Adds ability to fetch Mesos Master leader (#7)

* Adds ability to fetch Mesos Master leader from Zookeeper

zk.go

@@ -16,6 +16,7 @@ package realis
 
 import (
 	"encoding/json"
+	"math"
 	"strconv"
 	"strings"
 	"time"
@@ -35,6 +36,18 @@ type ServiceInstance struct {
 	Status              string              `json:"status"`
 }
 
+type MesosAddress struct {
+	Hostname string `json:"hostname"`
+	IP       string `json:"ip"`
+	Port     uint16 `json:"port"`
+}
+
+// MesosInstance is defined for mesos json stored in ZK.
+type MesosInstance struct {
+	Address MesosAddress `json:"address"`
+	Version string       `json:"version"`
+}
+
 type zkConfig struct {
 	endpoints []string
 	path      string
@@ -176,3 +189,100 @@ func LeaderFromZKOpts(options ...ZKOpt) (string, error) {
 
 	return leaderURL, nil
 }
+
+// Retrieves current mesos leader from ZK with a custom configuration.
+func MesosFromZKOpts(options ...ZKOpt) (string, error) {
+	var mesosURL string
+
+	// Load the default configuration for Zookeeper followed by overriding values with those provided by the caller.
+	config := &zkConfig{backoff: defaultBackoff, timeout: time.Second * 10, logger: NoopLogger{}}
+	for _, opt := range options {
+		opt(config)
+	}
+
+	if len(config.endpoints) == 0 {
+		return "", errors.New("no Zookeeper endpoints supplied")
+	}
+
+	if config.path == "" {
+		return "", errors.New("no Zookeeper path supplied")
+	}
+
+	// Create a closure that allows us to use the ExponentialBackoff function.
+	retryErr := ExponentialBackoff(config.backoff, config.logger, func() (bool, error) {
+
+		c, _, err := zk.Connect(config.endpoints, config.timeout, func(c *zk.Conn) { c.SetLogger(config.logger) })
+		if err != nil {
+			return false, NewTemporaryError(errors.Wrap(err, "Failed to connect to Zookeeper"))
+		}
+
+		defer c.Close()
+
+		// Open up descriptor for the ZK path given
+		children, _, _, err := c.ChildrenW(config.path)
+		if err != nil {
+
+			// Sentinel error check as there is no other way to check.
+			if err == zk.ErrInvalidPath {
+				return false, errors.Wrapf(err, "path %s is an invalid Zookeeper path", config.path)
+			}
+
+			return false,
+				NewTemporaryError(errors.Wrapf(err, "path %s doesn't exist on Zookeeper ", config.path))
+		}
+
+		// Search for the leader through all the children in the given path
+		minScore := math.MaxInt64
+		var mesosInstance MesosInstance
+		for _, child := range children {
+			// Only the leader will start with json.info_
+			if strings.HasPrefix(child, "json.info_") {
+				strs := strings.Split(child, "_")
+				if len(strs) < 2 {
+					config.logger.Printf("Zk node %v/%v's name is malformed.", config.path, child)
+					continue
+				}
+				score, err := strconv.Atoi(strs[1])
+				if err != nil {
+					return false, NewTemporaryError(errors.Wrap(err, "unable to read the zk node for Mesos."))
+				}
+
+				// get the leader from the child with the smallest score.
+				if score < minScore {
+					minScore = score
+					childPath := config.path + "/" + child
+					data, _, err := c.Get(childPath)
+					if err != nil {
+						if err == zk.ErrInvalidPath {
+							return false, errors.Wrapf(err, "path %s is an invalid Zookeeper path", childPath)
+						}
+
+						return false, NewTemporaryError(errors.Wrap(err, "error fetching contents of leader"))
+					}
+
+					err = json.Unmarshal([]byte(data), &mesosInstance)
+					if err != nil {
+						config.logger.Printf("%s", err)
+						return false,
+							NewTemporaryError(errors.Wrap(err, "unable to unmarshal contents of leader"))
+					}
+
+					mesosURL = mesosInstance.Address.IP + ":" + strconv.Itoa(int(mesosInstance.Address.Port))
+				}
+			}
+		}
+		if len(mesosURL) > 0 {
+			return true, nil
+		}
+
+		// Leader data might not be available yet, try to fetch again.
+		return false, NewTemporaryError(errors.New("no leader found"))
+	})
+
+	if retryErr != nil {
+		config.logger.Printf("Failed to determine leader after %v attempts", config.backoff.Steps)
+		return "", retryErr
+	}
+
+	return mesosURL, nil
+}