Adding ClosableTransport.

transport/transport.go

@@ -83,6 +83,24 @@ type Transport interface {
 	Proxy() bool
 }
 
+// ClosableTransport is for transport that run a service beyond a listener
+// (overlay networks, ...).
+type ClosableTransport interface {
+	Transport
+
+	// Close is called when the libp2p node doesn't need the transport anymore.
+	// (libp2p closing, transport unhooked from a node, ...)
+	//
+	// Once a Close is called you can safely assume no more Dial or Listen are
+	// gonna be issued on this instance.
+	//
+	// If Close return an error the transport is still considered as dead
+	// (ex: transport already closed or transport never started).
+	//
+	// No more than one Close will ever be issued on an instance.
+	Close() error
+}
+
 // Listener is an interface closely resembling the net.Listener interface. The
 // only real difference is that Accept() returns Conn's of the type in this
 // package, and also exposes a Multiaddr method as opposed to a regular Addr