Fix dockerfile parser failing silently on long tokens

Signed-off-by: Daniel Nephin <dnephin@docker.com>

builder/dockerfile/parser/parser.go

@@ -321,7 +321,7 @@ func Parse(rwc io.Reader) (*Result, error) {
 		Warnings:    warnings,
 		EscapeToken: d.escapeToken,
 		OS:          d.platformToken,
-	}, nil
+	}, handleScannerError(scanner.Err())
 }
 
 func trimComments(src []byte) []byte {
@@ -358,3 +358,12 @@ func processLine(d *Directive, token []byte, stripLeftWhitespace bool) ([]byte,
 	}
 	return trimComments(token), d.possibleParserDirective(string(token))
 }
+
+func handleScannerError(err error) error {
+	switch err {
+	case bufio.ErrTooLong:
+		return errors.Errorf("dockerfile line greater than max allowed size of %d", bufio.MaxScanTokenSize-1)
+	default:
+		return err
+	}
+}

builder/dockerfile/parser/parser_test.go

@@ -1,12 +1,14 @@
 package parser
 
 import (
+	"bufio"
 	"bytes"
 	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
 	"runtime"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -159,3 +161,14 @@ RUN indented \
 	assert.Contains(t, warnings[1], "RUN another     thing")
 	assert.Contains(t, warnings[2], "will become errors in a future release")
 }
+
+func TestParseReturnsScannerErrors(t *testing.T) {
+	label := strings.Repeat("a", bufio.MaxScanTokenSize)
+
+	dockerfile := strings.NewReader(fmt.Sprintf(`
+		FROM image
+		LABEL test=%s
+`, label))
+	_, err := Parse(dockerfile)
+	assert.EqualError(t, err, "dockerfile line greater than max allowed size of 65535")
+}