Only start from the skipped slot cache if better

ProcessSlots takes a state from the skipped slot cache and assumes that it's better than the passed current state.

If the incoming state.Slot() is higher than the skipped slot cache, then the highest slot is set to be the cache, we process slots again for the same state and then we put back to the cache.

beacon-chain/core/transition/transition.go

@@ -211,9 +211,12 @@ func ProcessSlots(ctx context.Context, state state.BeaconState, slot primitives.
 	if err != nil {
 		return nil, err
 	}
-
-	if cachedState != nil && !cachedState.IsNil() && cachedState.Slot() < slot {
-		highestSlot = cachedState.Slot()
+	cachedSlot := primitives.Slot(0)
+	if cachedState != nil {
+		cachedSlot = cachedState.Slot()
+	}
+	if cachedState != nil && !cachedState.IsNil() && cachedSlot <= slot && cachedSlot > highestSlot {
+		highestSlot = cachedSlot
 		state = cachedState
 	}
 	if err := SkipSlotCache.MarkInProgress(key); errors.Is(err, cache.ErrAlreadyInProgress) {
@@ -221,8 +224,10 @@ func ProcessSlots(ctx context.Context, state state.BeaconState, slot primitives.
 		if err != nil {
 			return nil, err
 		}
-		if cachedState != nil && !cachedState.IsNil() && cachedState.Slot() < slot {
-			highestSlot = cachedState.Slot()
+		if cachedState != nil {
+			cachedSlot = cachedState.Slot()
+		}
+		if cachedState != nil && !cachedState.IsNil() && cachedSlot <= slot && cachedSlot > highestSlot {
 			state = cachedState
 		}
 	} else if err != nil {
@@ -236,7 +241,7 @@ func ProcessSlots(ctx context.Context, state state.BeaconState, slot primitives.
 		if ctx.Err() != nil {
 			tracing.AnnotateError(span, ctx.Err())
 			// Cache last best value.
-			if highestSlot < state.Slot() {
+			if cachedSlot < state.Slot() {
 				if SkipSlotCache.Put(ctx, key, state); err != nil {
 					log.WithError(err).Error("Failed to put skip slot cache value")
 				}
@@ -277,7 +282,7 @@ func ProcessSlots(ctx context.Context, state state.BeaconState, slot primitives.
 		}
 	}
 
-	if highestSlot < state.Slot() {
+	if cachedSlot < state.Slot() {
 		SkipSlotCache.Put(ctx, key, state)
 	}