First attempt data feed side quote throttling

Adding binance's "hft" ws feeds has resulted in a lot of context
switching in our Qt charts, so much so it's chewin CPU and definitely
worth it to throttle to the detected display rate as per discussion in
issue #192.

This is a first very very naive attempt at throttling L1 tick feeds on
the `brokerd` end (producer side) using a constant and uniform delivery
rate by way of a `trio` task + mem chan.  The new func is
`data._sampling.uniform_rate_send()`. Basically if a client request
a feed and provides a throttle rate we just spawn a task and queue up
ticks until approximately the next display rate's worth period of time
has passed before forwarding. It's definitely nothing fancy but does
provide fodder and a start point for an up and coming queueing eng to
start digging into both #107 and #109 ;)

piker/data/_sampling.py

@@ -17,6 +17,7 @@
 """
 Data buffers for fast shared humpy.
 """
+import time
 from typing import Dict, List
 
 import tractor
@@ -152,10 +153,12 @@ async def iter_ohlc_periods(
 
 
 async def sample_and_broadcast(
+
     bus: '_FeedBus',  # noqa
     shm: ShmArray,
     quote_stream: trio.abc.ReceiveChannel,
     sum_tick_vlm: bool = True,
+
 ) -> None:
 
     log.info("Started shared mem bar writer")
@@ -177,11 +180,10 @@ async def sample_and_broadcast(
             # trade data
             for tick in quote['ticks']:
 
-                # if tick['type'] in ('utrade',):
+                ticktype = tick['type']
-                #     print(tick)
 
                 # write trade events to shm last OHLC sample
-                if tick['type'] in ('trade', 'utrade'):
+                if ticktype in ('trade', 'utrade'):
 
                     last = tick['price']
 
@@ -229,8 +231,12 @@ async def sample_and_broadcast(
             # thus other consumers still attached.
             subs = bus._subscribers[sym.lower()]
 
-            for stream in subs:
+            for (stream, tick_throttle) in subs:
-                # print(f'sub is {ctx.chan.uid}')
+
+                if tick_throttle:
+                    await stream.send(quote)
+
+                else:
                     try:
                         await stream.send({sym: quote})
                     except (
@@ -242,3 +248,54 @@ async def sample_and_broadcast(
                         # so far seems like no since this should all
                         # be single-threaded.
                         log.error(f'{stream._ctx.chan.uid} dropped connection')
+
+
+async def uniform_rate_send(
+    rate: float,
+    quote_stream: trio.abc.ReceiveChannel,
+    stream: tractor.MsgStream,
+) -> None:
+
+    sleep_period = 1/rate - 0.000616
+    last_send = time.time()
+
+    while True:
+
+        first_quote = await quote_stream.receive()
+        start = time.time()
+
+        # append quotes since last iteration into the last quote's
+        # tick array/buffer.
+
+        # TODO: once we decide to get fancy really we should have
+        # a shared mem tick buffer that is just continually filled and
+        # the UI just ready from it at it's display rate.
+        # we'll likely head toward this once we get this issue going:
+        #
+        while True:
+            try:
+                next_quote = quote_stream.receive_nowait()
+                ticks = next_quote.get('ticks')
+
+                if ticks:
+                    first_quote['ticks'].extend(ticks)
+
+            except trio.WouldBlock:
+                now = time.time()
+                rate = 1 / (now - last_send)
+                last_send = now
+
+                # print(f'{rate} Hz sending quotes\n{first_quote}')
+
+                # TODO: now if only we could sync this to the display
+                # rate timing exactly lul
+                await stream.send({first_quote['symbol']: first_quote})
+                break
+
+        end = time.time()
+        diff = end - start
+
+        # throttle to provided transmit rate
+        period = max(sleep_period - diff, 0)
+        if period > 0:
+            await trio.sleep(period)