added newsfragment

tests/core/pubsub/test_gossipsub_px_and_backoff.py

@@ -113,3 +113,53 @@ async def test_unsubscribe_backoff():
         assert host_0.get_id() in gsub1.mesh[topic], (
             "peer should be able to rejoin after backoff"
         )
+
+
+@pytest.mark.trio
+async def test_peer_exchange():
+    async with PubsubFactory.create_batch_with_gossipsub(
+        3,
+        heartbeat_interval=0.5,
+        do_px=True,
+        px_peers_count=1,
+    ) as pubsubs:
+        gsub0 = pubsubs[0].router
+        gsub1 = pubsubs[1].router
+        gsub2 = pubsubs[2].router
+        assert isinstance(gsub0, GossipSub)
+        assert isinstance(gsub1, GossipSub)
+        assert isinstance(gsub2, GossipSub)
+        host_0 = pubsubs[0].host
+        host_1 = pubsubs[1].host
+        host_2 = pubsubs[2].host
+
+        topic = "test_peer_exchange"
+
+        # connect hosts
+        await connect(host_1, host_0)
+        await connect(host_1, host_2)
+        await trio.sleep(0.5)
+
+        # all join the topic and 0 <-> 1 and 1 <-> 2 graft
+        await pubsubs[1].subscribe(topic)
+        await pubsubs[0].subscribe(topic)
+        await pubsubs[2].subscribe(topic)
+        await gsub1.emit_graft(topic, host_0.get_id())
+        await gsub1.emit_graft(topic, host_2.get_id())
+        await gsub0.emit_graft(topic, host_1.get_id())
+        await gsub2.emit_graft(topic, host_1.get_id())
+        await trio.sleep(1)
+
+        # ensure peer is registered in mesh
+        assert host_0.get_id() in gsub1.mesh[topic]
+        assert host_2.get_id() in gsub1.mesh[topic]
+        assert host_2.get_id() not in gsub0.mesh[topic]
+
+        # host_1 unsubscribes from the topic
+        await gsub1.leave(topic)
+        await trio.sleep(1)  # Wait for heartbeat to update mesh
+        assert topic not in gsub1.mesh
+
+        # Wait for gsub0 to graft host_2 into its mesh via PX
+        await trio.sleep(1)
+        assert host_2.get_id() in gsub0.mesh[topic]