fix(gossipsub): make sure we have fanout peers when publish

This PR is to submit a fix in Lighthouse. sigp/lighthouse#6738

An `InsufficientPeers` error can occur under a particular condition, even if we have peers subscribed to a topic.

Pull-Request: #5793.

Author: ackintosh

protocols/gossipsub/CHANGELOG.md

@@ -35,6 +35,9 @@
 - Fix `cargo clippy` warnings in `rustc 1.84.0-beta.1`.
   See [PR 5700](https://github.com/libp2p/rust-libp2p/pull/5700).
 
+- Fixe an issue where an `InsufficientPeers` error could occur under certain conditions, despite having peers subscribed to a topic.
+  See [PR 5793](https://github.com/libp2p/rust-libp2p/pull/5793).
+
 ## 0.47.0
 
 <!-- Update to libp2p-swarm v0.45.0 -->

protocols/gossipsub/src/behaviour.rs

@@ -675,9 +675,14 @@ where
                 // Gossipsub peers
                 None => {
                     tracing::debug!(topic=%topic_hash, "Topic not in the mesh");
+                    // `fanout_peers` is always non-empty if it's `Some`.
+                    let fanout_peers = self
+                        .fanout
+                        .get(&topic_hash)
+                        .filter(|peers| !peers.is_empty());
                     // If we have fanout peers add them to the map.
-                    if self.fanout.contains_key(&topic_hash) {
-                        for peer in self.fanout.get(&topic_hash).expect("Topic must exist") {
+                    if let Some(peers) = fanout_peers {
+                        for peer in peers {
                             recipient_peers.insert(*peer);
                         }
                     } else {