refactor: do not use gossip subscribe_all in iroh sync engine (#2265)

## Description

Changes gossip usage in `iroh::docs_engine` to not use
`Gossip::subscribe_all` but subscribe to individual topics instead.

## Breaking Changes

<!-- Optional, if there are any breaking changes document them,
including how to migrate older code. -->

## Notes & open questions

<!-- Any notes, remarks or open questions you have to make about the PR.
-->

## Change checklist

- [x] Self-review.
- [x] Documentation updates if relevant.
- [x] Tests if relevant.
- [x] All breaking changes documented.

iroh/src/docs_engine/gossip.rs

@@ -1,18 +1,20 @@
 use std::collections::HashSet;
 
-use anyhow::{anyhow, Context, Result};
+use anyhow::{Context, Result};
 use futures_lite::StreamExt;
+use futures_util::FutureExt;
 use iroh_docs::{actor::SyncHandle, ContentStatus, NamespaceId};
-use iroh_gossip::{
-    net::{Event, Gossip},
-    proto::TopicId,
-};
+use iroh_gossip::net::{Event, Gossip};
 use iroh_net::key::PublicKey;
 use tokio::{
-    sync::{broadcast::error::RecvError, mpsc},
+    sync::{broadcast, mpsc},
     task::JoinSet,
 };
-use tracing::{debug, error, trace};
+use tokio_stream::{
+    wrappers::{errors::BroadcastStreamRecvError, BroadcastStream},
+    StreamMap,
+};
+use tracing::{debug, error, trace, warn};
 
 use super::live::{Op, ToLiveActor};
 
@@ -37,7 +39,8 @@ pub struct GossipActor {
     to_sync_actor: mpsc::Sender<ToLiveActor>,
     joined: HashSet<NamespaceId>,
     want_join: HashSet<NamespaceId>,
-    pending_joins: JoinSet<(NamespaceId, Result<TopicId>)>,
+    pending_joins: JoinSet<(NamespaceId, Result<broadcast::Receiver<Event>>)>,
+    gossip_events: StreamMap<NamespaceId, BroadcastStream<Event>>,
 }
 
 impl GossipActor {
@@ -55,16 +58,16 @@ impl GossipActor {
             joined: Default::default(),
             want_join: Default::default(),
             pending_joins: Default::default(),
+            gossip_events: Default::default(),
         }
     }
     pub async fn run(&mut self) -> anyhow::Result<()> {
-        let mut gossip_events = self.gossip.clone().subscribe_all();
         let mut i = 0;
         loop {
             i += 1;
             trace!(?i, "tick wait");
             tokio::select! {
-                next = gossip_events.next() => {
+                next = self.gossip_events.next(), if !self.gossip_events.is_empty() => {
                     trace!(?i, "tick: gossip_event");
                     if let Err(err) = self.on_gossip_event(next).await {
                         error!("gossip actor died: {err:?}");
@@ -82,9 +85,11 @@ impl GossipActor {
                     trace!(?i, "tick: pending_joins");
                     let (namespace, res) = res.context("pending_joins closed")?;
                     match res {
-                        Ok(_topic) => {
+                        Ok(stream) => {
                             debug!(namespace = %namespace.fmt_short(), "joined gossip");
                             self.joined.insert(namespace);
+                            let stream = BroadcastStream::new(stream);
+                            self.gossip_events.insert(namespace, stream);
                         },
                         Err(err) => {
                             if self.want_join.contains(&namespace) {
@@ -112,13 +117,11 @@ impl GossipActor {
                 let gossip = self.gossip.clone();
                 // join gossip for the topic to receive and send message
                 let fut = async move {
-                    let res = gossip.join(namespace.into(), peers).await;
-                    let res = match res {
-                        Ok(fut) => fut.await,
-                        Err(err) => Err(err),
-                    };
-                    (namespace, res)
+                    let stream = gossip.subscribe(namespace.into()).await?;
+                    let _topic = gossip.join(namespace.into(), peers).await?.await?;
+                    Ok(stream)
                 };
+                let fut = fut.map(move |res| (namespace, res));
                 self.want_join.insert(namespace);
                 self.pending_joins.spawn(fut);
             }
@@ -132,22 +135,20 @@ impl GossipActor {
     }
     async fn on_gossip_event(
         &mut self,
-        event: Option<Result<(TopicId, Event), RecvError>>,
+        event: Option<(NamespaceId, Result<Event, BroadcastStreamRecvError>)>,
     ) -> Result<()> {
-        let (topic, event) = match event {
-            Some(Ok(event)) => event,
-            None => return Err(anyhow!("Gossip event channel closed")),
-            Some(Err(err)) => match err {
-                RecvError::Lagged(n) => {
-                    error!("GossipActor too slow (lagged by {n}) - dropping gossip event");
-                    return Ok(());
-                }
-                RecvError::Closed => {
-                    return Err(anyhow!("Gossip event channel closed"));
-                }
-            },
+        let (namespace, event) = event.context("Gossip event channel closed")?;
+        let event = match event {
+            Ok(event) => event,
+            Err(BroadcastStreamRecvError::Lagged(n)) => {
+                warn!("GossipActor too slow (lagged by {n}) - dropping gossip event");
+                return Ok(());
+            }
         };
-        let namespace: NamespaceId = topic.as_bytes().into();
+        if !self.joined.contains(&namespace) && !self.want_join.contains(&namespace) {
+            error!(namespace = %namespace.fmt_short(), "received gossip event for unknown topic");
+            return Ok(());
+        }
         if let Err(err) = self.on_gossip_event_inner(namespace, event).await {
             error!(namespace = %namespace.fmt_short(), ?err, "Failed to process gossip event");
         }