fix: acquire write lock on iroh_channels before checking for subscribe_loop

Author: link2xt

src/peer_channels.rs

@@ -81,7 +81,14 @@ impl Iroh {
         msg_id: MsgId,
     ) -> Result<Option<JoinTopicFut>> {
         let topic = get_iroh_topic_for_msg(ctx, msg_id).await?;
-        let seq = if let Some(channel_state) = self.iroh_channels.read().await.get(&topic) {
+
+        // Take exclusive lock to make sure
+        // no other thread can create a second gossip subscription
+        // after we check that it does not exist and before we create a new one.
+        // Otherwise we would receive every message twice or more times.
+        let mut iroh_channels = self.iroh_channels.write().await;
+
+        let seq = if let Some(channel_state) = iroh_channels.get(&topic) {
             if channel_state.subscribe_loop.is_some() {
                 return Ok(None);
             }
@@ -115,10 +122,7 @@ impl Iroh {
             }
         });
 
-        self.iroh_channels
-            .write()
-            .await
-            .insert(topic, ChannelState::new(seq, subscribe_loop));
+        iroh_channels.insert(topic, ChannelState::new(seq, subscribe_loop));
 
         Ok(Some(connect_future))
     }