.md

@@ -237,6 +237,11 @@ workflows with many-to-many dependencies (e.g. `<a> => <b>`).
 
 ### Enhancements
 
+[#5184](https://github.com/cylc/cylc-flow/pull/5184) - Scan for active
+runs of the same workflow at install time.
+
+[#5032](https://github.com/cylc/cylc-flow/pull/5032) - Set a default limit of
+
 [#5229](https://github.com/cylc/cylc-flow/pull/5229) -
 - Added a single command to validate a previously run workflow against changes
   to its source and reinstall a workflow.

changes.d/5090.feat.md

@@ -0,0 +1 @@
+Implement initial and final graphs, distinct from the main cycling graph.

cylc/flow/config.py

@@ -54,6 +54,11 @@
     get_sequence, get_sequence_cls, init_cyclers, get_dump_format,
     INTEGER_CYCLING_TYPE, ISO8601_CYCLING_TYPE
 )
+from cylc.flow.cycling.nocycle import (
+    NocycleSequence,
+    NOCYCLE_SEQ_ALPHA,
+    NOCYCLE_SEQ_OMEGA
+)
 from cylc.flow.id import Tokens
 from cylc.flow.cycling.integer import IntegerInterval
 from cylc.flow.cycling.iso8601 import ingest_time, ISO8601Interval
@@ -277,6 +282,7 @@ def __init__(
         self.start_point: 'PointBase'
         self.stop_point: Optional['PointBase'] = None
         self.final_point: Optional['PointBase'] = None
+        self.nocycle_sequences: Set['NocycleSequence'] = set()
         self.sequences: List['SequenceBase'] = []
         self.actual_first_point: Optional['PointBase'] = None
         self._start_point_for_actual_first_point: Optional['PointBase'] = None
@@ -616,9 +622,16 @@ def prelim_process_graph(self) -> None:
         if (
             'cycling mode' not in self.cfg['scheduling'] and
             self.cfg['scheduling'].get('initial cycle point', '1') == '1' and
-            all(item in ['graph', '1', 'R1'] for item in graphdict)
+            all(
+                item in [
+                    'graph', '1', 'R1',
+                    str(NOCYCLE_SEQ_ALPHA),
+                    str(NOCYCLE_SEQ_OMEGA)
+                ]
+                for item in graphdict
+            )
         ):
-            # Pure acyclic graph, assume integer cycling mode with '1' cycle
+            # Non-cycling graph, assume integer cycling mode with '1' cycle
             self.cfg['scheduling']['cycling mode'] = INTEGER_CYCLING_TYPE
             for key in ('initial cycle point', 'final cycle point'):
                 if key not in self.cfg['scheduling']:
@@ -2106,15 +2119,24 @@ def load_graph(self):
             try:
                 seq = get_sequence(section, icp, fcp)
             except (AttributeError, TypeError, ValueError, CylcError) as exc:
-                if cylc.flow.flags.verbosity > 1:
-                    traceback.print_exc()
-                msg = 'Cannot process recurrence %s' % section
-                msg += ' (initial cycle point=%s)' % icp
-                msg += ' (final cycle point=%s)' % fcp
-                if isinstance(exc, CylcError):
-                    msg += ' %s' % exc.args[0]
-                raise WorkflowConfigError(msg)
-            self.sequences.append(seq)
+                try:
+                    seq = NocycleSequence(section)
+                except ValueError:
+                    if cylc.flow.flags.verbosity > 1:
+                        traceback.print_exc()
+                    msg = (
+                        f"Cannot process recurrence {section}"
+                        f" (initial cycle point={icp})"
+                        f" (final cycle point={fcp})"
+                    )
+                    if isinstance(exc, CylcError):
+                        msg += ' %s' % exc.args[0]
+                    raise WorkflowConfigError(msg)
+                else:
+                    self.nocycle_sequences.add(seq)
+            else:
+                self.sequences.append(seq)
+
             parser = GraphParser(
                 family_map,
                 self.parameters,

cylc/flow/cycling/__init__.py

@@ -345,7 +345,11 @@ def TYPE_SORT_KEY(self) -> int:
     @classmethod
     @abstractmethod  # Note: stacked decorator not strictly enforced in Py2.x
     def get_async_expr(cls, start_point=0):
-        """Express a one-off sequence at the initial cycle point."""
+        """Express a one-off sequence at the initial cycle point.
+
+        Note "async" has nothing to do with asyncio. It was a (bad)
+        name for one-off (non-cycling) graphs in early Cylc versions.
+        """
         pass
 
     @abstractmethod

cylc/flow/cycling/integer.py

@@ -21,7 +21,12 @@
 import re
 
 from cylc.flow.cycling import (
-    PointBase, IntervalBase, SequenceBase, ExclusionBase, parse_exclusion, cmp
+    PointBase,
+    IntervalBase,
+    SequenceBase,
+    ExclusionBase,
+    parse_exclusion,
+    cmp
 )
 from cylc.flow.exceptions import (
     CylcMissingContextPointError,

cylc/flow/cycling/nocycle.py

@@ -0,0 +1,159 @@
+# 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/>.
+
+"""
+Cycling logic for isolated non-cycling startup and shutdown graphs.
+"""
+
+from cylc.flow.cycling import PointBase, SequenceBase, cmp
+
+# TODO: scheduler check DB to be sure alpha and omega sections have run or not.
+
+# cycle point values
+NOCYCLE_PT_ALPHA = "alpha"
+NOCYCLE_PT_OMEGA = "omega"
+
+NOCYCLE_POINTS = (
+    NOCYCLE_PT_ALPHA,
+    NOCYCLE_PT_OMEGA
+)
+
+CYCLER_TYPE_NOCYCLE = "nocycle"
+CYCLER_TYPE_SORT_KEY_NOCYCLE = 1
+
+
+class NocyclePoint(PointBase):
+    """A string-valued point."""
+
+    TYPE = CYCLER_TYPE_NOCYCLE
+    TYPE_SORT_KEY = CYCLER_TYPE_SORT_KEY_NOCYCLE
+
+    __slots__ = ('value')
+
+    def __init__(self, value: str) -> None:
+        if value not in [NOCYCLE_PT_ALPHA, NOCYCLE_PT_OMEGA]:
+            raise ValueError(f"Illegal Nocycle value {value}")
+        self.value = value
+
+    def __hash__(self):
+        return hash(self.value)
+
+    def __eq__(self, other):
+        return str(other) == self.value
+
+    def __le__(self, other):
+        """less than or equal only if equal."""
+        return str(other) == self.value
+
+    def __lt__(self, other):
+        """never less than."""
+        return False
+
+    def __gt__(self, other):
+        """never greater than."""
+        return False
+
+    def __str__(self):
+        return self.value
+
+    def _cmp(self, other):
+        return cmp(int(self), int(other))
+
+    def add(self, other):
+        # NOT USED
+        return None
+
+    def sub(self, other):
+        # NOT USED
+        return None
+
+
+class NocycleSequence(SequenceBase):
+    """A single point sequence."""
+
+    def __init__(self, dep_section, p_context_start=None, p_context_stop=None):
+        """Workflow cycling context is ignored."""
+        self.point = NocyclePoint(dep_section)
+
+    def __hash__(self):
+        return hash(str(self.point))
+
+    def is_valid(self, point):
+        """Is point on-sequence and in-bounds?"""
+        return str(point) == self.point
+
+    def get_first_point(self, point):
+        """First point is the only point"""
+        return self.point
+
+    def get_next_point(self, point):
+        """There is no next point"""
+        return None
+
+    def get_next_point_on_sequence(self, point):
+        """There is no next point"""
+        return None
+
+    def __eq__(self, other):
+        try:
+            return other.point == self.point
+        except AttributeError:
+            # (other is not a nocycle sequence)
+            return False
+
+    def __str__(self):
+        return str(self.point)
+
+    def TYPE(self):
+        raise NotImplementedError
+
+    def TYPE_SORT_KEY(self):
+        raise NotImplementedError
+
+    def get_async_expr(cls, start_point=0):
+        raise NotImplementedError
+
+    def get_interval(self):
+        """Return the cycling interval of this sequence."""
+        raise NotImplementedError
+
+    def get_offset(self):
+        """Deprecated: return the offset used for this sequence."""
+        raise NotImplementedError
+
+    def set_offset(self, i_offset):
+        """Deprecated: alter state to offset the entire sequence."""
+        raise NotImplementedError
+
+    def is_on_sequence(self, point):
+        """Is point on-sequence, disregarding bounds?"""
+        raise NotImplementedError
+
+    def get_prev_point(self, point):
+        """Return the previous point < point, or None if out of bounds."""
+        raise NotImplementedError
+
+    def get_nearest_prev_point(self, point):
+        """Return the largest point < some arbitrary point."""
+        raise NotImplementedError
+
+    def get_stop_point(self):
+        """Return the last point in this sequence, or None if unbounded."""
+        raise NotImplementedError
+
+
+NOCYCLE_SEQ_ALPHA = NocycleSequence(NOCYCLE_PT_ALPHA)
+NOCYCLE_SEQ_OMEGA = NocycleSequence(NOCYCLE_PT_OMEGA)