dockerfile: fix syntax forwarding for files with BOM

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

Author: tonistiigi

frontend/dockerfile/parser/directives.go

@@ -116,6 +116,7 @@ func DetectSyntax(dt []byte) (string, string, []Range, bool) {
 }
 
 func ParseDirective(key string, dt []byte) (string, string, []Range, bool) {
+	dt = discardBOM(dt)
 	dt, hadShebang, err := discardShebang(dt)
 	if err != nil {
 		return "", "", nil, false
@@ -171,3 +172,7 @@ func discardShebang(dt []byte) ([]byte, bool, error) {
 	}
 	return dt, false, nil
 }
+
+func discardBOM(dt []byte) []byte {
+	return bytes.TrimPrefix(dt, []byte{0xEF, 0xBB, 0xBF})
+}

frontend/dockerfile/parser/directives_test.go

@@ -104,6 +104,20 @@ RUN ls
 	require.False(t, ok)
 }
 
+func TestDetectSyntaxBOM(t *testing.T) {
+	t.Parallel()
+
+	dt := append([]byte{0xEF, 0xBB, 0xBF}, []byte(`# syntax = myfrontend
+FROM busybox
+`)...)
+	ref, cmdline, loc, ok := DetectSyntax(dt)
+	require.True(t, ok)
+	require.Equal(t, "myfrontend", ref)
+	require.Equal(t, "myfrontend", cmdline)
+	require.Equal(t, 1, loc[0].Start.Line)
+	require.Equal(t, 1, loc[0].End.Line)
+}
+
 func TestParseDirective(t *testing.T) {
 	t.Parallel()
 

frontend/dockerfile/parser/parser.go

@@ -291,7 +291,7 @@ func Parse(rwc io.Reader) (*Result, error) {
 		bytesRead := scanner.Bytes()
 		if currentLine == 0 {
 			// First line, strip the byte-order-marker if present
-			bytesRead = bytes.TrimPrefix(bytesRead, utf8bom)
+			bytesRead = discardBOM(bytesRead)
 		}
 		if isComment(bytesRead) {
 			comment := strings.TrimSpace(string(bytesRead[1:]))
@@ -522,8 +522,6 @@ func isEmptyContinuationLine(line []byte) bool {
 	return len(trimLeadingWhitespace(trimNewline(line))) == 0
 }
 
-var utf8bom = []byte{0xEF, 0xBB, 0xBF}
-
 func trimContinuationCharacter(line []byte, d *directives) ([]byte, bool) {
 	if d.lineContinuationRegex.Match(line) {
 		line = d.lineContinuationRegex.ReplaceAll(line, []byte("$1"))