return placeholder chan for zookeeper lock, remove check in Lock/Unlock test

Signed-off-by: Alexandre Beslic <abronan@docker.com>

pkg/store/store_test.go

@@ -222,13 +222,7 @@ func testLockUnlock(t *testing.T, kv Store) {
 	// Lock should successfully succeed or block
 	lockChan, err := lock.Lock()
 	assert.NoError(t, err)
-
-	// If Zookeeper, chan is Nil and its OK
-	if _, ok := kv.(*Zookeeper); ok {
-		assert.Nil(t, lockChan)
-	} else {
-		assert.NotNil(t, lockChan)
-	}
+	assert.NotNil(t, lockChan)
 
 	// Get should work
 	pair, err := kv.Get(key)

pkg/store/zookeeper.go

@@ -296,7 +296,7 @@ func (l *zookeeperLock) Lock() (<-chan struct{}, error) {
 		_, err = l.client.Set(l.key, l.value, -1)
 	}
 
-	return nil, err
+	return make(chan struct{}), err
 }
 
 // Unlock released the lock. It is an error to call this