Add configurable discovery

This PR allows to configure the discovery path using the
--discovery-opt flag (with "kv.path=path/to/nodes"). We
can point to "docker/nodes" and use the docker discovery.

If docker instances are advertising to the cluster using
the `--cluster-advertise` flag, the swarm join command
becomes unnecessary.

Signed-off-by: Alexandre Beslic <abronan@docker.com>

discovery/kv/kv.go

@@ -17,7 +17,7 @@ import (
 )
 
 const (
-	discoveryPath = "docker/swarm/nodes"
+	defaultDiscoveryPath = "docker/swarm/nodes"
 )
 
 // Discovery is exported
@@ -62,7 +62,14 @@ func (s *Discovery) Initialize(uris string, heartbeat time.Duration, ttl time.Du
 
 	s.heartbeat = heartbeat
 	s.ttl = ttl
-	s.path = path.Join(s.prefix, discoveryPath)
+
+	// Use a custom path if specified in discovery options
+	dpath := defaultDiscoveryPath
+	if discoveryOpt["kv.path"] != "" {
+		dpath = discoveryOpt["kv.path"]
+	}
+
+	s.path = path.Join(s.prefix, dpath)
 
 	var config *store.Config
 	if discoveryOpt["kv.cacertfile"] != "" && discoveryOpt["kv.certfile"] != "" && discoveryOpt["kv.keyfile"] != "" {

discovery/kv/kv_test.go

@@ -28,7 +28,7 @@ func TestInitialize(t *testing.T) {
 	s := d.store.(*libkvmock.Mock)
 	assert.Len(t, s.Endpoints, 1)
 	assert.Equal(t, s.Endpoints[0], "127.0.0.1")
-	assert.Equal(t, d.path, discoveryPath)
+	assert.Equal(t, d.path, defaultDiscoveryPath)
 
 	storeMock, err = libkvmock.New([]string{"127.0.0.1:1234"}, nil)
 	assert.NotNil(t, storeMock)
@@ -41,7 +41,7 @@ func TestInitialize(t *testing.T) {
 	s = d.store.(*libkvmock.Mock)
 	assert.Len(t, s.Endpoints, 1)
 	assert.Equal(t, s.Endpoints[0], "127.0.0.1:1234")
-	assert.Equal(t, d.path, "path/"+discoveryPath)
+	assert.Equal(t, d.path, "path/"+defaultDiscoveryPath)
 
 	storeMock, err = libkvmock.New([]string{"127.0.0.1:1234", "127.0.0.2:1234", "127.0.0.3:1234"}, nil)
 	assert.NotNil(t, storeMock)
@@ -57,7 +57,7 @@ func TestInitialize(t *testing.T) {
 		assert.Equal(t, s.Endpoints[1], "127.0.0.2:1234")
 		assert.Equal(t, s.Endpoints[2], "127.0.0.3:1234")
 	}
-	assert.Equal(t, d.path, "path/"+discoveryPath)
+	assert.Equal(t, d.path, "path/"+defaultDiscoveryPath)
 }
 
 func TestInitializeWithCerts(t *testing.T) {
@@ -147,19 +147,19 @@ func TestWatch(t *testing.T) {
 	mockCh := make(chan []*store.KVPair)
 
 	// The first watch will fail on those three calls
-	s.On("Exists", "path/"+discoveryPath).Return(false, errors.New("test error"))
-	s.On("Put", "path/"+discoveryPath, mock.Anything, mock.Anything).Return(errors.New("test error"))
-	s.On("WatchTree", "path/"+discoveryPath, mock.Anything).Return(mockCh, errors.New("test error")).Once()
+	s.On("Exists", "path/"+defaultDiscoveryPath).Return(false, errors.New("test error"))
+	s.On("Put", "path/"+defaultDiscoveryPath, mock.Anything, mock.Anything).Return(errors.New("test error"))
+	s.On("WatchTree", "path/"+defaultDiscoveryPath, mock.Anything).Return(mockCh, errors.New("test error")).Once()
 
 	// The second one will succeed.
-	s.On("WatchTree", "path/"+discoveryPath, mock.Anything).Return(mockCh, nil).Once()
+	s.On("WatchTree", "path/"+defaultDiscoveryPath, mock.Anything).Return(mockCh, nil).Once()
 	expected := discovery.Entries{
 		&discovery.Entry{Host: "1.1.1.1", Port: "1111"},
 		&discovery.Entry{Host: "2.2.2.2", Port: "2222"},
 	}
 	kvs := []*store.KVPair{
-		{Key: path.Join("path", discoveryPath, "1.1.1.1"), Value: []byte("1.1.1.1:1111")},
-		{Key: path.Join("path", discoveryPath, "2.2.2.2"), Value: []byte("2.2.2.2:2222")},
+		{Key: path.Join("path", defaultDiscoveryPath, "1.1.1.1"), Value: []byte("1.1.1.1:1111")},
+		{Key: path.Join("path", defaultDiscoveryPath, "2.2.2.2"), Value: []byte("2.2.2.2:2222")},
 	}
 
 	stopCh := make(chan struct{})
@@ -179,13 +179,13 @@ func TestWatch(t *testing.T) {
 
 	// Add a new entry.
 	expected = append(expected, &discovery.Entry{Host: "3.3.3.3", Port: "3333"})
-	kvs = append(kvs, &store.KVPair{Key: path.Join("path", discoveryPath, "3.3.3.3"), Value: []byte("3.3.3.3:3333")})
+	kvs = append(kvs, &store.KVPair{Key: path.Join("path", defaultDiscoveryPath, "3.3.3.3"), Value: []byte("3.3.3.3:3333")})
 	mockCh <- kvs
 	assert.Equal(t, <-ch, expected)
 
 	// Make sure that if an error occurs it retries.
 	// This third call to WatchTree will be checked later by AssertExpectations.
-	s.On("WatchTree", "path/"+discoveryPath, mock.Anything).Return(mockCh, nil)
+	s.On("WatchTree", "path/"+defaultDiscoveryPath, mock.Anything).Return(mockCh, nil)
 	close(mockCh)
 	// Give it enough time to call WatchTree.
 	time.Sleep(3)