informers: don't treat relist same as sync

Background:

Before this change, DeltaFIFO emits the Sync DeltaType on Resync() and
Replace(). Seperately, the SharedInformer will only pass that event
on to handlers that have a ResyncInterval and are due for Resync. This
can cause updates to be lost if an object changes as part of the Replace(),
as it may be incorrectly discarded if the handler does not want a Resync.

What this change does:

Creates a new DeltaType, Replaced, which is emitted by DeltaFIFO on
Replace(). For backwards compatability concerns, the old behavior of
always emitting Sync is preserved unless explicity overridden.

As a result, if an object changes (or is added) on Replace(), now all
SharedInformer handlers will get a correct Add() or Update()
notification.

One additional side-effect is that handlers which do not ever want
Resyncs will now see them for all objects that have not changed during
the Replace.

Kubernetes-commit: ca1eeb99b530a6d76b464dad545abc18d4508c49

tools/cache/controller.go

@@ -364,7 +364,10 @@ func newInformer(
 	// This will hold incoming changes. Note how we pass clientState in as a
 	// KeyLister, that way resync operations will result in the correct set
 	// of update/delete deltas.
-	fifo := NewDeltaFIFO(MetaNamespaceKeyFunc, clientState)
+	fifo := NewDeltaFIFOWithOptions(DeltaFIFOOptions{
+		KnownObjects:          clientState,
+		EmitDeltaTypeReplaced: true,
+	})
 
 	cfg := &Config{
 		Queue:            fifo,
@@ -377,7 +380,7 @@ func newInformer(
 			// from oldest to newest
 			for _, d := range obj.(Deltas) {
 				switch d.Type {
-				case Sync, Added, Updated:
+				case Sync, Replaced, Added, Updated:
 					if old, exists, err := clientState.Get(d.Object); err == nil && exists {
 						if err := clientState.Update(d.Object); err != nil {
 							return err