Implement cylc pause

Now:
- cylc pause/play pauses/resumes the workflow
- cylc hold/release holds/releases tasks

pause/play is decoupled from hold/release, i.e. they don't affect task 
states

cylc/flow/etc/cylc-bash-completion

@@ -38,7 +38,7 @@ _cylc() {
     cur="${COMP_WORDS[COMP_CWORD]}"
     sec="${COMP_WORDS[1]}"
     opts="$(cylc scan -t name 2>/dev/null)"
-    suite_cmds="broadcast|bcast|cat-log|check-versions|clean|compare|diff|dump|edit|ext-trigger|external-trigger|get-suite-config|get-config|get-suite-version|get-cylc-version|graph|graph-diff|hold|insert|install|kill|list|log|ls|tui|ping|play|poll|print|release|unhold|reload|remove|report-timings|reset|scan|search|grep|set-verbosity|show|set-outputs|stop|shutdown|single|suite-state|test-battery|trigger|validate|view|warranty"
+    suite_cmds="broadcast|bcast|cat-log|check-versions|clean|compare|diff|dump|edit|ext-trigger|external-trigger|get-suite-config|get-config|get-suite-version|get-cylc-version|graph|graph-diff|hold|insert|install|kill|list|log|ls|tui|pause|ping|play|poll|print|release|unhold|reload|remove|report-timings|reset|scan|search|grep|set-verbosity|show|set-outputs|stop|shutdown|single|suite-state|test-battery|trigger|validate|view|warranty"
 
 
     if [[ ${COMP_CWORD} -eq 1 ]]; then

cylc/flow/network/resolvers.py

@@ -21,6 +21,7 @@
 import logging
 import queue
 from time import time
+from typing import Iterable, Optional, Tuple, TYPE_CHECKING
 from uuid import uuid4
 
 from graphene.utils.str_converters import to_snake_case
@@ -34,6 +35,10 @@
     NodesEdges, PROXY_NODES, SUB_RESOLVERS, parse_node_id, sort_elements
 )
 
+if TYPE_CHECKING:
+    from cylc.flow.scheduler import Scheduler
+
+
 logger = logging.getLogger(__name__)
 
 DELTA_SLEEP_INTERVAL = 0.5
@@ -436,7 +441,7 @@ async def subscribe_delta(self, root, info, args):
 class Resolvers(BaseResolvers):
     """Workflow Service context GraphQL query and mutation resolvers."""
 
-    schd = None
+    schd: 'Scheduler' = None
 
     def __init__(self, data, **kwargs):
         super().__init__(data)
@@ -518,8 +523,13 @@ def broadcast(
                 cutoff)
         raise ValueError('Unsupported broadcast mode')
 
-    def hold(self, tasks=None, time=None):
-        """Hold the workflow."""
+    def hold(
+            self, tasks: Optional[Iterable[str]] = None,
+            time: Optional[str] = None
+    ) -> Tuple[bool, str]:
+        """Hold tasks."""
+        if (tasks and time) or not (tasks or time):
+            return (False, 'Argument must be either tasks or time (not both)')
         self.schd.command_queue.put((
             'hold',
             tuple(),
@@ -530,6 +540,11 @@ def hold(self, tasks=None, time=None):
         ))
         return (True, 'Command queued')
 
+    def pause(self) -> Tuple[bool, str]:
+        """Pause the workflow."""
+        self.schd.command_queue.put(('pause', tuple(), {}))
+        return (True, 'Command queued')
+
     def kill_tasks(self, tasks):
         """Kill task jobs.
 
@@ -647,17 +662,23 @@ def reload_suite(self):
         self.schd.command_queue.put(("reload_suite", (), {}))
         return (True, 'Command queued')
 
-    def release(self, tasks=None):
-        """Release (un-hold) the workflow."""
+    def release(
+            self, tasks: Optional[Iterable[str]] = None) -> Tuple[bool, str]:
+        """Release held tasks."""
         self.schd.command_queue.put((
-            "release",
-            (),
+            'release',
+            tuple(),
             filter_none({
                 'ids': tasks
             })
         ))
         return (True, 'Command queued')
 
+    def resume(self) -> Tuple[bool, str]:
+        """Resume the workflow."""
+        self.schd.command_queue.put(('resume', tuple(), {}))
+        return (True, 'Command queued')
+
     def set_verbosity(self, level):
         """Set suite verbosity to new level (for suite logs).
 

cylc/flow/network/scan.py

@@ -343,8 +343,8 @@ async def graphql_query(flow, fields, filters=None):
                # state must be running
                [('state',), 'running']
 
-               # state must be running or held
-               [('state',), ('running', 'held')]
+               # state must be running or paused
+               [('state',), ('running', 'paused')]
 
     """
     query = f'query {{ workflows(ids: ["{flow["name"]}"]) {{ {fields} }} }}'

cylc/flow/network/schema.py

@@ -1508,24 +1508,37 @@ class Arguments:
 class Hold(Mutation):
     class Meta:
         description = sstrip('''
-            Hold a workflow or tasks within it.
+            Hold tasks within a workflow.
         ''')
         resolver = partial(mutator, command='hold')
 
     class Arguments:
         workflows = List(WorkflowID, required=True)
         tasks = List(
             NamespaceIDGlob,
-            description='Hold the specified tasks rather than the workflow.'
+            description='Hold matching tasks.'
         )
         time = TimePoint(description=sstrip('''
-            Get the workflow to hold after the specified wallclock time
+            Hold all tasks only after the specified wallclock time
             has passed.
         '''))
 
     result = GenericScalar()
 
 
+class Pause(Mutation):
+    class Meta:
+        description = sstrip('''
+            Pause a workflow.
+        ''')
+        resolver = partial(mutator, command='pause')
+
+    class Arguments:
+        workflows = List(WorkflowID, required=True)
+
+    result = GenericScalar()
+
+
 class Ping(Mutation):
     class Meta:
         description = sstrip('''
@@ -1572,7 +1585,7 @@ class Arguments:
 class Release(Mutation):
     class Meta:
         description = sstrip('''
-            Release a held workflow or tasks within it.
+            Release held tasks within a workflow.
 
             See also the opposite command `hold`.
         ''')
@@ -1583,13 +1596,29 @@ class Arguments:
         tasks = List(
             NamespaceIDGlob,
             description=sstrip('''
-                Release matching tasks rather than the workflow as whole.
+                Release matching tasks or, if not specified, release all tasks
+                and remove the hold point if set.
             ''')
         )
 
     result = GenericScalar()
 
 
+class Resume(Mutation):
+    class Meta:
+        description = sstrip('''
+            Resume a paused workflow.
+
+            See also the opposite command `pause`.
+        ''')
+        resolver = partial(mutator, command='resume')
+
+    class Arguments:
+        workflows = List(WorkflowID, required=True)
+
+    result = GenericScalar()
+
+
 class Reload(Mutation):
     class Meta:
         description = sstrip('''
@@ -1824,18 +1853,20 @@ class Mutations(ObjectType):
     # workflow actions
     broadcast = _mut_field(Broadcast)
     ext_trigger = _mut_field(ExtTrigger)
-    hold = _mut_field(Hold)
     message = _mut_field(Message)
+    pause = _mut_field(Pause)
     ping = _mut_field(Ping)
-    release = _mut_field(Release)
     reload = _mut_field(Reload)
+    resume = _mut_field(Resume)
     set_verbosity = _mut_field(SetVerbosity)
     set_graph_window_extent = _mut_field(SetGraphWindowExtent)
     stop = _mut_field(Stop)
 
     # task actions
+    hold = _mut_field(Hold)
     kill = _mut_field(Kill)
     poll = _mut_field(Poll)
+    release = _mut_field(Release)
     remove = _mut_field(Remove)
     set_outputs = _mut_field(SetOutputs)
     trigger = _mut_field(Trigger)