feat(iroh)!: add passive mode for mdns discovery (#3401) (#3403)

## Description

This pull request adds support for passive mDNS discovery, enabling
nodes to listen for peer advertisements without broadcasting their own
presence on the local network.

### Summary of changes:
- Added `advertise(bool)` setter to `MdnsDiscoveryBuilder` (default
`true`).
- Modified `MdnsDiscovery::new` and `spawn_discoverer` to respect the
`advertise` flag.

## Breaking Changes

- `MdnsDiscovery::new(node_id: NodeId)` → `MdnsDiscovery::new(node_id:
NodeId, advertise: bool)`
Code using `MdnsDiscovery::new` must now provide the `advertise`
argument explicitly.

## Notes & Open Questions

## Change Checklist

- [x] Self-review.
- [x] Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text).
- [x] Tests if relevant.
- [x] All breaking changes documented.

---------

Co-authored-by: ramfox <kasey@n0.computer>

Author: Le-Maz

iroh/src/discovery/mdns.rs

@@ -73,6 +73,7 @@ pub struct MdnsDiscovery {
     #[allow(dead_code)]
     handle: AbortOnDropHandle<()>,
     sender: mpsc::Sender<Message>,
+    advertise: bool,
     /// When `local_addrs` changes, we re-publish our info.
     local_addrs: Watchable<Option<NodeData>>,
 }
@@ -127,39 +128,73 @@ impl Subscribers {
 
 /// Builder for [`MdnsDiscovery`].
 #[derive(Debug)]
-pub struct MdnsDiscoveryBuilder;
+pub struct MdnsDiscoveryBuilder {
+    advertise: bool,
+}
+
+impl MdnsDiscoveryBuilder {
+    /// Creates a new [`MdnsDiscoveryBuilder`] with default settings.
+    pub fn new() -> Self {
+        Self { advertise: true }
+    }
+
+    /// Sets whether this node should advertise its presence.
+    ///
+    /// Default is true.
+    pub fn advertise(mut self, advertise: bool) -> Self {
+        self.advertise = advertise;
+        self
+    }
+
+    /// Builds an [`MdnsDiscovery`] instance with the configured settings.
+    pub fn build(self, node_id: NodeId) -> Result<MdnsDiscovery, IntoDiscoveryError> {
+        MdnsDiscovery::new(node_id, self.advertise)
+    }
+}
+
+impl Default for MdnsDiscoveryBuilder {
+    fn default() -> Self {
+        Self::new()
+    }
+}
 
 impl IntoDiscovery for MdnsDiscoveryBuilder {
     fn into_discovery(
         self,
         context: &DiscoveryContext,
     ) -> Result<impl Discovery, IntoDiscoveryError> {
-        MdnsDiscovery::new(context.node_id())
+        self.build(context.node_id())
     }
 }
 
 impl MdnsDiscovery {
     /// Returns a [`MdnsDiscoveryBuilder`] that implements [`IntoDiscovery`].
     pub fn builder() -> MdnsDiscoveryBuilder {
-        MdnsDiscoveryBuilder
+        MdnsDiscoveryBuilder::new()
     }
 
     /// Create a new [`MdnsDiscovery`] Service.
     ///
-    /// This starts a [`Discoverer`] that broadcasts your addresses and receives addresses from other nodes in your local network.
+    /// This starts a [`Discoverer`] that broadcasts your addresses (if advertise is set to true)
+    /// and receives addresses from other nodes in your local network.
     ///
     /// # Errors
     /// Returns an error if the network does not allow ipv4 OR ipv6.
     ///
     /// # Panics
     /// This relies on [`tokio::runtime::Handle::current`] and will panic if called outside of the context of a tokio runtime.
-    pub fn new(node_id: NodeId) -> Result<Self, IntoDiscoveryError> {
+    pub fn new(node_id: NodeId, advertise: bool) -> Result<Self, IntoDiscoveryError> {
         debug!("Creating new MdnsDiscovery service");
         let (send, mut recv) = mpsc::channel(64);
         let task_sender = send.clone();
         let rt = tokio::runtime::Handle::current();
-        let discovery =
-            MdnsDiscovery::spawn_discoverer(node_id, task_sender.clone(), BTreeSet::new(), &rt)?;
+        let discovery = MdnsDiscovery::spawn_discoverer(
+            node_id,
+            advertise,
+            task_sender.clone(),
+            BTreeSet::new(),
+            &rt,
+        )?;
 
         let local_addrs: Watchable<Option<NodeData>> = Watchable::default();
         let mut addrs_change = local_addrs.watch();
@@ -311,13 +346,15 @@ impl MdnsDiscovery {
         let handle = task::spawn(discovery_fut.instrument(info_span!("swarm-discovery.actor")));
         Ok(Self {
             handle: AbortOnDropHandle::new(handle),
+            advertise,
             sender: send,
             local_addrs,
         })
     }
 
     fn spawn_discoverer(
         node_id: PublicKey,
+        advertise: bool,
         sender: mpsc::Sender<Message>,
         socketaddrs: BTreeSet<SocketAddr>,
         rt: &tokio::runtime::Handle,
@@ -337,15 +374,17 @@ impl MdnsDiscovery {
                 sender.send(Message::Discovery(node_id, peer)).await.ok();
             });
         };
-        let addrs = MdnsDiscovery::socketaddrs_to_addrs(&socketaddrs);
         let node_id_str = data_encoding::BASE32_NOPAD
             .encode(node_id.as_bytes())
             .to_ascii_lowercase();
         let mut discoverer = Discoverer::new_interactive(N0_LOCAL_SWARM.to_string(), node_id_str)
             .with_callback(callback)
             .with_ip_class(IpClass::Auto);
-        for addr in addrs {
-            discoverer = discoverer.with_addrs(addr.0, addr.1);
+        if advertise {
+            let addrs = MdnsDiscovery::socketaddrs_to_addrs(&socketaddrs);
+            for addr in addrs {
+                discoverer = discoverer.with_addrs(addr.0, addr.1);
+            }
         }
         discoverer
             .spawn(rt)
@@ -406,7 +445,9 @@ impl Discovery for MdnsDiscovery {
     }
 
     fn publish(&self, data: &NodeData) {
-        self.local_addrs.set(Some(data.clone())).ok();
+        if self.advertise {
+            self.local_addrs.set(Some(data.clone())).ok();
+        }
     }
 
     fn subscribe(&self) -> Option<BoxStream<DiscoveryItem>> {
@@ -440,8 +481,10 @@ mod tests {
         #[tokio::test]
         #[traced_test]
         async fn mdns_publish_resolve() -> Result {
-            let (_, discovery_a) = make_discoverer()?;
-            let (node_id_b, discovery_b) = make_discoverer()?;
+            // Create discoverer A with advertise=false (only listens)
+            let (_, discovery_a) = make_discoverer(false)?;
+            // Create discoverer B with advertise=true (will broadcast)
+            let (node_id_b, discovery_b) = make_discoverer(true)?;
 
             // make addr info for discoverer b
             let user_data: UserData = "foobar".parse()?;
@@ -477,11 +520,11 @@ mod tests {
             let mut node_ids = BTreeSet::new();
             let mut discoverers = vec![];
 
-            let (_, discovery) = make_discoverer()?;
+            let (_, discovery) = make_discoverer(false)?;
             let node_data = NodeData::new(None, BTreeSet::from(["0.0.0.0:11111".parse().unwrap()]));
 
             for i in 0..num_nodes {
-                let (node_id, discovery) = make_discoverer()?;
+                let (node_id, discovery) = make_discoverer(true)?;
                 let user_data: UserData = format!("node{i}").parse()?;
                 let node_data = node_data.clone().with_user_data(Some(user_data.clone()));
                 node_ids.insert((node_id, Some(user_data)));
@@ -513,9 +556,38 @@ mod tests {
                 .context("timeout")?
         }
 
-        fn make_discoverer() -> Result<(PublicKey, MdnsDiscovery)> {
+        #[tokio::test]
+        #[traced_test]
+        async fn non_advertising_node_not_discovered() -> Result {
+            let (_, discovery_a) = make_discoverer(false)?;
+            let (node_id_b, discovery_b) = make_discoverer(false)?;
+
+            let (node_id_c, discovery_c) = make_discoverer(true)?;
+            let node_data_c =
+                NodeData::new(None, BTreeSet::from(["0.0.0.0:22222".parse().unwrap()]));
+            discovery_c.publish(&node_data_c);
+
+            let node_data_b =
+                NodeData::new(None, BTreeSet::from(["0.0.0.0:11111".parse().unwrap()]));
+            discovery_b.publish(&node_data_b);
+
+            let mut stream_c = discovery_a.resolve(node_id_c).unwrap();
+            let result_c = tokio::time::timeout(Duration::from_secs(2), stream_c.next()).await;
+            assert!(result_c.is_ok(), "Advertising node should be discoverable");
+
+            let mut stream_b = discovery_a.resolve(node_id_b).unwrap();
+            let result_b = tokio::time::timeout(Duration::from_secs(2), stream_b.next()).await;
+            assert!(
+                result_b.is_err(),
+                "Expected timeout since node b isn't advertising"
+            );
+
+            Ok(())
+        }
+
+        fn make_discoverer(advertise: bool) -> Result<(PublicKey, MdnsDiscovery)> {
             let node_id = SecretKey::generate(rand::thread_rng()).public();
-            Ok((node_id, MdnsDiscovery::new(node_id)?))
+            Ok((node_id, MdnsDiscovery::new(node_id, advertise)?))
         }
     }
 }