Don't recreate cached layers

pkg/commands/base_command.go

@@ -22,10 +22,7 @@ import (
 )
 
 type BaseCommand struct {
-}
-
-func (b *BaseCommand) CacheCommand(v1.Image) DockerCommand {
-	return nil
+	cache v1.Image
 }
 
 func (b *BaseCommand) FilesToSnapshot() []string {
@@ -44,6 +41,18 @@ func (b *BaseCommand) RequiresUnpackedFS() bool {
 	return false
 }
 
+func (b *BaseCommand) CacheCommand() DockerCommand {
+	return nil
+}
+
+func (b *BaseCommand) CacheImage() v1.Image {
+	return b.cache
+}
+
+func (b *BaseCommand) SetCacheImage(cache v1.Image) {
+	b.cache = cache
+}
+
 func (b *BaseCommand) ShouldCacheOutput() bool {
 	return false
 }

pkg/commands/commands.go

@@ -37,9 +37,6 @@ type DockerCommand interface {
 	// A list of files to snapshot, empty for metadata commands or nil if we don't know
 	FilesToSnapshot() []string
 
-	// Return a cache-aware implementation of this command, if it exists.
-	CacheCommand(v1.Image) DockerCommand
-
 	// Return true if this command depends on the build context.
 	FilesUsedFromContext(*v1.Config, *dockerfile.BuildArgs) ([]string, error)
 
@@ -48,6 +45,18 @@ type DockerCommand interface {
 	RequiresUnpackedFS() bool
 
 	ShouldCacheOutput() bool
+
+	// Return an implementation of the command to be executed if the cached layer
+	// exists. This command will be executed after the layer cache is unpacked,
+	// so it only needs to contain additional (for example, metadata changes)
+	// operatons.
+	CacheCommand() DockerCommand
+
+	// Returns a cache image for the layer created by this command, if set
+	CacheImage() v1.Image
+
+	// Sets the cache image for the command's layer
+	SetCacheImage(cache v1.Image)
 }
 
 func GetCommand(cmd instructions.Command, buildcontext string) (DockerCommand, error) {

pkg/commands/run.go

@@ -152,15 +152,6 @@ func (r *RunCommand) FilesToSnapshot() []string {
 	return nil
 }
 
-// CacheCommand returns true since this command should be cached
-func (r *RunCommand) CacheCommand(img v1.Image) DockerCommand {
-
-	return &CachingRunCommand{
-		img: img,
-		cmd: r.cmd,
-	}
-}
-
 func (r *RunCommand) MetadataOnly() bool {
 	return false
 }
@@ -172,28 +163,3 @@ func (r *RunCommand) RequiresUnpackedFS() bool {
 func (r *RunCommand) ShouldCacheOutput() bool {
 	return true
 }
-
-type CachingRunCommand struct {
-	BaseCommand
-	img            v1.Image
-	extractedFiles []string
-	cmd            *instructions.RunCommand
-}
-
-func (cr *CachingRunCommand) ExecuteCommand(config *v1.Config, buildArgs *dockerfile.BuildArgs) error {
-	logrus.Infof("Found cached layer, extracting to filesystem")
-	var err error
-	cr.extractedFiles, err = util.GetFSFromImage(constants.RootDir, cr.img)
-	if err != nil {
-		return errors.Wrap(err, "extracting fs from image")
-	}
-	return nil
-}
-
-func (cr *CachingRunCommand) FilesToSnapshot() []string {
-	return cr.extractedFiles
-}
-
-func (cr *CachingRunCommand) String() string {
-	return cr.cmd.String()
-}

pkg/commands/volume.go

@@ -50,7 +50,7 @@ func (v *VolumeCommand) ExecuteCommand(config *v1.Config, buildArgs *dockerfile.
 		existingVolumes[volume] = x
 		util.AddVolumePathToWhitelist(volume)
 
-		// Only create and snapshot the dir if it didn't exist already
+		// Only create the dir if it didn't exist already
 		if _, err := os.Stat(volume); os.IsNotExist(err) {
 			logrus.Infof("Creating directory %s", volume)
 			if err := os.MkdirAll(volume, 0755); err != nil {

pkg/commands/workdir.go

@@ -37,9 +37,7 @@ type WorkdirCommand struct {
 // For testing
 var mkdir = os.MkdirAll
 
-func (w *WorkdirCommand) ExecuteCommand(config *v1.Config, buildArgs *dockerfile.BuildArgs) error {
-	logrus.Info("cmd: workdir")
-	workdirPath := w.cmd.Path
+func updateWorkdir(workdirPath string, config *v1.Config, buildArgs *dockerfile.BuildArgs) error {
 	replacementEnvs := buildArgs.ReplacementEnvs(config.Env)
 	resolvedWorkingDir, err := util.ResolveEnvironmentReplacement(workdirPath, replacementEnvs, true)
 	if err != nil {
@@ -51,14 +49,25 @@ func (w *WorkdirCommand) ExecuteCommand(config *v1.Config, buildArgs *dockerfile
 		config.WorkingDir = filepath.Join(config.WorkingDir, resolvedWorkingDir)
 	}
 	logrus.Infof("Changed working directory to %s", config.WorkingDir)
+	return nil
+}
+
+func (w *WorkdirCommand) ExecuteCommand(config *v1.Config, buildArgs *dockerfile.BuildArgs) error {
+	logrus.Info("cmd: workdir")
+
+	workdirPath := w.cmd.Path
+	err := updateWorkdir(workdirPath, config, buildArgs)
+	if err != nil {
+		return err
+	}
 
-	// Only create and snapshot the dir if it didn't exist already
-	w.snapshotFiles = []string{}
 	if _, err := os.Stat(config.WorkingDir); os.IsNotExist(err) {
 		logrus.Infof("Creating directory %s", config.WorkingDir)
 		w.snapshotFiles = append(w.snapshotFiles, config.WorkingDir)
 		return mkdir(config.WorkingDir, 0755)
 	}
+	// Cache the empty layer so we don't have to unpack FS on rerun
+	w.snapshotFiles = nil
 	return nil
 }
 
@@ -75,3 +84,34 @@ func (w *WorkdirCommand) String() string {
 func (w *WorkdirCommand) MetadataOnly() bool {
 	return false
 }
+
+func (w *WorkdirCommand) RequiresUnpackedFS() bool {
+	return true
+}
+
+func (w *WorkdirCommand) ShouldCacheOutput() bool {
+	return true
+}
+
+func (w *WorkdirCommand) CacheCommand() DockerCommand {
+	return &CachedWorkdirCommand{cmd: w.cmd}
+}
+
+type CachedWorkdirCommand struct {
+	BaseCommand
+	cmd *instructions.WorkdirCommand
+}
+
+func (cw *CachedWorkdirCommand) ExecuteCommand(config *v1.Config, buildArgs *dockerfile.BuildArgs) error {
+	logrus.Info("cmd: workdir")
+
+	return updateWorkdir(cw.cmd.Path, config, buildArgs)
+}
+
+func (cw *CachedWorkdirCommand) MetadataOnly() bool {
+	return true
+}
+
+func (cw *CachedWorkdirCommand) String() string {
+	return cw.cmd.String()
+}

pkg/commands/workdir_test.go

@@ -63,7 +63,7 @@ var workdirTests = []struct {
 	{
 		path:          "$home",
 		expectedPath:  "/root",
-		snapshotFiles: []string{},
+		snapshotFiles: nil,
 	},
 	{
 		path:          "/foo/$path/$home",
@@ -73,7 +73,7 @@ var workdirTests = []struct {
 	{
 		path:          "/tmp",
 		expectedPath:  "/tmp",
-		snapshotFiles: []string{},
+		snapshotFiles: nil,
 	},
 }