Node: Add support for NEO cluster with > 1 master

Some NEO clusters have more than one master to gain a higher availability.
Before this patch NEO/go Node type only handled one master address. This
commit adjusts the node type and related bits so that it can support more than
one master node.

go/neo/client.go

@@ -80,9 +80,9 @@ var _ zodb.IStorageDriver = (*Client)(nil)
 //
 // It will connect to master @masterAddr and identify with specified cluster name.
 // Use Run to actually start running the node.
-func NewClient(clusterName, masterAddr string, net xnet.Networker) *Client {
+func NewClient(clusterName, masterAddrSlice []string, net xnet.Networker) *Client {
 	c := &Client{
-		node:     newMasteredNode(proto.CLIENT, clusterName, net, masterAddr),
+		node:     newMasteredNode(proto.CLIENT, clusterName, net, masterAddrSlice),
 		at0Ready: make(chan struct{}),
 		closed:   make(chan struct{}),
 	}
@@ -483,7 +483,7 @@ func openClientByURL(ctx context.Context, u *url.URL, opt *zodb.DriverOptions) (
 		return nil, zodb.InvalidTid, err
 	}
 
-	c := NewClient(name, u.Host, net)
+	c := NewClient(name, []string{u.Host}, net)
 	c.ownNet = true
 	c.watchq = opt.Watchq
 	defer func() {

go/neo/master.go

@@ -160,7 +160,7 @@ func (_ *_ΔStateCode) δClusterState() {}
 // Use Run to actually start running the node.
 func NewMaster(clusterName string, net xnet.Networker) *Master {
 	return &Master{
-		node: xneo.NewNode(proto.MASTER, clusterName, net, ""),
+		node: xneo.NewNode(proto.MASTER, clusterName, net, []string{}),
 
 		ctlStart:   make(chan chan error),
 		ctlStop:    make(chan chan struct{}),
@@ -219,7 +219,9 @@ func (m *Master) Run(ctx context.Context, l xnet.Listener) (err error) {
 	if err != nil {
 		return err
 	}
-	m.node.MasterAddr = addr.String()
+    // XXX: How can this master node know the address of the
+    //      other master nodes?
+	m.node.MasterAddrSlice = []string{addr.String()}
 	m.node.MyInfo = proto.NodeInfo{
 		Type:	proto.MASTER,
 		Addr:	naddr,

go/neo/mastered.go

@@ -85,9 +85,9 @@ const (
 )
 
 // newMasteredNode creates new _MasteredNode that connects to masterAddr/cluster via net.
-func newMasteredNode(typ proto.NodeType, clusterName string, net xnet.Networker, masterAddr string) *_MasteredNode {
+func newMasteredNode(typ proto.NodeType, clusterName string, net xnet.Networker, masterAddrSlice []string) *_MasteredNode {
 	node := &_MasteredNode{
-		Node: xneo.NewNode(typ, clusterName, net, masterAddr),
+		Node: xneo.NewNode(typ, clusterName, net, masterAddrSlice),
 		opReady: make(chan struct{}),
 	}
 
@@ -108,13 +108,24 @@ func newMasteredNode(typ proto.NodeType, clusterName string, net xnet.Networker,
 func (node *_MasteredNode) TalkMaster(ctx context.Context, f func(context.Context, *_MasterLink) error) (err error) {
 	// start logging with initial NID (that might be temporary, and which master can tell us to change)
 	ctx0 := ctx
+
+	// When a node is created with "NewNode", we don't now yet, which of the
+	// the provided master nodes is the primary master. We'll figure this our here
+	// and simply start with the first node.
+	// We don't assign 'MasterAddr' in 'NewNode', because master nodes are also
+	// created with the 'NewNode' method and master nodes don't know their Addr yet
+	// during initialization time: it's only assigned later when 'Run' is called.
+	if (node.MasterAddr == "") {
+		node.MasterAddr = node.MasterAddrSlice[0]
+	}
+
 	defer task.Runningf(&ctx, "%s: talk master(%s)", node.MyInfo.NID, node.MasterAddr)(&err)
 
 	for {
 		node.updateOperational(func() {
 			node.mlink = nil
 		})
-		err := node.talkMaster1(ctx, ctx0, f)
+		err := node.talkMaster1(ctx, ctx0, f, node.MasterAddr)
 		log.Warning(ctx, err)	// XXX Warning -> Error?
 		if errors.Is(err, cmdShutdown) {
 			return err // M commands to shutdown
@@ -134,7 +145,7 @@ func (node *_MasteredNode) TalkMaster(ctx context.Context, f func(context.Contex
 	}
 }
 
-func (node *_MasteredNode) talkMaster1(ctx, ctxPreTalkM context.Context, f func(context.Context, *_MasterLink) error) error {
+func (node *_MasteredNode) talkMaster1(ctx, ctxPreTalkM context.Context, f func(context.Context, *_MasterLink) error, masterAddr string) error {
 	reqID := &proto.RequestIdentification{
 		NodeType:	node.MyInfo.Type,
 		NID:		node.MyInfo.NID,
@@ -144,7 +155,7 @@ func (node *_MasteredNode) talkMaster1(ctx, ctxPreTalkM context.Context, f func(
 		DevPath:        nil,                // XXX stub
 		NewNID:         nil,	            // XXX stub
 	}
-	mlink, accept, err := xneo.Dial(ctx, proto.MASTER, node.Net, node.MasterAddr, reqID)
+	mlink, accept, err := xneo.Dial(ctx, proto.MASTER, node.Net, masterAddr, reqID)
 	if err != nil {
 		return err
 	}

go/neo/xneo/xneo.go

@@ -80,6 +80,7 @@ type Node struct {
 	ClusterName     string
 	Net             xnet.Networker // network AP we are sending/receiving on
 	MasterAddr      string         // address of current master  TODO -> masterRegistry
+	MasterAddrSlice []string       // address of all known masters
 
 	// XXX reconsider not using State and have just .NodeTab, .PartTab, .ClusterState
 //	StateMu	sync.RWMutex	// <- XXX unexport ?   XXX not used -> move to MasteredNode ?
@@ -90,7 +91,7 @@ type Node struct {
 }
 
 // NewNode creates new node.
-func NewNode(typ proto.NodeType, clusterName string, net xnet.Networker, masterAddr string) *Node {
+func NewNode(typ proto.NodeType, clusterName string, net xnet.Networker, masterAddrSlice []string) *Node {
 	node := &Node{
 		MyInfo:	proto.NodeInfo{
 			Type:   typ,
@@ -101,7 +102,7 @@ func NewNode(typ proto.NodeType, clusterName string, net xnet.Networker, masterA
 		ClusterName: clusterName,
 
 		Net:             net,
-		MasterAddr: masterAddr,
+		MasterAddrSlice: masterAddrSlice,
 
 		State: ClusterState{
 			NodeTab: &NodeTable{},