Remove contemplation of invariant violations from delta_fifo.go

Some comments and code incorrectly contemplated violating the
invariant that a keys is in `f.items` if and only if it is in
`f.queue`.

Also fixed up some comment wording.

staging/src/k8s.io/client-go/tools/cache/delta_fifo.go

@@ -145,7 +145,7 @@ func NewDeltaFIFOWithOptions(opts DeltaFIFOOptions) *DeltaFIFO {
 // DeltaFIFO's Pop(), Get(), and GetByKey() methods return
 // interface{} to satisfy the Store/Queue interfaces, but they
 // will always return an object of type Deltas. List() returns
-// the newest objects currently in the FIFO.
+// the newest object from each accumulator in the FIFO.
 //
 // A DeltaFIFO's knownObjects KeyListerGetter provides the abilities
 // to list Store keys and to get objects by Store key.  The objects in
@@ -161,12 +161,13 @@ type DeltaFIFO struct {
 	lock sync.RWMutex
 	cond sync.Cond
 
-	// `items` maps keys to Deltas.
-	// `queue` maintains FIFO order of keys for consumption in Pop().
-	// We maintain the property that keys in the `items` and `queue` are
-	// strictly 1:1 mapping, and that all Deltas in `items` should have
-	// at least one Delta.
+	// `items` maps a key to a Deltas.
+	// Each such Deltas has at least one Delta.
 	items map[string]Deltas
+
+	// `queue` maintains FIFO order of keys for consumption in Pop().
+	// There are no duplicates in `queue`.
+	// A key is in `queue` if and only if it is in `items`.
 	queue []string
 
 	// populated is true if the first batch of items inserted by Replace() has been populated
@@ -389,10 +390,7 @@ func (f *DeltaFIFO) queueActionLocked(actionType DeltaType, obj interface{}) err
 	} else {
 		// This never happens, because dedupDeltas never returns an empty list
 		// when given a non-empty list (as it is here).
-		// But if somehow it ever does return an empty list, then
-		// We need to remove this from our map (extra items in the queue are
-		// ignored if they are not in the map).
-		delete(f.items, id)
+		panic(fmt.Sprintf("Impossible dedupDeltas for id=%q: old items=%#+v, obj=%#+v", id, f.items[id], obj))
 	}
 	return nil
 }
@@ -476,37 +474,35 @@ func (f *DeltaFIFO) IsClosed() bool {
 func (f *DeltaFIFO) Pop(process PopProcessFunc) (interface{}, error) {
 	f.lock.Lock()
 	defer f.lock.Unlock()
-	for {
-		for len(f.queue) == 0 {
-			// When the queue is empty, invocation of Pop() is blocked until new item is enqueued.
-			// When Close() is called, the f.closed is set and the condition is broadcasted.
-			// Which causes this loop to continue and return from the Pop().
-			if f.closed {
-				return nil, ErrFIFOClosed
-			}
+	for len(f.queue) == 0 {
+		// When the queue is empty, invocation of Pop() is blocked until new item is enqueued.
+		// When Close() is called, the f.closed is set and the condition is broadcasted.
+		// Which causes this loop to continue and return from the Pop().
+		if f.closed {
+			return nil, ErrFIFOClosed
+		}
 
-			f.cond.Wait()
-		}
-		id := f.queue[0]
-		f.queue = f.queue[1:]
-		if f.initialPopulationCount > 0 {
-			f.initialPopulationCount--
-		}
-		item, ok := f.items[id]
-		if !ok {
-			// Item may have been deleted subsequently.
-			continue
-		}
-		delete(f.items, id)
-		err := process(item)
-		if e, ok := err.(ErrRequeue); ok {
-			f.addIfNotPresent(id, item)
-			err = e.Err
-		}
-		// Don't need to copyDeltas here, because we're transferring
-		// ownership to the caller.
-		return item, err
+		f.cond.Wait()
 	}
+	id := f.queue[0]
+	f.queue = f.queue[1:]
+	if f.initialPopulationCount > 0 {
+		f.initialPopulationCount--
+	}
+	item, ok := f.items[id]
+	if !ok {
+		// This should never happen
+		panic(fmt.Sprintf("Inconceivable! %q was in f.queue but not f.items!", id))
+	}
+	delete(f.items, id)
+	err := process(item)
+	if e, ok := err.(ErrRequeue); ok {
+		f.addIfNotPresent(id, item)
+		err = e.Err
+	}
+	// Don't need to copyDeltas here, because we're transferring
+	// ownership to the caller.
+	return item, err
 }
 
 // Replace atomically does two things: (1) it adds the given objects