Feed detach must explicitly unsub throttled streams

If a client attaches to a quotes data feed and requests a throttle rate,
be sure to unsub that side-band memchan + task when it detaches and
especially so on any transport connection error.

Also, use an explicit `tractor.Context.cancel()` on the client feed
block exit since we removed the implicit cancel option from the
`tractor` api.

piker/data/_sampling.py

@@ -233,22 +233,26 @@ async def sample_and_broadcast(
 
             for (stream, tick_throttle) in subs:
 
-                if tick_throttle:
-                    await stream.send(quote)
+                try:
+                    if tick_throttle:
+                        await stream.send(quote)
 
-                else:
-                    try:
+                    else:
                         await stream.send({sym: quote})
-                    except (
-                        trio.BrokenResourceError,
-                        trio.ClosedResourceError
-                    ):
-                        # XXX: do we need to deregister here
-                        # if it's done in the fee bus code?
-                        # so far seems like no since this should all
-                        # be single-threaded.
-                        log.error(f'{stream._ctx.chan.uid} dropped connection')
-                        subs.remove((stream, tick_throttle))
+
+                except (
+                    trio.BrokenResourceError,
+                    trio.ClosedResourceError
+                ):
+                    # XXX: do we need to deregister here
+                    # if it's done in the fee bus code?
+                    # so far seems like no since this should all
+                    # be single-threaded.
+                    log.warning(
+                        f'{stream._ctx.chan.uid} dropped  '
+                        '`brokerd`-quotes-feed connection'
+                    )
+                    subs.remove((stream, tick_throttle))
 
 
 async def uniform_rate_send(

piker/data/feed.py

@@ -305,6 +305,11 @@ async def attach_feed_bus(
     ):
 
         if tick_throttle:
+
+            # open a bg task which receives quotes over a mem chan
+            # and only pushes them to the target actor-consumer at
+            # a max ``tick_throttle`` instantaneous rate.
+
             send, recv = trio.open_memory_channel(2**10)
             n.start_soon(
                 uniform_rate_send,
@@ -321,7 +326,12 @@ async def attach_feed_bus(
 
         try:
             await trio.sleep_forever()
+
         finally:
+            log.info(
+                f'Stopping {symbol}.{brokername} feed for {ctx.chan.uid}')
+            if tick_throttle:
+                n.cancel_scope.cancel()
             bus._subscribers[symbol].remove(sub)
 
 
@@ -473,11 +483,6 @@ async def open_feed(
         ctx.open_stream() as stream,
     ):
 
-        # TODO: can we make this work better with the proposed
-        # context based bidirectional streaming style api proposed in:
-        # https://github.com/goodboy/tractor/issues/53
-        # init_msg = await stream.receive()
-
         # we can only read from shm
         shm = attach_shm_array(
             token=init_msg[sym]['shm_token'],
@@ -520,4 +525,8 @@ async def open_feed(
 
         feed._max_sample_rate = max(ohlc_sample_rates)
 
-        yield feed
+        try:
+            yield feed
+        finally:
+            # drop the infinite stream connection
+            await ctx.cancel()