builder: avoid decoding "null" string in ADD, COPY and VOLUME step to nil slice

Signed-off-by: Soshi Katsuta <katsuta_soshi@cyberagent.co.jp>

builder/parser/line_parsers.go

@@ -232,6 +232,11 @@ func parseString(rest string) (*Node, map[string]bool, error) {
 
 // parseJSON converts JSON arrays to an AST.
 func parseJSON(rest string) (*Node, map[string]bool, error) {
+	rest = strings.TrimLeftFunc(rest, unicode.IsSpace)
+	if !strings.HasPrefix(rest, "[") {
+		return nil, nil, fmt.Errorf("Error parsing \"%s\" as a JSON array", rest)
+	}
+
 	var myJSON []interface{}
 	if err := json.NewDecoder(strings.NewReader(rest)).Decode(&myJSON); err != nil {
 		return nil, nil, err

builder/parser/testfiles/ADD-COPY-with-JSON/Dockerfile

@@ -3,6 +3,8 @@ MAINTAINER	Seongyeol Lim <seongyeol37@gmail.com>
 
 COPY	.	/go/src/github.com/docker/docker
 ADD		.	/
+ADD		null /
+COPY	nullfile /tmp
 ADD		[ "vimrc", "/tmp" ]
 COPY	[ "bashrc", "/tmp" ]
 COPY	[ "test file", "/tmp" ]

builder/parser/testfiles/ADD-COPY-with-JSON/result

@@ -2,6 +2,8 @@
 (maintainer "Seongyeol Lim <seongyeol37@gmail.com>")
 (copy "." "/go/src/github.com/docker/docker")
 (add "." "/")
+(add "null" "/")
+(copy "nullfile" "/tmp")
 (add "vimrc" "/tmp")
 (copy "bashrc" "/tmp")
 (copy "test file" "/tmp")