Fix a race in pkg/discovery/memory

Fix #22964

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

discovery/memory/memory.go

@@ -1,16 +1,18 @@
 package memory
 
 import (
+	"sync"
 	"time"
 
 	"github.com/docker/docker/pkg/discovery"
 )
 
-// Discovery implements a descovery backend that keeps
+// Discovery implements a discovery backend that keeps
 // data in memory.
 type Discovery struct {
 	heartbeat time.Duration
 	values    []string
+	mu        sync.Mutex
 }
 
 func init() {
@@ -41,21 +43,27 @@ func (s *Discovery) Watch(stopCh <-chan struct{}) (<-chan discovery.Entries, <-c
 
 		// Send the initial entries if available.
 		var currentEntries discovery.Entries
-		if len(s.values) > 0 {
 		var err error
+
+		s.mu.Lock()
+		if len(s.values) > 0 {
 			currentEntries, err = discovery.CreateEntries(s.values)
+		}
+		s.mu.Unlock()
+
 		if err != nil {
 			errCh <- err
-			} else {
+		} else if currentEntries != nil {
 			ch <- currentEntries
 		}
-		}
 
 		// Periodically send updates.
 		for {
 			select {
 			case <-ticker.C:
+				s.mu.Lock()
 				newEntries, err := discovery.CreateEntries(s.values)
+				s.mu.Unlock()
 				if err != nil {
 					errCh <- err
 					continue
@@ -78,6 +86,8 @@ func (s *Discovery) Watch(stopCh <-chan struct{}) (<-chan discovery.Entries, <-c
 
 // Register adds a new address to the discovery.
 func (s *Discovery) Register(addr string) error {
+	s.mu.Lock()
 	s.values = append(s.values, addr)
+	s.mu.Unlock()
 	return nil
 }