parser: avoid excessive memory usage and bogus type conversions

Don't reallocate memory again for each string append.

Remove conversion between strings and byte slices where they
are actually reconverted back right away.

Signed-off-by: Tonis Tiigi <[email protected]>

Author: tonistiigi

frontend/dockerfile/parser/parser.go

@@ -283,6 +283,7 @@ func Parse(rwc io.Reader) (*Result, error) {
 	scanner.Split(scanLines)
 	warnings := []Warning{}
 	var comments []string
+	buf := &bytes.Buffer{}
 
 	var err error
 	for scanner.Scan() {
@@ -306,10 +307,12 @@ func Parse(rwc io.Reader) (*Result, error) {
 		currentLine++
 
 		startLine := currentLine
-		line, isEndOfLine := trimContinuationCharacter(string(bytesRead), d)
-		if isEndOfLine && line == "" {
+		bytesRead, isEndOfLine := trimContinuationCharacter(bytesRead, d)
+		if isEndOfLine && len(bytesRead) == 0 {
 			continue
 		}
+		buf.Reset()
+		buf.Write(bytesRead)
 
 		var hasEmptyContinuationLine bool
 		for !isEndOfLine && scanner.Scan() {
@@ -328,11 +331,12 @@ func Parse(rwc io.Reader) (*Result, error) {
 				continue
 			}
 
-			continuationLine := string(bytesRead)
-			continuationLine, isEndOfLine = trimContinuationCharacter(continuationLine, d)
-			line += continuationLine
+			bytesRead, isEndOfLine = trimContinuationCharacter(bytesRead, d)
+			buf.Write(bytesRead)
 		}
 
+		line := buf.String()
+
 		if hasEmptyContinuationLine {
 			warnings = append(warnings, Warning{
 				Short:    "Empty continuation line found in: " + line,
@@ -513,9 +517,9 @@ func isEmptyContinuationLine(line []byte) bool {
 
 var utf8bom = []byte{0xEF, 0xBB, 0xBF}
 
-func trimContinuationCharacter(line string, d *directives) (string, bool) {
-	if d.lineContinuationRegex.MatchString(line) {
-		line = d.lineContinuationRegex.ReplaceAllString(line, "$1")
+func trimContinuationCharacter(line []byte, d *directives) ([]byte, bool) {
+	if d.lineContinuationRegex.Match(line) {
+		line = d.lineContinuationRegex.ReplaceAll(line, []byte("$1"))
 		return line, false
 	}
 	return line, true