Remove cache.ErrRequeue

cache.ErrRequeue advertised itself as a way to requeue failures on a
FIFO, but it suffers the same problems as AddIfNotPresent.  If we do
requeue an item at the end, we'll move the informer back in time.  If we
requeue at the beginning we'll simply wedge FIFO.

We didn't find examples in the wild, but by removing the error type
those impacted will get a compile error and get to decide what action is
most appropriate for their failure.  Most of the time, proceeding to the
next item is best.

Kubernetes-commit: 238c32a1d9b2c72d648193fa8642a53a2884975f

tools/cache/fifo.go

@@ -27,23 +27,9 @@ import (
 // It is supposed to process the accumulator popped from the queue.
 type PopProcessFunc func(obj interface{}, isInInitialList bool) error
 
-// ErrRequeue may be returned by a PopProcessFunc to safely requeue
-// the current item. The value of Err will be returned from Pop.
-type ErrRequeue struct {
-	// Err is returned by the Pop function
-	Err error
-}
-
 // ErrFIFOClosed used when FIFO is closed
 var ErrFIFOClosed = errors.New("DeltaFIFO: manipulating with closed queue")
 
-func (e ErrRequeue) Error() string {
-	if e.Err == nil {
-		return "the popped item should be requeued without returning an error"
-	}
-	return e.Err.Error()
-}
-
 // Queue extends ReflectorStore with a collection of Store keys to "process".
 // Every Add, Update, or Delete may put the object's key in that collection.
 // A Queue has a way to derive the corresponding key given an accumulator.
@@ -172,19 +158,6 @@ func (f *FIFO) Add(obj interface{}) error {
 	return nil
 }
 
-// addIfNotPresent assumes the fifo lock is already held and adds the provided
-// item to the queue under id if it does not already exist.
-func (f *FIFO) addIfNotPresent(id string, obj interface{}) {
-	f.populated = true
-	if _, exists := f.items[id]; exists {
-		return
-	}
-
-	f.queue = append(f.queue, id)
-	f.items[id] = obj
-	f.cond.Broadcast()
-}
-
 // Update is the same as Add in this implementation.
 func (f *FIFO) Update(obj interface{}) error {
 	return f.Add(obj)
@@ -245,10 +218,6 @@ func (f *FIFO) Pop(process PopProcessFunc) (interface{}, error) {
 		}
 		delete(f.items, id)
 		err := process(item, isInInitialList)
-		if e, ok := err.(ErrRequeue); ok {
-			f.addIfNotPresent(id, item)
-			err = e.Err
-		}
 		return item, err
 	}
 }