client: Sync() pin the endpoint when member list doesn't change

This helps client to pin the same endpoint as long as cluster doesn't change.

client/client.go

@@ -22,6 +22,8 @@ import (
 	"net"
 	"net/http"
 	"net/url"
+	"reflect"
+	"sort"
 	"sync"
 	"time"
 
@@ -313,6 +315,18 @@ func (c *httpClusterClient) Sync(ctx context.Context) error {
 	for _, m := range ms {
 		eps = append(eps, m.ClientURLs...)
 	}
+	sort.Sort(sort.StringSlice(eps))
+
+	ceps := make([]string, len(c.endpoints))
+	for i, cep := range c.endpoints {
+		ceps[i] = cep.String()
+	}
+	sort.Sort(sort.StringSlice(ceps))
+	// fast path if no change happens
+	// this helps client to pin the endpoint when no cluster change
+	if reflect.DeepEqual(eps, ceps) {
+		return nil
+	}
 
 	return c.reset(eps)
 }

client/client_test.go

@@ -802,6 +802,46 @@ func TestHTTPClusterClientAutoSyncFail(t *testing.T) {
 	}
 }
 
+// TestHTTPClusterClientSyncPinEndpoint tests that Sync() pins the endpoint when
+// it gets the exactly same member list as before.
+func TestHTTPClusterClientSyncPinEndpoint(t *testing.T) {
+	cf := newStaticHTTPClientFactory([]staticHTTPResponse{
+		staticHTTPResponse{
+			resp: http.Response{StatusCode: http.StatusOK, Header: http.Header{"Content-Type": []string{"application/json"}}},
+			body: []byte(`{"members":[{"id":"2745e2525fce8fe","peerURLs":["http://127.0.0.1:7003"],"name":"node3","clientURLs":["http://127.0.0.1:4003"]},{"id":"42134f434382925","peerURLs":["http://127.0.0.1:2380","http://127.0.0.1:7001"],"name":"node1","clientURLs":["http://127.0.0.1:2379","http://127.0.0.1:4001"]},{"id":"94088180e21eb87b","peerURLs":["http://127.0.0.1:7002"],"name":"node2","clientURLs":["http://127.0.0.1:4002"]}]}`),
+		},
+		staticHTTPResponse{
+			resp: http.Response{StatusCode: http.StatusOK, Header: http.Header{"Content-Type": []string{"application/json"}}},
+			body: []byte(`{"members":[{"id":"2745e2525fce8fe","peerURLs":["http://127.0.0.1:7003"],"name":"node3","clientURLs":["http://127.0.0.1:4003"]},{"id":"42134f434382925","peerURLs":["http://127.0.0.1:2380","http://127.0.0.1:7001"],"name":"node1","clientURLs":["http://127.0.0.1:2379","http://127.0.0.1:4001"]},{"id":"94088180e21eb87b","peerURLs":["http://127.0.0.1:7002"],"name":"node2","clientURLs":["http://127.0.0.1:4002"]}]}`),
+		},
+		staticHTTPResponse{
+			resp: http.Response{StatusCode: http.StatusOK, Header: http.Header{"Content-Type": []string{"application/json"}}},
+			body: []byte(`{"members":[{"id":"2745e2525fce8fe","peerURLs":["http://127.0.0.1:7003"],"name":"node3","clientURLs":["http://127.0.0.1:4003"]},{"id":"42134f434382925","peerURLs":["http://127.0.0.1:2380","http://127.0.0.1:7001"],"name":"node1","clientURLs":["http://127.0.0.1:2379","http://127.0.0.1:4001"]},{"id":"94088180e21eb87b","peerURLs":["http://127.0.0.1:7002"],"name":"node2","clientURLs":["http://127.0.0.1:4002"]}]}`),
+		},
+	})
+
+	hc := &httpClusterClient{
+		clientFactory: cf,
+		rand:          rand.New(rand.NewSource(0)),
+	}
+	err := hc.reset([]string{"http://127.0.0.1:4003", "http://127.0.0.1:2379", "http://127.0.0.1:4001", "http://127.0.0.1:4002"})
+	if err != nil {
+		t.Fatalf("unexpected error during setup: %#v", err)
+	}
+	pinnedEndpoint := hc.endpoints[hc.pinned]
+
+	for i := 0; i < 3; i++ {
+		err = hc.Sync(context.Background())
+		if err != nil {
+			t.Fatalf("#%d: unexpected error during Sync: %#v", i, err)
+		}
+
+		if g := hc.endpoints[hc.pinned]; g != pinnedEndpoint {
+			t.Errorf("#%d: pinned endpoint = %s, want %s", i, g, pinnedEndpoint)
+		}
+	}
+}
+
 func TestHTTPClusterClientResetFail(t *testing.T) {
 	tests := [][]string{
 		// need at least one endpoint