Use only the necessary files in the cache keys. (#387)

pkg/executor/build.go

@@ -125,10 +125,6 @@ func (s *stageBuilder) build() error {
 
 	// Set the initial cache key to be the base image digest, the build args and the SrcContext.
 	compositeKey := NewCompositeCache(s.baseImageDigest)
-	contextHash, err := HashDir(s.opts.SrcContext)
-	if err != nil {
-		return err
-	}
 	compositeKey.AddKey(s.opts.BuildArgs...)
 
 	cmds := []commands.DockerCommand{}
@@ -148,8 +144,16 @@ func (s *stageBuilder) build() error {
 
 		// Add the next command to the cache key.
 		compositeKey.AddKey(command.String())
-		if command.UsesContext() {
-			compositeKey.AddKey(contextHash)
+
+		// If the command uses files from the context, add them.
+		files, err := command.FilesUsedFromContext(&s.cf.Config, args)
+		if err != nil {
+			return err
+		}
+		for _, f := range files {
+			if err := compositeKey.AddPath(f); err != nil {
+				return err
+			}
 		}
 		logrus.Info(command.String())
 
@@ -172,7 +176,7 @@ func (s *stageBuilder) build() error {
 		if err := command.ExecuteCommand(&s.cf.Config, args); err != nil {
 			return err
 		}
-		files := command.FilesToSnapshot()
+		files = command.FilesToSnapshot()
 		var contents []byte
 
 		if !s.shouldTakeSnapshot(index, files) {

pkg/executor/composite_cache.go

@@ -53,6 +53,32 @@ func (s *CompositeCache) Hash() (string, error) {
 	return util.SHA256(strings.NewReader(s.Key()))
 }
 
+func (s *CompositeCache) AddPath(p string) error {
+	sha := sha256.New()
+	fi, err := os.Lstat(p)
+	if err != nil {
+		return err
+	}
+	if fi.Mode().IsDir() {
+		k, err := HashDir(p)
+		if err != nil {
+			return err
+		}
+		s.keys = append(s.keys, k)
+		return nil
+	}
+	fh, err := util.CacheHasher()(p)
+	if err != nil {
+		return err
+	}
+	if _, err := sha.Write([]byte(fh)); err != nil {
+		return err
+	}
+
+	s.keys = append(s.keys, string(sha.Sum(nil)))
+	return nil
+}
+
 // HashDir returns a hash of the directory.
 func HashDir(p string) (string, error) {
 	sha := sha256.New()