Implement missing bits for an external servertype

Make ServerListeners public and add two helper methods to get access
to the address they listen on. This is useful for tests (among other
things)

Also make DefaultConfigFile a var so it can be overridden by server
types.

caddy.go

@@ -76,7 +76,7 @@ type Instance struct {
 	context Context
 
 	// servers is the list of servers with their listeners.
-	servers []serverListener
+	servers []ServerListener
 
 	// these callbacks execute when certain events occur
 	onFirstStartup  []func() error // starting, not as part of a restart
@@ -86,6 +86,9 @@ type Instance struct {
 	onFinalShutdown []func() error // stopping, not as part of a restart
 }
 
+// Servers returns the ServerListeners in i.
+func (i *Instance) Servers() []ServerListener { return i.servers }
+
 // Stop stops all servers contained in i. It does NOT
 // execute shutdown callbacks.
 func (i *Instance) Stop() error {
@@ -202,7 +205,7 @@ func (i *Instance) Restart(newCaddyfile Input) (*Instance, error) {
 // internally-kept list of servers that is running. For
 // saved servers, graceful restarts will be provided.
 func (i *Instance) SaveServer(s Server, ln net.Listener) {
-	i.servers = append(i.servers, serverListener{server: s, listener: ln})
+	i.servers = append(i.servers, ServerListener{server: s, listener: ln})
 }
 
 // HasListenerWithAddress returns whether this package is
@@ -644,7 +647,7 @@ func startServers(serverList []Server, inst *Instance, restartFds map[string]res
 			errChan <- s.ServePacket(pc)
 		}(s, ln, pc, inst)
 
-		inst.servers = append(inst.servers, serverListener{server: s, listener: ln, packet: pc})
+		inst.servers = append(inst.servers, ServerListener{server: s, listener: ln, packet: pc})
 	}
 
 	// Log errors that may be returned from Serve() calls,
@@ -836,7 +839,7 @@ var (
 	instancesMu sync.Mutex
 )
 
-const (
+var (
 	// DefaultConfigFile is the name of the configuration file that is loaded
 	// by default if no other file is specified.
 	DefaultConfigFile = "Caddyfile"

plugins.go

@@ -82,13 +82,31 @@ func ValidDirectives(serverType string) []string {
 	return stype.Directives
 }
 
-// serverListener pairs a server to its listener and/or packetconn.
-type serverListener struct {
+// ServerListener pairs a server to its listener and/or packetconn.
+type ServerListener struct {
 	server   Server
 	listener net.Listener
 	packet   net.PacketConn
 }
 
+// LocalAddr returns the local network address of the packetconn. It returns
+// nil when it is not set.
+func (s ServerListener) LocalAddr() net.Addr {
+	if s.packet == nil {
+		return nil
+	}
+	return s.packet.LocalAddr()
+}
+
+// Addr returns the listener's network address. It returns nil when it is
+// not set.
+func (s ServerListener) Addr() net.Addr {
+	if s.listener == nil {
+		return nil
+	}
+	return s.listener.Addr()
+}
+
 // Context is a type which carries a server type through
 // the load and setup phase; it maintains the state
 // between loading the Caddyfile, then executing its