Makes the default protocol 2 and lets 3 interoperate with 2.

command/agent/agent.go

@@ -545,6 +545,22 @@ func (a *Agent) WANMembers() []serf.Member {
 	}
 }
 
+// CanServersUnderstandProtocol checks to see if all the servers understand the
+// given protocol version.
+func (a *Agent) CanServersUnderstandProtocol(version uint8) bool {
+	numServers, numWhoGrok := 0, 0
+	members := a.LANMembers()
+	for _, member := range members {
+		if member.Tags["role"] == "consul" {
+			numServers++
+			if member.ProtocolMax >= version {
+				numWhoGrok++
+			}
+		}
+	}
+	return (numServers > 0) && (numWhoGrok == numServers)
+}
+
 // StartSync is called once Services and Checks are registered.
 // This is called to prevent a race between clients and the anti-entropy routines
 func (a *Agent) StartSync() {
@@ -562,6 +578,16 @@ func (a *Agent) ResumeSync() {
 	a.state.Resume()
 }
 
+// Returns the coordinate of this node in the local pool (assumes coordinates
+// are enabled, so check that before calling).
+func (a *Agent) GetCoordinate() (*coordinate.Coordinate, error) {
+	if a.config.Server {
+		return a.server.GetLANCoordinate()
+	} else {
+		return a.client.GetCoordinate()
+	}
+}
+
 // sendCoordinate is a long-running loop that periodically sends our coordinate
 // to the server. Closing the agent's shutdownChannel will cause this to exit.
 func (a *Agent) sendCoordinate() {
@@ -573,14 +599,13 @@ func (a *Agent) sendCoordinate() {
 
 		select {
 		case <-time.After(intv):
+			if !a.CanServersUnderstandProtocol(3) {
+				continue
+			}
+
 			var c *coordinate.Coordinate
 			var err error
-			if a.config.Server {
+			if c, err = a.GetCoordinate(); err != nil {
-				c, err = a.server.GetLANCoordinate()
-			} else {
-				c, err = a.client.GetCoordinate()
-			}
-			if err != nil {
 				a.logger.Printf("[ERR] agent: failed to get coordinate: %s", err)
 				continue
 			}

command/agent/agent_endpoint.go

@@ -17,18 +17,17 @@ type AgentSelf struct {
 }
 
 func (s *HTTPServer) AgentSelf(resp http.ResponseWriter, req *http.Request) (interface{}, error) {
-	var coord *coordinate.Coordinate
+	var c *coordinate.Coordinate
 	if !s.agent.config.DisableCoordinates {
 		var err error
-		coord, err = s.agent.server.GetLANCoordinate()
+		if c, err = s.agent.GetCoordinate(); err != nil {
-		if err != nil {
 			return nil, err
 		}
 	}
 
 	return AgentSelf{
 		Config: s.agent.config,
-		Coord:  coord,
+		Coord:  c,
 		Member: s.agent.LocalMember(),
 	}, nil
 }

command/agent/agent_test.go

@@ -1581,3 +1581,51 @@ func TestAgent_purgeCheckState(t *testing.T) {
 		t.Fatalf("should have removed file")
 	}
 }
+
+func TestAgent_GetCoordinate(t *testing.T) {
+	check := func(server bool) {
+		config := nextConfig()
+		config.Server = server
+		dir, agent := makeAgent(t, config)
+		defer os.RemoveAll(dir)
+		defer agent.Shutdown()
+
+		// This doesn't verify the returned coordinate, but it makes
+		// sure that the agent chooses the correct Serf instance,
+		// depending on how it's configured as a client or a server.
+		// If it chooses the wrong one, this will crash.
+		if _, err := agent.GetCoordinate(); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
+	check(true)
+	check(false)
+}
+
+func TestAgent_CanServersUnderstandProtocol(t *testing.T) {
+	config := nextConfig()
+	dir, agent := makeAgent(t, config)
+	defer os.RemoveAll(dir)
+	defer agent.Shutdown()
+
+	min := uint8(consul.ProtocolVersionMin)
+	if !agent.CanServersUnderstandProtocol(min) {
+		t.Fatalf("should grok %d", min)
+	}
+
+	max := uint8(consul.ProtocolVersionMax)
+	if !agent.CanServersUnderstandProtocol(max) {
+		t.Fatalf("should grok %d", max)
+	}
+
+	current := uint8(config.Protocol)
+	if !agent.CanServersUnderstandProtocol(current) {
+		t.Fatalf("should grok %d", current)
+	}
+
+	future := max + 1
+	if agent.CanServersUnderstandProtocol(future) {
+		t.Fatalf("should not grok %d", future)
+	}
+}

command/agent/config.go

@@ -478,7 +478,7 @@ func DefaultConfig() *Config {
 		},
 		StatsitePrefix:      "consul",
 		SyslogFacility:      "LOCAL0",
-		Protocol:            consul.ProtocolVersionMax,
+		Protocol:            consul.ProtocolVersion2Compatible,
 		CheckUpdateInterval: 5 * time.Minute,
 		AEInterval:          time.Minute,
 		DisableCoordinates:  false,

consul/config.go

@@ -32,6 +32,7 @@ func init() {
 	protocolVersionMap = map[uint8]uint8{
 		1: 4,
 		2: 4,
+		3: 4,
 	}
 }
 
@@ -267,7 +268,7 @@ func DefaultConfig() *Config {
 		SerfLANConfig:           serf.DefaultConfig(),
 		SerfWANConfig:           serf.DefaultConfig(),
 		ReconcileInterval:       60 * time.Second,
-		ProtocolVersion:         ProtocolVersionMax,
+		ProtocolVersion:         ProtocolVersion2Compatible,
 		ACLTTL:                  30 * time.Second,
 		ACLDefaultPolicy:        "allow",
 		ACLDownPolicy:           "extend-cache",

consul/server.go

@@ -28,7 +28,15 @@ import (
 // protocol versions.
 const (
 	ProtocolVersionMin uint8 = 1
-	ProtocolVersionMax       = 2
+
+	// Version 3 added support for network coordinates but we kept the
+	// default protocol version at 2 to ease the transition to this new
+	// feature. A Consul agent speaking version 2 of the protocol will
+	// attempt to send its coordinates to a server who understands version
+	// 3 or greater.
+	ProtocolVersion2Compatible = 2
+
+	ProtocolVersionMax = 3
 )
 
 const (