From d66fb36f32fec957add9568c9f62db588f73f568 Mon Sep 17 00:00:00 2001
From: Tonis Tiigi <tonistiigi@gmail.com>
Date: Fri, 10 Jan 2025 16:42:50 -0800
Subject: [PATCH] dockerfile: fix syntax forwarding for files with BOM

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
---
 frontend/dockerfile/parser/directives.go      |  5 +++++
 frontend/dockerfile/parser/directives_test.go | 14 ++++++++++++++
 frontend/dockerfile/parser/parser.go          |  4 +---
 3 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/frontend/dockerfile/parser/directives.go b/frontend/dockerfile/parser/directives.go
index 651b13148c3a..c29d18b8f11d 100644
--- a/frontend/dockerfile/parser/directives.go
+++ b/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})
+}
diff --git a/frontend/dockerfile/parser/directives_test.go b/frontend/dockerfile/parser/directives_test.go
index 61ede0ac407f..651120b29118 100644
--- a/frontend/dockerfile/parser/directives_test.go
+++ b/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()
 
diff --git a/frontend/dockerfile/parser/parser.go b/frontend/dockerfile/parser/parser.go
index 740f03b708f3..1b0a96228656 100644
--- a/frontend/dockerfile/parser/parser.go
+++ b/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"))