support empty images in cache to handle WORKDIR w/o implicit folders created

pkg/commands/workdir.go

@@ -88,11 +88,7 @@ func (w *WorkdirCommand) ExecuteCommand(config *v1.Config, buildArgs *dockerfile
 
 // FilesToSnapshot returns the workingdir, which should have been created if it didn't already exist
 func (w *WorkdirCommand) FilesToSnapshot() []string {
-	return nil
+	return w.snapshotFiles
-}
-
-func (r *WorkdirCommand) ProvidesFilesToSnapshot() bool {
-	return false
 }
 
 // String returns some information about the command for the image config history
@@ -154,8 +150,11 @@ func (wr *CachingWorkdirCommand) ExecuteCommand(config *v1.Config, buildArgs *do
 		return errors.Wrap(err, "retrieving image layers")
 	}
 
-	if len(layers) != 1 {
+	if len(layers) > 1 {
 		return errors.New(fmt.Sprintf("expected %d layers but got %d", 1, len(layers)))
+	} else if len(layers) == 0 {
+		// an empty image in cache indicates that no directory was created by WORKDIR
+		return nil
 	}
 
 	wr.layer = layers[0]

pkg/executor/build.go

@@ -414,11 +414,13 @@ func (s *stageBuilder) build() error {
 			continue
 		}
 		if isCacheCommand {
-			if files != nil && len(files) == 0 {
-				logrus.Info("No files were changed, appending empty layer to config. No layer added to image.")
-			} else {
 			v := command.(commands.Cached)
 			layer := v.Layer()
+			if (files != nil || layer == nil) && len(files) == 0 {
+				// a cache image with a layer with no files indicates that no files were changed, ie. by 'RUN echo hello'
+				// a cache image without a layer indicates that no files were changed too, ie. by 'WORKDIR /'
+				logrus.Info("No files were changed, appending empty layer to config. No layer added to image.")
+			} else {
 				if err := s.saveLayerToImage(layer, command.String()); err != nil {
 					return errors.Wrap(err, "failed to save layer")
 				}

pkg/executor/push.go

@@ -369,11 +369,6 @@ func pushLayerToCache(opts *config.KanikoOptions, cacheKey string, tarPath strin
 		// layer already gzipped by default
 	}
 
-	layer, err := tarball.LayerFromFile(tarPath, layerOpts...)
-	if err != nil {
-		return err
-	}
-
 	cache, err := cache.Destination(opts, cacheKey)
 	if err != nil {
 		return errors.Wrap(err, "getting cache destination")
@@ -385,6 +380,13 @@ func pushLayerToCache(opts *config.KanikoOptions, cacheKey string, tarPath strin
 		return errors.Wrap(err, "setting empty image created time")
 	}
 
+	// WORKDIR can create empty layers by design, yet still we must cache them
+	// to transfer the knowledge that they are empty.
+	if tarPath != "" {
+		layer, err := tarball.LayerFromFile(tarPath, layerOpts...)
+		if err != nil {
+			return err
+		}
 		empty, err = mutate.Append(empty,
 			mutate.Addendum{
 				Layer: layer,
@@ -397,6 +399,8 @@ func pushLayerToCache(opts *config.KanikoOptions, cacheKey string, tarPath strin
 		if err != nil {
 			return errors.Wrap(err, "appending layer onto empty image")
 		}
+	}
+
 	cacheOpts := *opts
 	cacheOpts.TarPath = ""              // tarPath doesn't make sense for Docker layers
 	cacheOpts.NoPush = opts.NoPushCache // we do not want to push cache if --no-push-cache is set.