Pubsub: change channel size

To `32` to conform to the go implementation.

libp2p/pubsub/pubsub.py

@@ -1,6 +1,5 @@
 import functools
 import logging
-import math
 import time
 from typing import TYPE_CHECKING, Dict, KeysView, List, NamedTuple, Set, Tuple, cast
 
@@ -32,6 +31,9 @@ if TYPE_CHECKING:
     from typing import Any  # noqa: F401
 
 
+# Ref: https://github.com/libp2p/go-libp2p-pubsub/blob/40e1c94708658b155f30cf99e4574f384756d83c/topic.go#L97  # noqa: E501
+SUBSCRIPTION_CHANNEL_SIZE = 32
+
 logger = logging.getLogger("libp2p.pubsub")
 
 
@@ -373,7 +375,13 @@ class Pubsub(Service, IPubsub):
                 # we are subscribed to a topic this message was sent for,
                 # so add message to the subscription output queue
                 # for each topic
-                await self.subscribed_topics_send[topic].send(publish_message)
+                try:
+                    self.subscribed_topics_send[topic].send_nowait(publish_message)
+                except trio.WouldBlock:
+                    # Channel is full, ignore this message.
+                    logger.warning(
+                        "fail to deliver message to subscription for topic %s", topic
+                    )
 
     async def subscribe(self, topic_id: str) -> ISubscriptionAPI:
         """
@@ -389,7 +397,7 @@ class Pubsub(Service, IPubsub):
             return self.subscribed_topics_receive[topic_id]
 
         send_channel, receive_channel = trio.open_memory_channel[rpc_pb2.Message](
-            math.inf
+            SUBSCRIPTION_CHANNEL_SIZE
         )
 
         subscription = TrioSubscriptionAPI(

tests/pubsub/test_pubsub.py

@@ -6,7 +6,7 @@ import trio
 
 from libp2p.exceptions import ValidationError
 from libp2p.pubsub.pb import rpc_pb2
-from libp2p.pubsub.pubsub import PUBSUB_SIGNING_PREFIX
+from libp2p.pubsub.pubsub import PUBSUB_SIGNING_PREFIX, SUBSCRIPTION_CHANNEL_SIZE
 from libp2p.tools.constants import MAX_READ_LEN
 from libp2p.tools.factories import IDFactory, PubsubFactory, net_stream_pair_factory
 from libp2p.tools.pubsub.utils import make_pubsub_msg
@@ -444,6 +444,38 @@ async def test_subscribe_and_publish():
             nursery.start_soon(publish_data, TESTING_TOPIC)
 
 
+@pytest.mark.trio
+async def test_subscribe_and_publish_full_channel():
+    async with PubsubFactory.create_batch_with_floodsub(1) as pubsubs_fsub:
+        pubsub = pubsubs_fsub[0]
+
+        extra_data_0 = b"extra_data_0"
+        extra_data_1 = b"extra_data_1"
+
+        # Test: Subscription channel is of size `SUBSCRIPTION_CHANNEL_SIZE`.
+        #   When the channel is full, new received messages are dropped.
+        #   After the channel has empty slot, the channel can receive new messages.
+
+        # Assume `SUBSCRIPTION_CHANNEL_SIZE` is smaller than `2**(4*8)`.
+        list_data = [i.to_bytes(4, "big") for i in range(SUBSCRIPTION_CHANNEL_SIZE)]
+        # Expect `extra_data_0` is dropped and `extra_data_1` is appended.
+        expected_list_data = list_data + [extra_data_1]
+
+        subscription = await pubsub.subscribe(TESTING_TOPIC)
+        for data in list_data:
+            await pubsub.publish(TESTING_TOPIC, data)
+
+        # Publish `extra_data_0` which should be dropped since the channel is already full.
+        await pubsub.publish(TESTING_TOPIC, extra_data_0)
+        # Consume a message and there is an empty slot in the channel.
+        assert (await subscription.get()).data == expected_list_data.pop(0)
+        # Publish `extra_data_1` which should be appended to the channel.
+        await pubsub.publish(TESTING_TOPIC, extra_data_1)
+
+        for expected_data in expected_list_data:
+            assert (await subscription.get()).data == expected_data
+
+
 @pytest.mark.trio
 async def test_publish_push_msg_is_called(monkeypatch):
     msg_forwarders = []