consul: Support a stale read query

consul/catalog_endpoint_test.go

@@ -232,6 +232,57 @@ func TestCatalogListNodes(t *testing.T) {
 	}
 }
 
+func TestCatalogListNodes_StaleRaad(t *testing.T) {
+	dir1, s1 := testServer(t)
+	defer os.RemoveAll(dir1)
+	defer s1.Shutdown()
+	client1 := rpcClient(t, s1)
+	defer client1.Close()
+
+	dir2, s2 := testServer(t)
+	defer os.RemoveAll(dir2)
+	defer s2.Shutdown()
+	client2 := rpcClient(t, s2)
+	defer client2.Close()
+
+	// Try to join
+	addr := fmt.Sprintf("127.0.0.1:%d",
+		s1.config.SerfLANConfig.MemberlistConfig.BindPort)
+	if _, err := s2.JoinLAN([]string{addr}); err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Wait for a leader
+	time.Sleep(100 * time.Millisecond)
+
+	// Use the follower as the client
+	var client *rpc.Client
+	if !s1.IsLeader() {
+		client = client1
+
+		// Inject fake data on the follower!
+		s1.fsm.State().EnsureNode(1, structs.Node{"foo", "127.0.0.1"})
+	} else {
+		client = client2
+
+		// Inject fake data on the follower!
+		s2.fsm.State().EnsureNode(1, structs.Node{"foo", "127.0.0.1"})
+	}
+
+	args := structs.DCSpecificRequest{
+		Datacenter:   "dc1",
+		QueryOptions: structs.QueryOptions{AllowStale: true},
+	}
+	var out structs.IndexedNodes
+	if err := client.Call("Catalog.ListNodes", &args, &out); err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	if len(out.Nodes) != 3 {
+		t.Fatalf("bad: %v", out)
+	}
+}
+
 func BenchmarkCatalogListNodes(t *testing.B) {
 	dir1, s1 := testServer(nil)
 	defer os.RemoveAll(dir1)

consul/rpc.go

@@ -142,6 +142,11 @@ func (s *Server) forward(method string, info structs.RPCInfo, args interface{},
 		return true, err
 	}
 
+	// Check if we can allow a stale read
+	if info.IsRead() && info.AllowStaleRead() {
+		return false, nil
+	}
+
 	// Handle leader forwarding
 	if !s.IsLeader() {
 		err := s.forwardLeader(method, args, reply)