selector: new selector interface with random & roundrobin implementation (#1761)

* selector: implement new selector interface plus random & roundrobin implementations

* selector/roundrobin: remove unused consts

* router: add close method to interface

* selector/roundrobin: fix concurrent map iteration and map write

* selector: replace variadic argument on Select

selector/tests.go

@@ -0,0 +1,45 @@
+package selector
+
+import (
+	"testing"
+
+	"github.com/micro/go-micro/v2/router"
+	"github.com/stretchr/testify/assert"
+)
+
+// Tests runs all the tests against a selector to ensure the implementations are consistent
+func Tests(t *testing.T, s Selector) {
+	r1 := &router.Route{Service: "go.micro.service.foo", Address: "127.0.0.1:8000"}
+	r2 := &router.Route{Service: "go.micro.service.foo", Address: "127.0.0.1:8001"}
+
+	t.Run("Select", func(t *testing.T) {
+		t.Run("NoRoutes", func(t *testing.T) {
+			srv, err := s.Select([]*router.Route{})
+			assert.Nil(t, srv, "Route should be nil")
+			assert.Equal(t, ErrNoneAvailable, err, "Expected error to be none available")
+		})
+
+		t.Run("OneRoute", func(t *testing.T) {
+			srv, err := s.Select([]*router.Route{r1})
+			assert.Nil(t, err, "Error should be nil")
+			assert.Equal(t, r1, srv, "Expected the route to be returned")
+		})
+
+		t.Run("MultipleRoutes", func(t *testing.T) {
+			srv, err := s.Select([]*router.Route{r1, r2})
+			assert.Nil(t, err, "Error should be nil")
+			if srv.Address != r1.Address && srv.Address != r2.Address {
+				t.Errorf("Expected the route to be one of the inputs")
+			}
+		})
+	})
+
+	t.Run("Record", func(t *testing.T) {
+		err := s.Record(r1, nil)
+		assert.Nil(t, err, "Expected the error to be nil")
+	})
+
+	t.Run("String", func(t *testing.T) {
+		assert.NotEmpty(t, s.String(), "String returned a blank string")
+	})
+}