Use `MktPair` for `Flume.symbol` when used by backend

Initial attempt at getting the sampling and shm layer to use the new mkt
info meta-data type. Draft out a potential `BackendInitMsg:
msgspec.Struct` for validating the init msg returned from the
`stream_quotes()` start value; obvs don't actually use it yet.

piker/data/feed.py

@@ -933,6 +933,24 @@ async def manage_history(
             await trio.sleep_forever()
 
 
+class BackendInitMsg(Struct, frozen=True):
+    '''
+    A stringent data provider startup msg schema validator.
+
+    The fields defined here are matched with those absolutely required
+    from each backend broker/data provider.
+
+    '''
+    fqsn: str
+    symbol_info: dict | None = None
+    mkt_info: MktPair | None = None
+    shm_write_opts: dict[str, Any] | None = None
+
+
+def validate_init_msg() -> None:
+    ...
+
+
 async def allocate_persistent_feed(
     bus: _FeedsBus,
     sub_registered: trio.Event,
@@ -977,7 +995,10 @@ async def allocate_persistent_feed(
 
     # establish broker backend quote stream by calling
     # ``stream_quotes()``, which is a required broker backend endpoint.
-    init_msg, first_quote = await bus.nursery.start(
+    (
+        init_msg,
+        first_quote,
+    ) = await bus.nursery.start(
         partial(
             mod.stream_quotes,
             send_chan=send,
@@ -1008,19 +1029,24 @@ async def allocate_persistent_feed(
     #   a small streaming machine around the remote feed which can then
     #   do the normal work of sampling and writing shm buffers
     #   (depending on if we want sampling done on the far end or not?)
-    msg = init_msg[symstr]
+    per_mkt_init_msg = init_msg[symstr]
 
     # the broker-specific fully qualified symbol name,
     # but ensure it is lower-cased for external use.
-    bs_mktid = msg['fqsn'].lower()
+    bs_mktid = per_mkt_init_msg['fqsn'].lower()
 
     # true fqme including broker/provider suffix
     fqme = '.'.join((bs_mktid, brokername))
 
-    mktinfo = msg.get('mkt_info')
+    mktinfo = per_mkt_init_msg.get('mkt_info')
     if not mktinfo:
 
-        mktinfo = msg['symbol_info']
+        log.warning(
+            f'BACKEND {brokername} is using old `Symbol` style API\n'
+            'IT SHOULD BE PORTED TO THE NEW `.accounting._mktinfo.MktPair`\n'
+            'STATTTTT!!!\n'
+        )
+        mktinfo = per_mkt_init_msg['symbol_info']
 
         # TODO: read out renamed/new tick size fields in block below!
         price_tick = mktinfo.get(
@@ -1042,10 +1068,15 @@ async def allocate_persistent_feed(
             _atype=mktinfo['asset_type']
         )
 
+        symbol = Symbol.from_fqsn(
+            fqsn=fqme,
+            info=mktinfo,
+        )
+
     else:
         # the new msg-protocol is to expect an already packed
         # ``Asset`` and ``MktPair`` object from the backend
-        mkt = mktinfo
+        symbol = mkt = mktinfo
         assert isinstance(mkt, MktPair)
         assert isinstance(mkt.dst, Asset)
 
@@ -1080,15 +1111,13 @@ async def allocate_persistent_feed(
     log.info(f'waiting on history to load: {fqme}')
     await some_data_ready.wait()
 
-    symbol = Symbol.from_fqsn(
-        fqsn=fqme,
-        info=msg['symbol_info'],
-    )
     flume = Flume(
+
         # TODO: we have to use this for now since currently the
         # MktPair above doesn't render the correct output key it seems
         # when we provide the `MktInfo` here?..?
         symbol=symbol,
+
         first_quote=first_quote,
         _rt_shm_token=rt_shm.token,
         _hist_shm_token=hist_shm.token,
@@ -1109,6 +1138,8 @@ async def allocate_persistent_feed(
     # the backend will indicate when real-time quotes have begun.
     await feed_is_live.wait()
 
+    # NOTE: if not configured otherwise, we always sum tick volume
+    # values in the OHLCV sampler.
     sum_tick_vlm: bool = init_msg.get(
         'shm_write_opts', {}
     ).get('sum_tick_vlm', True)
@@ -1132,7 +1163,7 @@ async def allocate_persistent_feed(
         rt_shm.array['time'][1] = ts + 1
 
     elif hist_shm.array.size == 0:
-        await tractor.breakpoint()
+        raise RuntimeError(f'History (1m) Shm for {fqme} is empty!?')
 
     # wait the spawning parent task to register its subscriber
     # send-stream entry before we start the sample loop.
@@ -1235,7 +1266,7 @@ async def open_feed_bus(
         flume = bus.feeds[symbol]
         sym = flume.symbol
         bs_mktid = sym.key
-        fqsn = sym.fqsn  # true fqsn
+        fqsn = sym.fqme  # true fqsn
         assert bs_mktid in fqsn and brokername in fqsn
 
         if sym.suffix: