feat: allow to get the network address server is listening on (open-t…

…elemetry#217)

Add an Addr() method to fetch the network address Server is listening on. I think this may be useful for the supervisor to start a server with 0 port to allocate an ephemeral port and get the allocated port for rendering an agent config.

server/server.go

@@ -62,4 +62,9 @@ type OpAMPServer interface {
 	// Stop accepting new connections and close all current connections. This should
 	// block until all connections are closed.
 	Stop(ctx context.Context) error
+
+	// Addr returns the network address Server is listening on. Nil if not started.
+	// Typically used to fetch the port when ListenEndpoint's port is specified as 0 to
+	// allocate an ephemeral port.
+	Addr() net.Addr
 }

server/serverimpl.go

@@ -40,6 +40,9 @@ type server struct {
 	// The listening HTTP Server after successful Start() call. Nil if Start()
 	// is not called or was not successful.
 	httpServer *http.Server
+
+	// The network address Server is listening on. Nil if not started.
+	addr net.Addr
 }
 
 var _ OpAMPServer = (*server)(nil)
@@ -118,6 +121,7 @@ func (s *server) startHttpServer(listenAddr string, serveFunc func(l net.Listene
 	if err != nil {
 		return err
 	}
+	s.addr = ln.Addr()
 
 	// Begin serving connections in the background.
 	go func() {
@@ -143,6 +147,10 @@ func (s *server) Stop(ctx context.Context) error {
 	return nil
 }
 
+func (s *server) Addr() net.Addr {
+	return s.addr
+}
+
 func (s *server) httpHandler(w http.ResponseWriter, req *http.Request) {
 	var connectionCallbacks serverTypes.ConnectionCallbacks
 	if s.settings.Callbacks != nil {

server/serverimpl_test.go

@@ -57,6 +57,48 @@ func TestServerStartStop(t *testing.T) {
 	assert.NoError(t, err)
 }
 
+func TestServerAddrWithNonZeroPort(t *testing.T) {
+	srv := New(&sharedinternal.NopLogger{})
+	require.NotNil(t, srv)
+
+	// Nil if not started
+	assert.Nil(t, srv.Addr())
+
+	addr := testhelpers.GetAvailableLocalAddress()
+
+	err := srv.Start(StartSettings{
+		ListenEndpoint: addr,
+		ListenPath:     "/",
+	})
+	assert.NoError(t, err)
+
+	assert.Equal(t, addr, srv.Addr().String())
+
+	err = srv.Stop(context.Background())
+	assert.NoError(t, err)
+}
+
+func TestServerAddrWithZeroPort(t *testing.T) {
+	srv := New(&sharedinternal.NopLogger{})
+	require.NotNil(t, srv)
+
+	// Nil if not started
+	assert.Nil(t, srv.Addr())
+
+	err := srv.Start(StartSettings{
+		ListenEndpoint: "127.0.0.1:0",
+		ListenPath:     "/",
+	})
+	assert.NoError(t, err)
+
+	// should be listening on an non-zero ephemeral port
+	assert.NotEqual(t, "127.0.0.1:0", srv.Addr().String())
+	assert.Regexp(t, `^127.0.0.1:\d+`, srv.Addr().String())
+
+	err = srv.Stop(context.Background())
+	assert.NoError(t, err)
+}
+
 func TestServerStartRejectConnection(t *testing.T) {
 	callbacks := CallbacksStruct{
 		OnConnectingFunc: func(request *http.Request) types.ConnectionResponse {