Expose the signaling connection id on accepted child channels

transport-nethernet/src/main/java/dev/kastle/netty/channel/nethernet/NetherNetChildChannel.java

@@ -3,12 +3,26 @@
 import dev.kastle.netty.channel.nethernet.config.DefaultNetherChannelConfig;
 import io.netty.channel.Channel;
 import io.netty.channel.ChannelPromise;
+import io.netty.util.AttributeKey;
 import tel.schich.libdatachannel.PeerConnection;
 
 import java.net.InetSocketAddress;
 import java.net.SocketAddress;
 
 public class NetherNetChildChannel extends NetherNetChannel {
+
+    /**
+     * The signaling connection ID this channel was accepted for.
+     * <p>
+     * A child arrives on the server pipeline carrying nothing that ties it back to the offer that
+     * produced it, and arrival order does not follow the order answers were produced in. Signaling
+     * implementations already hold per connection ID state, since that is the key
+     * {@code setSignalHandler} and {@code removeSignalHandler} use, so exposing it here lets them
+     * attribute a channel without reaching into internals.
+     */
+    public static final AttributeKey<Long> CONNECTION_ID =
+            AttributeKey.valueOf(NetherNetChildChannel.class, "connectionId");
+
     public NetherNetChildChannel(Channel parent, PeerConnection peerConnection, InetSocketAddress remote, InetSocketAddress local) {
         super(parent, remote, local);
         this.peerConnection = peerConnection;

transport-nethernet/src/main/java/dev/kastle/netty/channel/nethernet/NetherNetServerChannel.java

@@ -106,6 +106,7 @@ public void acceptConnection(long connectionId, String offerSdp, String remoteNe
         observer.setPeerConnection(pc);
 
         NetherNetChildChannel child = new NetherNetChildChannel(this, pc, new InetSocketAddress(0), localAddress);
+        child.attr(NetherNetChildChannel.CONNECTION_ID).set(connectionId);
         observer.setChildChannel(child);
 
         child.closeFuture().addListener(future -> signaling.removeSignalHandler(connectionId));