Fix bug flushing Data frames from asyncio client

When AsyncioClient.data() was called with non-zero flush_period
then it only yielded the Data after flush when a new packet of data
appeared on the wire or at the end of acquisition. This changes it to be
more eager and yield as soon as it is flushed.

pandablocks/asyncio.py

@@ -1,8 +1,7 @@
 import asyncio
 import logging
 from asyncio.streams import StreamReader, StreamWriter
-from collections import deque
-from typing import AsyncGenerator, Deque, Dict, Optional
+from typing import AsyncGenerator, Dict, Iterable, Optional
 
 from .commands import Command, T
 from .connections import ControlConnection, DataConnection
@@ -142,35 +141,41 @@ async def data(
                 `asyncio.TimeoutError`
         """
 
-        data_stream = _StreamHelper()
-        await data_stream.connect(self._host, 8889),
-
+        stream = _StreamHelper()
         connection = DataConnection()
-        data: Deque[Data] = deque()
-        reader = data_stream.reader
-        # Should we flush every FrameData?
-        flush_every_frame = flush_period is None
+        queue: asyncio.Queue[Iterable[Data]] = asyncio.Queue()
 
-        async def queue_flushed_data():
-            data.extend(connection.flush())
+        def raise_timeouterror():
+            raise asyncio.TimeoutError(f"No data received for {frame_timeout}s")
+            yield
 
         async def periodic_flush():
-            if not flush_every_frame:
+            if flush_period is not None:
                 while True:
                     # Every flush_period seconds flush and queue data
-                    await asyncio.gather(
-                        asyncio.sleep(flush_period), queue_flushed_data()
-                    )
+                    await asyncio.sleep(flush_period)
+                    queue.put_nowait(connection.flush())
 
-        flush_task = asyncio.create_task(periodic_flush())
+        async def read_from_stream():
+            reader = stream.reader
+            # Should we flush every FrameData?
+            flush_every_frame = flush_period is None
+            while True:
+                try:
+                    recv = await asyncio.wait_for(reader.read(4096), frame_timeout)
+                except asyncio.TimeoutError:
+                    queue.put_nowait(raise_timeouterror())
+                    break
+                else:
+                    queue.put_nowait(connection.receive_bytes(recv, flush_every_frame))
+
+        await stream.connect(self._host, 8889)
+        await stream.write_and_drain(connection.connect(scaled))
+        fut = asyncio.gather(periodic_flush(), read_from_stream())
         try:
-            await data_stream.write_and_drain(connection.connect(scaled))
             while True:
-                received = await asyncio.wait_for(reader.read(4096), frame_timeout)
-                for d in connection.receive_bytes(received, flush_every_frame):
-                    data.append(d)
-                while data:
-                    yield data.popleft()
+                for data in await queue.get():
+                    yield data
         finally:
-            flush_task.cancel()
-            await data_stream.close()
+            fut.cancel()
+            await stream.close()

tests/test_asyncio.py

@@ -31,17 +31,33 @@ async def test_asyncio_bad_put_raises(dummy_server_async):
 
 
 @pytest.mark.asyncio
-async def test_asyncio_data(dummy_server_async, fast_dump, fast_dump_expected):
+@pytest.mark.parametrize("disarmed", [True, False])
+@pytest.mark.parametrize("flush_period", [0.1, None])
+async def test_asyncio_data(
+    dummy_server_async, fast_dump, fast_dump_expected, disarmed, flush_period
+):
+    if not disarmed:
+        # simulate getting the data without the END marker as if arm was not pressed
+        fast_dump = map(lambda x: x.split(b"END")[0], fast_dump)
+        fast_dump_expected = list(fast_dump_expected)[:-1]
     dummy_server_async.data = fast_dump
     events = []
     async with AsyncioClient("localhost") as client:
-        async for data in client.data(frame_timeout=1):
+        async for data in client.data(frame_timeout=1, flush_period=flush_period):
             events.append(data)
-            if len(events) == 9:
+            if len(events) == len(fast_dump_expected):
                 break
     assert fast_dump_expected == events
 
 
+async def test_asyncio_data_timeout(dummy_server_async, fast_dump):
+    dummy_server_async.data = fast_dump
+    async with AsyncioClient("localhost") as client:
+        with pytest.raises(asyncio.TimeoutError, match="No data received for 0.1s"):
+            async for data in client.data(frame_timeout=0.1):
+                "This goes forever, when it runs out of data we will get our timeout"
+
+
 @pytest.mark.asyncio
 async def test_asyncio_connects(dummy_server_async: DummyServer):
     async with AsyncioClient("localhost") as client: