inmem.StackSender.Add returns a reference to the previous chain

Signed-off-by: Solomon Hykes <solomon@docker.com>

inmem/stack.go

@@ -2,6 +2,7 @@ package inmem
 
 import (
 	"container/list"
+	"fmt"
 	"sync"
 )
 
@@ -11,6 +12,7 @@ import (
 // are removed from the queue.
 type StackSender struct {
 	stack *list.List
+	front *list.Element
 	l     sync.RWMutex
 }
 
@@ -21,28 +23,77 @@ func NewStackSender() *StackSender {
 	}
 }
 
-func (s *StackSender) Send(msg *Message, mode int) (Receiver, Sender, error) {
-	s.l.RLock()
-	e := s.stack.Front()
-	s.l.RUnlock()
-	for e != nil {
-		h := e.Value.(Sender)
-		r, w, err := h.Send(msg, mode)
+func (s *StackSender) Send(msg *Message, mode int) (r Receiver, w Sender, err error) {
+	ok := s.walk(func(h Sender) (ok bool) {
+		r, w, err = h.Send(msg, mode)
+		fmt.Printf("sending %v to %#v returned %v\n", msg, h, err)
 		if err == nil {
-			return r, w, nil
+			return true
 		}
+		return false
+	})
+	if ok {
+		return r, w, err
+	}
+	// Silently drop messages if no valid backend is available.
+	return nil, nil, nil
+}
+
+func (s *StackSender) Add(dst Sender) *StackSender {
+	s.l.Lock()
+	defer s.l.Unlock()
+	prev := &StackSender{
+		stack: s.stack,
+		front: s.stack.Front(),
+	}
+	s.stack.PushFront(dst)
+	return prev
+}
+
+func (s *StackSender) Close() error {
+	s.walk(func(h Sender) bool {
+		h.Close()
+		// remove all handlers
+		return false
+	})
+	return nil
+}
+
+func (s *StackSender) walk(f func(Sender) bool) bool {
+	var e *list.Element
+	if s.front == nil {
+		s.l.RLock()
+		e = s.stack.Front()
+		s.l.RUnlock()
+	} else {
+		e = s.front
+	}
+	for e != nil {
+		fmt.Printf("[StackSender.Walk] %v\n", e.Value.(Sender))
+		ok := f(e.Value.(Sender))
+		if ok {
+			fmt.Printf("[StackSender.Walk] %v -> OK\n", e.Value.(Sender))
+			return true
+		}
+		fmt.Printf("[StackSender.Walk] %v invalid: removing\n", e.Value.(Sender))
 		s.l.Lock()
 		next := e.Next()
 		s.stack.Remove(e)
 		s.l.Unlock()
 		e = next
 	}
-	// Silently drop messages if no valid backend is available.
-	return nil, nil, nil
+	fmt.Printf("[STackSender.Walk] -> no valid handler\n")
+	return false
 }
-
-func (s *StackSender) Add(dst Sender) {
-	s.l.Lock()
-	defer s.l.Unlock()
-	s.stack.PushFront(dst)
+func (s *StackSender) Len() int {
+	s.l.RLock()
+	defer s.l.RUnlock()
+	if s.front == nil {
+		return s.stack.Len()
+	}
+	var len int
+	for e := s.front; e != nil ; e = e.Next() {
+		len++
+	}
+	return len
 }