fix(mdns): Handle problems on Apple platforms

protocols/mdns/CHANGELOG.md

@@ -1,5 +1,8 @@
 ## 0.46.0
 
+- Issue a warning if configured for IPv6 on Apple platforms and a more explanatory error when
+  binding to that port fails. See [PR 5527](https://github.com/libp2p/rust-libp2p/pull/5526).
+
 <!-- Update to libp2p-swarm v0.45.0 -->
 ## 0.45.2
 

protocols/mdns/src/behaviour.rs

@@ -219,6 +219,23 @@ where
         }
         self.closest_expiration = Some(P::Timer::at(now));
     }
+
+    #[cfg(not(target_vendor = "apple"))]
+    fn apple_warning(&self) {}
+
+    #[cfg(target_vendor = "apple")]
+    fn apple_warning(&self) {
+        static APPLE_WARNING: std::sync::atomic::AtomicBool =
+            std::sync::atomic::AtomicBool::new(false);
+        let is_ipv6 = self.config.enable_ipv6;
+        if APPLE_WARNING.load(std::sync::atomic::Ordering::Relaxed) {
+            return;
+        }
+        APPLE_WARNING.store(true, std::sync::atomic::Ordering::Relaxed);
+        if is_ipv6 {
+            tracing::warn!("You are using IPv6 with mDNS on an Apple device. This may fail due to the mDNSResponder, a system service, using that port. For more infos: `man mdnsresponder`");
+        }
+    }
 }
 
 impl<P> NetworkBehaviour for Behaviour<P>
@@ -290,6 +307,8 @@ where
         &mut self,
         cx: &mut Context<'_>,
     ) -> Poll<ToSwarm<Self::ToSwarm, THandlerInEvent<Self>>> {
+        self.apple_warning();
+
         // Poll ifwatch.
         while let Poll::Ready(Some(event)) = Pin::new(&mut self.if_watch).poll_next(cx) {
             match event {
@@ -315,7 +334,12 @@ where
                                 e.insert(P::spawn(iface_state));
                             }
                             Err(err) => {
-                                tracing::error!("failed to create `InterfaceState`: {}", err)
+                                const IS_APPLE: bool = cfg!(target_vendor = "apple");
+                                if IS_APPLE && addr.is_ipv6() {
+                                    tracing::error!("failed to create `InterfaceState`: {}. This is likely because the mDNSResponder, a system service, is using that port. For more infos: `man mdnsresponder`", err)
+                                } else {
+                                    tracing::error!("failed to create `InterfaceState`: {}", err)
+                                }
                             }
                         }
                     }

protocols/mdns/tests/use-async-std.rs

@@ -35,6 +35,9 @@ async fn test_discovery_async_std_ipv4() {
     run_discovery_test(Config::default()).await
 }
 
+// This test (typically) doesn't work on macOS because that port and address are already used execlusively by
+// mdnsResponder, which is a system service.
+#[cfg_attr(target_vendor = "apple", ignore)]
 #[async_std::test]
 async fn test_discovery_async_std_ipv6() {
     let _ = tracing_subscriber::fmt()

protocols/mdns/tests/use-tokio.rs

@@ -33,6 +33,7 @@ async fn test_discovery_tokio_ipv4() {
     run_discovery_test(Config::default()).await
 }
 
+#[cfg_attr(target_vendor = "apple", ignore)]
 #[tokio::test]
 async fn test_discovery_tokio_ipv6() {
     let _ = tracing_subscriber::fmt()