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/roundrobin/roundrobin_test.go

@@ -0,0 +1,49 @@
+package roundrobin
+
+import (
+	"testing"
+
+	"github.com/micro/go-micro/v2/router"
+	"github.com/micro/go-micro/v2/selector"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRoundRobin(t *testing.T) {
+	selector.Tests(t, NewSelector())
+
+	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"}
+	r3 := &router.Route{Service: "go.micro.service.foo", Address: "127.0.0.1:8002"}
+
+	sel := NewSelector()
+
+	// By passing r1 and r2 first, it forces a set sequence of (r1 => r2 => r3 => r1)
+
+	r, err := sel.Select([]*router.Route{r1})
+	assert.Nil(t, err, "Error should be nil")
+	assert.Equal(t, r1, r, "Expected route to be r1")
+
+	r, err = sel.Select([]*router.Route{r2})
+	assert.Nil(t, err, "Error should be nil")
+	assert.Equal(t, r2, r, "Expected route to be r2")
+
+	// Because r1 and r2 have been recently called, r3 should be chosen
+
+	r, err = sel.Select([]*router.Route{r1, r2, r3})
+	assert.Nil(t, err, "Error should be nil")
+	assert.Equal(t, r3, r, "Expected route to be r3")
+
+	// r1 was called longest ago, so it should be prioritised
+
+	r, err = sel.Select([]*router.Route{r1, r2, r3})
+	assert.Nil(t, err, "Error should be nil")
+	assert.Equal(t, r1, r, "Expected route to be r1")
+
+	r, err = sel.Select([]*router.Route{r1, r2, r3})
+	assert.Nil(t, err, "Error should be nil")
+	assert.Equal(t, r2, r, "Expected route to be r2")
+
+	r, err = sel.Select([]*router.Route{r1, r2, r3})
+	assert.Nil(t, err, "Error should be nil")
+	assert.Equal(t, r3, r, "Expected route to be r3")
+}