Add pkg/discovery for nodes discovery

Absorb Swarm's discovery package in order to provide a common node
discovery mechanism to be used by both Swarm and networking code.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>

discovery/backends.go

@@ -0,0 +1,53 @@
+package discovery
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	log "github.com/Sirupsen/logrus"
+)
+
+var (
+	// Backends is a global map of discovery backends indexed by their
+	// associated scheme.
+	backends map[string]Backend
+)
+
+func init() {
+	backends = make(map[string]Backend)
+}
+
+// Register makes a discovery backend available by the provided scheme.
+// If Register is called twice with the same scheme an error is returned.
+func Register(scheme string, d Backend) error {
+	if _, exists := backends[scheme]; exists {
+		return fmt.Errorf("scheme already registered %s", scheme)
+	}
+	log.WithField("name", scheme).Debug("Registering discovery service")
+	backends[scheme] = d
+	return nil
+}
+
+func parse(rawurl string) (string, string) {
+	parts := strings.SplitN(rawurl, "://", 2)
+
+	// nodes:port,node2:port => nodes://node1:port,node2:port
+	if len(parts) == 1 {
+		return "nodes", parts[0]
+	}
+	return parts[0], parts[1]
+}
+
+// New returns a new Discovery given a URL, heartbeat and ttl settings.
+// Returns an error if the URL scheme is not supported.
+func New(rawurl string, heartbeat time.Duration, ttl time.Duration) (Backend, error) {
+	scheme, uri := parse(rawurl)
+	if backend, exists := backends[scheme]; exists {
+		log.WithFields(log.Fields{"name": scheme, "uri": uri}).Debug("Initializing discovery service")
+		err := backend.Initialize(uri, heartbeat, ttl)
+		return backend, err
+	}
+
+	return nil, ErrNotSupported
+}