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/file/file.go

@@ -0,0 +1,109 @@
+package file
+
+import (
+	"fmt"
+	"io/ioutil"
+	"strings"
+	"time"
+
+	"github.com/docker/docker/pkg/discovery"
+)
+
+// Discovery is exported
+type Discovery struct {
+	heartbeat time.Duration
+	path      string
+}
+
+func init() {
+	Init()
+}
+
+// Init is exported
+func Init() {
+	discovery.Register("file", &Discovery{})
+}
+
+// Initialize is exported
+func (s *Discovery) Initialize(path string, heartbeat time.Duration, ttl time.Duration) error {
+	s.path = path
+	s.heartbeat = heartbeat
+	return nil
+}
+
+func parseFileContent(content []byte) []string {
+	var result []string
+	for _, line := range strings.Split(strings.TrimSpace(string(content)), "\n") {
+		line = strings.TrimSpace(line)
+		// Ignoring line starts with #
+		if strings.HasPrefix(line, "#") {
+			continue
+		}
+		// Inlined # comment also ignored.
+		if strings.Contains(line, "#") {
+			line = line[0:strings.Index(line, "#")]
+			// Trim additional spaces caused by above stripping.
+			line = strings.TrimSpace(line)
+		}
+		for _, ip := range discovery.Generate(line) {
+			result = append(result, ip)
+		}
+	}
+	return result
+}
+
+func (s *Discovery) fetch() (discovery.Entries, error) {
+	fileContent, err := ioutil.ReadFile(s.path)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read '%s': %v", s.path, err)
+	}
+	return discovery.CreateEntries(parseFileContent(fileContent))
+}
+
+// Watch is exported
+func (s *Discovery) Watch(stopCh <-chan struct{}) (<-chan discovery.Entries, <-chan error) {
+	ch := make(chan discovery.Entries)
+	errCh := make(chan error)
+	ticker := time.NewTicker(s.heartbeat)
+
+	go func() {
+		defer close(errCh)
+		defer close(ch)
+
+		// Send the initial entries if available.
+		currentEntries, err := s.fetch()
+		if err != nil {
+			errCh <- err
+		} else {
+			ch <- currentEntries
+		}
+
+		// Periodically send updates.
+		for {
+			select {
+			case <-ticker.C:
+				newEntries, err := s.fetch()
+				if err != nil {
+					errCh <- err
+					continue
+				}
+
+				// Check if the file has really changed.
+				if !newEntries.Equals(currentEntries) {
+					ch <- newEntries
+				}
+				currentEntries = newEntries
+			case <-stopCh:
+				ticker.Stop()
+				return
+			}
+		}
+	}()
+
+	return ch, errCh
+}
+
+// Register is exported
+func (s *Discovery) Register(addr string) error {
+	return discovery.ErrNotImplemented
+}