fix(gossipsub): fix mesh/fanout inconsistencies

When a peer unsubscribes also remove it from fanout.

Pull-Request: #5690.

Author: jxs

protocols/gossipsub/CHANGELOG.md

@@ -1,15 +1,17 @@
 ## 0.48.0
 
+- Correct state inconsistencies with the mesh and fanout when unsubscribing.
+  See [PR 5690](https://github.com/libp2p/rust-libp2p/pull/5690)
+
 - Deprecate `futures-ticker` and use `futures-timer` instead.
   See [PR 5674](https://github.com/libp2p/rust-libp2p/pull/5674).
+
 - Apply `max_transmit_size` to the inner message instead of the final payload.
   See [PR 5642](https://github.com/libp2p/rust-libp2p/pull/5642).
 
 - Deprecate `void` crate.
   See [PR 5676](https://github.com/libp2p/rust-libp2p/pull/5676).
 
-## 0.47.1
-
 - Attempt to publish to at least mesh_n peers when flood publish is disabled.
   See [PR 5578](https://github.com/libp2p/rust-libp2p/pull/5578).
 

protocols/gossipsub/src/behaviour.rs

@@ -1962,8 +1962,11 @@ where
             }
         }
 
-        // remove unsubscribed peers from the mesh if it exists
+        // remove unsubscribed peers from the mesh and fanout if they exist there.
         for (peer_id, topic_hash) in unsubscribed_peers {
+            self.fanout
+                .get_mut(&topic_hash)
+                .map(|peers| peers.remove(&peer_id));
             self.remove_peer_from_mesh(&peer_id, &topic_hash, None, false, Churn::Unsub);
         }