Flatten the client-request handler loop with `match:`

piker/clearing/_ems.py

@@ -822,34 +822,25 @@ async def process_client_order_cmds(
 
     # cmd: dict
     async for cmd in client_order_stream:
-
         log.info(f'Received order cmd:\n{pformat(cmd)}')
 
-        action = cmd['action']
         oid = cmd['oid']
-
-        # TODO: make ``tractor.MsgStream`` a frozen type again such that it
-        # can be stored in sets like the old context was.
-        # wait, maybe this **is** already working thanks to our parent
-        # `trio` type?
-
         # register this stream as an active dialogue for this order id
         # such that translated message from the brokerd backend can be
         # routed (relayed) to **just** that client stream (and in theory
         # others who are registered for such order affiliated msgs).
         client_dialogues[oid] = client_order_stream
-
         reqid = dark_book._ems2brokerd_ids.inverse.get(oid)
         live_entry = dark_book._ems_entries.get(oid)
 
-        # TODO: can't wait for this stuff to land in 3.10
+        match cmd:
-        # https://www.python.org/dev/peps/pep-0636/#going-to-the-cloud-mappings
-        if action in ('cancel',):
 
-            # check for live-broker order
+            # existing live-broker order cancel
-            if live_entry:
+            case {
+                'action': 'cancel',
+                'oid': oid,
+            } if live_entry:
                 reqid = live_entry.reqid
-
                 msg = BrokerdCancel(
                     oid=oid,
                     reqid=reqid,
@@ -860,12 +851,10 @@ async def process_client_order_cmds(
                 # NOTE: cancel response will be relayed back in messages
                 # from corresponding broker
                 if reqid is not None:
-
                     # send cancel to brokerd immediately!
                     log.info(
                         f'Submitting cancel for live order {reqid}'
                     )
-
                     await brokerd_order_stream.send(msg)
 
                 else:
@@ -876,7 +865,10 @@ async def process_client_order_cmds(
                     dark_book._ems_entries[oid] = msg
 
             # dark trigger cancel
-            else:
+            case {
+                'action': 'cancel',
+                'oid': oid,
+            } if not live_entry:
                 try:
                     # remove from dark book clearing
                     dark_book.orders[symbol].pop(oid, None)
@@ -896,25 +888,27 @@ async def process_client_order_cmds(
                 except KeyError:
                     log.exception(f'No dark order for {symbol}?')
 
-        # TODO: 3.10 struct-pattern matching and unpacking here
+            # live order submission
-        elif action in ('alert', 'buy', 'sell',):
+            case {
-
+                'oid': oid,
+                'symbol': fqsn,
+                'price': trigger_price,
+                'size': size,
+                'action': ('buy' | 'sell') as action,
+                'exec_mode': 'live',
+            }:
+                # TODO: eventually we should be receiving
+                # this struct on the wire unpacked in a scoped protocol
+                # setup with ``tractor``.
                 msg = Order(**cmd)
-
-            fqsn = msg.symbol
-            trigger_price = msg.price
-            size = msg.size
-            exec_mode = msg.exec_mode
                 broker = msg.brokers[0]
+
                 # remove the broker part before creating a message
                 # to send to the specific broker since they probably
                 # aren't expectig their own name, but should they?
                 sym = fqsn.replace(f'.{broker}', '')
 
-            if exec_mode == 'live' and action in ('buy', 'sell',):
-
                 if live_entry is not None:
-
                     # sanity check on emsd id
                     assert live_entry.oid == oid
                     reqid = live_entry.reqid
@@ -954,12 +948,21 @@ async def process_client_order_cmds(
                 # that live order asap.
                 dark_book._ems_entries[oid] = msg
 
+            # dark-order / alert submission
+            case {
+                'oid': oid,
+                'symbol': fqsn,
+                'price': trigger_price,
+                'size': size,
+                'exec_mode': exec_mode,
+                'action': action,
+            } if (
                     # "DARK" triggers
                     # submit order to local EMS book and scan loop,
                     # effectively a local clearing engine, which
                     # scans for conditions and triggers matching executions
-            elif exec_mode in ('dark', 'paper') or (
+                    exec_mode in ('dark', 'paper')
-                action in ('alert')
+                    or action == 'alert'
             ):
                 # Auto-gen scanner predicate:
                 # we automatically figure out what the alert check
@@ -999,10 +1002,8 @@ async def process_client_order_cmds(
                     abs_diff_away = 0
 
                 # submit execution/order to EMS scan loop
-
                 # NOTE: this may result in an override of an existing
                 # dark book entry if the order id already exists
-
                 dark_book.orders.setdefault(
                     fqsn, {}
                 )[oid] = (