raft: Allow Join to be called multiple times for the same cluster member

If Join is called and the member already exists in the cluster, instead
of returning an error, it will update the node's address with the new
one provided to the RPC.

This will allow managers to update their addresses automatically on
startup, if they were configured to autodetect the addresses.

It will also be possible to manually repeat the "docker swarm join"
command, to specify a different advertise address, or rejoin a cluster
when all known manager IPs have changed.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

manager/state/raft/raft.go

@@ -909,24 +909,6 @@ func (n *Node) Join(ctx context.Context, req *api.JoinRequest) (*api.JoinRespons
 		return nil, grpc.Errorf(codes.FailedPrecondition, "%s", ErrLostLeadership.Error())
 	}
 
-	// A single manager must not be able to join the raft cluster twice. If
-	// it did, that would cause the quorum to be computed incorrectly. This
-	// could happen if the WAL was deleted from an active manager.
-	for _, m := range n.cluster.Members() {
-		if m.NodeID == nodeInfo.NodeID {
-			return nil, grpc.Errorf(codes.AlreadyExists, "%s", "a raft member with this node ID already exists")
-		}
-	}
-
-	// Find a unique ID for the joining member.
-	var raftID uint64
-	for {
-		raftID = uint64(rand.Int63()) + 1
-		if n.cluster.GetMember(raftID) == nil && !n.cluster.IsIDRemoved(raftID) {
-			break
-		}
-	}
-
 	remoteAddr := req.Addr
 
 	// If the joining node sent an address like 0.0.0.0:4242, automatically
@@ -953,12 +935,52 @@ func (n *Node) Join(ctx context.Context, req *api.JoinRequest) (*api.JoinRespons
 		return nil, err
 	}
 
+	// If the peer is already a member of the cluster, we will only update
+	// its information, not add it as a new member. Adding it again would
+	// cause the quorum to be computed incorrectly.
+	for _, m := range n.cluster.Members() {
+		if m.NodeID == nodeInfo.NodeID {
+			if remoteAddr == m.Addr {
+				return n.joinResponse(m.RaftID), nil
+			}
+			updatedRaftMember := &api.RaftMember{
+				RaftID: m.RaftID,
+				NodeID: m.NodeID,
+				Addr:   remoteAddr,
+			}
+			if err := n.cluster.UpdateMember(m.RaftID, updatedRaftMember); err != nil {
+				return nil, err
+			}
+
+			if err := n.updateNodeBlocking(ctx, m.RaftID, remoteAddr); err != nil {
+				return nil, err
+			}
+
+			return n.joinResponse(m.RaftID), nil
+		}
+	}
+
+	// Find a unique ID for the joining member.
+	var raftID uint64
+	for {
+		raftID = uint64(rand.Int63()) + 1
+		if n.cluster.GetMember(raftID) == nil && !n.cluster.IsIDRemoved(raftID) {
+			break
+		}
+	}
+
 	err = n.addMember(ctx, remoteAddr, raftID, nodeInfo.NodeID)
 	if err != nil {
 		log.WithError(err).Errorf("failed to add member %x", raftID)
 		return nil, err
 	}
 
+	log.Debugf("node joined")
+
+	return n.joinResponse(raftID), nil
+}
+
+func (n *Node) joinResponse(raftID uint64) *api.JoinResponse {
 	var nodes []*api.RaftMember
 	for _, node := range n.cluster.Members() {
 		nodes = append(nodes, &api.RaftMember{
@@ -967,9 +989,8 @@ func (n *Node) Join(ctx context.Context, req *api.JoinRequest) (*api.JoinRespons
 			Addr:   node.Addr,
 		})
 	}
-	log.Debugf("node joined")
 
-	return &api.JoinResponse{Members: nodes, RaftID: raftID}, nil
+	return &api.JoinResponse{Members: nodes, RaftID: raftID}
 }
 
 // checkHealth tries to contact an aspiring member through its advertised address

manager/state/raft/raft_test.go

@@ -6,14 +6,14 @@ import (
 	"io/ioutil"
 	"log"
 	"math/rand"
+	"net"
 	"os"
 	"reflect"
 	"strconv"
 	"testing"
 	"time"
 
 	"google.golang.org/grpc"
-	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/grpclog"
 
 	"golang.org/x/net/context"
@@ -88,9 +88,19 @@ func dial(n *raftutils.TestNode, addr string) (*grpc.ClientConn, error) {
 func TestRaftJoinTwice(t *testing.T) {
 	t.Parallel()
 
-	nodes, _ := raftutils.NewRaftCluster(t, tc)
+	nodes, clockSource := raftutils.NewRaftCluster(t, tc)
 	defer raftutils.TeardownCluster(nodes)
 
+	// Node 3's address changes
+	nodes[3].Server.Stop()
+	nodes[3].ShutdownRaft()
+	nodes[3].Listener.CloseListener()
+
+	l, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err, "can't bind to raft service port")
+	nodes[3].Listener = raftutils.NewWrappedListener(l)
+	nodes[3] = raftutils.RestartNode(t, clockSource, nodes[3], false)
+
 	// Node 3 tries to join again
 	// Use gRPC instead of calling handler directly because of
 	// authorization check.
@@ -99,10 +109,23 @@ func TestRaftJoinTwice(t *testing.T) {
 	raftClient := api.NewRaftMembershipClient(cc)
 	defer cc.Close()
 	ctx, _ := context.WithTimeout(context.Background(), 10*time.Second)
-	_, err = raftClient.Join(ctx, &api.JoinRequest{})
-	assert.Error(t, err, "expected error on duplicate Join")
-	assert.Equal(t, grpc.Code(err), codes.AlreadyExists)
-	assert.Equal(t, grpc.ErrorDesc(err), "a raft member with this node ID already exists")
+	_, err = raftClient.Join(ctx, &api.JoinRequest{Addr: l.Addr().String()})
+	assert.NoError(t, err)
+
+	// Propose a value and wait for it to propagate
+	value, err := raftutils.ProposeValue(t, nodes[1], DefaultProposalTime)
+	assert.NoError(t, err, "failed to propose value")
+	raftutils.CheckValue(t, clockSource, nodes[2], value)
+
+	// Restart node 2
+	nodes[2].Server.Stop()
+	nodes[2].ShutdownRaft()
+	nodes[2] = raftutils.RestartNode(t, clockSource, nodes[2], false)
+	raftutils.WaitForCluster(t, clockSource, nodes)
+
+	// Node 2 should have the updated address for node 3 in its member list
+	require.NotNil(t, nodes[2].GetMemberlist()[nodes[3].Config.ID])
+	require.Equal(t, l.Addr().String(), nodes[2].GetMemberlist()[nodes[3].Config.ID].Addr)
 }
 
 func TestRaftLeader(t *testing.T) {

manager/state/raft/testutils/testutils.go

@@ -159,8 +159,8 @@ func (l *WrappedListener) Close() error {
 	return nil
 }
 
-// CloseListener closes the listener
-func (l *WrappedListener) close() error {
+// CloseListener closes the underlying listener
+func (l *WrappedListener) CloseListener() error {
 	return l.Listener.Close()
 }
 
@@ -471,7 +471,7 @@ func ShutdownNode(node *TestNode) {
 		<-node.Done()
 	}
 	os.RemoveAll(node.StateDir)
-	node.Listener.close()
+	node.Listener.CloseListener()
 }
 
 // ShutdownRaft shutdowns only raft part of node.
@@ -487,7 +487,7 @@ func (n *TestNode) ShutdownRaft() {
 func CleanupNonRunningNode(node *TestNode) {
 	node.Server.Stop()
 	os.RemoveAll(node.StateDir)
-	node.Listener.close()
+	node.Listener.CloseListener()
 }
 
 // Leader determines who is the leader amongst a set of raft nodes