Implement lock and unlock mechanism

nodesync.go

@@ -19,6 +19,9 @@ var defaultAcl = zk.WorldACL(zk.PermAll)
 // The length of a postfix sequence, see http://zookeeper.apache.org/doc/current/zookeeperProgrammers.html#Sequence+Nodes+--+Unique+Naming
 var sequenceLen = 10
 
+// Name is used by default to create keys
+var defaultKey = "sync"
+
 // New creates a new nodesync instance
 func New(zkconn *zk.Conn, rootPath string) (nodeSync *NodeSync, err error) {
 
@@ -61,3 +64,76 @@ func (s *NodeSync) FetchAndSet(where string, key string, value []byte) (old []by
 
 	return old, err
 }
+
+// Lock creates zookeeper ephemeral key to ensure itself in the node list.
+// This is used by the locking mechanism to emulate mutex in a cluster.
+func (s *NodeSync) Lock(where string) (iam string, err error) {
+
+	workingPath := path.Join(s.rootPath, where)
+
+	err = s.createRecursively(workingPath, defaultAcl)
+	if err != nil {
+		return
+	}
+
+	created, err := s.Zk.CreateProtectedEphemeralSequential(path.Join(workingPath, defaultKey), nil, defaultAcl)
+	if err != nil {
+		return
+	}
+
+	_, iam = path.Split(created)
+
+	return
+}
+
+// Unlock waits for the previous worker be unlocked, then removes itself
+func (s *NodeSync) Unlock(where string, iam string) (err error) {
+
+	workingPath := path.Join(s.rootPath, where)
+
+	children, _, err := s.Zk.Children(workingPath)
+	if err != nil {
+		return err
+	}
+	if len(children) == 0 {
+		return
+	}
+
+	children = sortChildren(children)
+
+	locked := children[0]
+	if locked == iam {
+		err = s.Zk.Delete(path.Join(workingPath, iam), 0)
+		return
+	}
+
+	for i := 1; i < len(children); i++ {
+		if children[i] == iam {
+			break
+		}
+		locked = children[i]
+	}
+
+	for {
+		var exists bool
+		var ev <-chan zk.Event
+
+		exists, _, ev, err = s.Zk.ExistsW(path.Join(workingPath, locked))
+		if err != nil {
+			return
+		}
+		if !exists {
+			break
+		}
+
+		change := <-ev
+
+		if change.Type == zk.EventNodeDeleted {
+			break
+		}
+	}
+
+	err = s.Zk.Delete(path.Join(workingPath, iam), 0)
+
+	return
+}