cast: don't drop the io loop on a single bad message, and don't let a scopeless link-local IPv6 clobber a working host

The receiver-status io loop treated any read error as a dead connection,
so one malformed/unexpected message (e.g. a MEDIA_STATUS missing a field
the struct requires) tore down the whole control channel for the rest of
the session even though the RTP stream was fine. Only end the loop on
Io/Tls/Dns errors now; log and keep going on Serialization/Parsing/etc.

mDNS resolves a single physical Cast device once per local address it
has, so the same device id can show up with a private IPv4 host and
again with a link-local IPv6 host. A bare fe80:: address has no zone id
attached, so connecting to it fails outright -- don't let it replace an
already-usable host in the device map just because it resolved more
recently.

breadcast-core/src/cast_sender.rs

@@ -326,10 +326,20 @@ fn run_io_loop(
                 // doc comment for why that's fine for this project's usage.
             }
             Ok(_) => {}
-            Err(e) => {
-                tracing::debug!(error = %e, "cast session io loop ending: receive error");
+            // Only end the loop on an error that means the connection
+            // itself is gone -- a single malformed/unexpected message (e.g.
+            // a MEDIA_STATUS missing a field this crate's struct treats as
+            // required) is a `Serialization`/`Parsing` error, not a dead
+            // socket, and used to take the whole receiver-status/control
+            // channel down with it for the rest of the session even though
+            // the RTP stream itself was unaffected.
+            Err(e @ (rust_cast::errors::Error::Io(_) | rust_cast::errors::Error::Tls(_) | rust_cast::errors::Error::Dns(_))) => {
+                tracing::debug!(error = %e, "cast session io loop ending: connection error");
                 return;
             }
+            Err(e) => {
+                tracing::warn!(error = %e, "cast session: ignoring unparseable/unexpected message");
+            }
         }
     }
 }

breadcastd/src/daemon.rs

@@ -73,6 +73,12 @@ impl ActiveSession {
     }
 }
 
+/// `host` strings come straight from mDNS resolution, so this just checks
+/// the address, not whether a zone id is attached (mDNS never gives us one).
+fn is_link_local_v6(host: &str) -> bool {
+    matches!(host.parse::<std::net::IpAddr>(), Ok(std::net::IpAddr::V6(v6)) if (v6.segments()[0] & 0xffc0) == 0xfe80)
+}
+
 struct Daemon {
     cast_devices: HashMap<String, CastDevice>,
     /// Keyed by `DlnaDevice::url`, the closest thing DLNA has to a stable
@@ -128,7 +134,20 @@ impl Daemon {
                 }
             }
             DaemonCommand::CastDeviceFound(device) => {
-                self.cast_devices.insert(device.id.clone(), device);
+                // mDNS resolves one physical device on every local address
+                // it has -- typically a private IPv4 and a link-local IPv6
+                // -- as separate events carrying the same id. A bare
+                // `fe80::` address has no interface scope attached, so
+                // `TcpStream::connect`ing to it fails outright; don't let
+                // one clobber an already-usable host just because it
+                // happened to resolve more recently.
+                let should_replace = match self.cast_devices.get(&device.id) {
+                    Some(existing) if is_link_local_v6(&device.host) && !is_link_local_v6(&existing.host) => false,
+                    _ => true,
+                };
+                if should_replace {
+                    self.cast_devices.insert(device.id.clone(), device);
+                }
                 self.broadcast_devices();
             }
             DaemonCommand::CastDeviceLost(id) => {