respect channel layer capacity in RedisChannelLayer.receive_buffer

respect the capacity setting so that the receive_buffer does not grow
without bounds

see: #212

channels_redis/core.py

@@ -2,6 +2,7 @@
 import base64
 import binascii
 import collections
+import functools
 import hashlib
 import itertools
 import logging
@@ -179,6 +180,19 @@ class UnsupportedRedis(Exception):
     pass
 
 
+class BoundedQueue(asyncio.Queue):
+    def put_nowait(self, item):
+        if self.full():
+            # see: https://github.com/django/channels_redis/issues/212
+            # if we actually get into this code block, it likely means that
+            # this specific consumer has stopped reading
+            # if we get into this code block, it's better to drop messages
+            # that exceed the channel layer capacity than to continue to
+            # malloc() forever
+            self.get_nowait()
+        return super(BoundedQueue, self).put_nowait(item)
+
+
 class RedisChannelLayer(BaseChannelLayer):
     """
     Redis channel layer.
@@ -226,7 +240,9 @@ def __init__(
         # Event loop they are trying to receive on
         self.receive_event_loop = None
         # Buffered messages by process-local channel name
-        self.receive_buffer = collections.defaultdict(asyncio.Queue)
+        self.receive_buffer = collections.defaultdict(
+            functools.partial(BoundedQueue, self.capacity)
+        )
         # Detached channel cleanup tasks
         self.receive_cleaners = []
         # Per-channel cleanup locks to prevent a receive starting and moving

tests/test_core.py

@@ -627,3 +627,18 @@ def test_custom_group_key_format():
     channel_layer = RedisChannelLayer(prefix="test_prefix")
     group_name = channel_layer._group_key("test_group")
     assert group_name == b"test_prefix:group:test_group"
+
+
+@pytest.mark.asyncio
+async def test_receive_buffer_respects_capacity():
+    channel_layer = RedisChannelLayer()
+    buff = channel_layer.receive_buffer['test-group']
+    for i in range(10000):
+        buff.put_nowait(i)
+
+    capacity = 100
+    assert channel_layer.capacity == capacity
+    assert buff.full() is True
+    assert buff.qsize() == capacity
+    messages = [buff.get_nowait() for _ in range(capacity)]
+    assert list(range(9900, 10000)) == messages