dockerfile: add syntax directive for introducing new features

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

frontend/dockerfile/builder/build.go

@@ -116,6 +116,13 @@ func Build(ctx context.Context, c client.Client) error {
 		return err
 	}
 
+	if _, ok := c.Opts()["cmdline"]; !ok {
+		ref, argv, ok := dockerfile2llb.DetectSyntax(bytes.NewBuffer(dtDockerfile))
+		if ok {
+			return forwardGateway(ctx, c, ref, argv)
+		}
+	}
+
 	st, img, err := dockerfile2llb.Dockerfile2LLB(ctx, dtDockerfile, dockerfile2llb.ConvertOpt{
 		Target:       opts[keyTarget],
 		MetaResolver: c,
@@ -158,6 +165,20 @@ func Build(ctx context.Context, c client.Client) error {
 	return nil
 }
 
+func forwardGateway(ctx context.Context, c client.Client, ref string, argv string) error {
+	opts := c.Opts()
+	if opts == nil {
+		opts = map[string]string{}
+	}
+	opts["cmdline"] = argv
+	opts["source"] = ref
+	_, err := c.Solve(ctx, client.SolveRequest{
+		Frontend:    "gateway.v0",
+		FrontendOpt: opts,
+	}, nil, true)
+	return err
+}
+
 func filter(opt map[string]string, key string) map[string]string {
 	m := map[string]string{}
 	for k, v := range opt {

frontend/dockerfile/dockerfile2llb/directives.go

@@ -0,0 +1,38 @@
+package dockerfile2llb
+
+import (
+	"bufio"
+	"io"
+	"regexp"
+	"strings"
+)
+
+const keySyntax = "syntax"
+
+var reDirective = regexp.MustCompile(`^#\s*([a-z][a-z0-9]*)\s*=\s*(.+?)\s*$`)
+
+func DetectSyntax(r io.Reader) (string, string, bool) {
+	directives := ParseDirectives(r)
+	if len(directives) == 0 {
+		return "", "", false
+	}
+	v, ok := directives[keySyntax]
+	if !ok {
+		return "", "", false
+	}
+	p := strings.SplitN(v, " ", 2)
+	return p[0], v, true
+}
+
+func ParseDirectives(r io.Reader) map[string]string {
+	m := map[string]string{}
+	s := bufio.NewScanner(r)
+	for s.Scan() {
+		match := reDirective.FindStringSubmatch(s.Text())
+		if len(match) == 0 {
+			return m
+		}
+		m[match[1]] = match[2]
+	}
+	return m
+}

frontend/dockerfile/dockerfile2llb/directives_test.go

@@ -0,0 +1,52 @@
+package dockerfile2llb
+
+import (
+	"bytes"
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDirectives(t *testing.T) {
+	t.Parallel()
+
+	dt := `#escape=\
+# key = FOO bar
+
+# smth
+`
+
+	d := ParseDirectives(bytes.NewBuffer([]byte(dt)))
+	require.Equal(t, len(d), 2, fmt.Sprintf("%+v", d))
+
+	v, ok := d["escape"]
+	require.True(t, ok)
+	require.Equal(t, v, "\\")
+
+	v, ok = d["key"]
+	require.True(t, ok)
+	require.Equal(t, v, "FOO bar")
+}
+
+func TestSyntaxDirective(t *testing.T) {
+	t.Parallel()
+
+	dt := `# syntax = dockerfile:experimental // opts
+FROM busybox
+`
+
+	ref, argv, ok := DetectSyntax(bytes.NewBuffer([]byte(dt)))
+	require.True(t, ok)
+	require.Equal(t, ref, "dockerfile:experimental")
+	require.Equal(t, argv, "dockerfile:experimental // opts")
+
+	dt = `FROM busybox
+RUN ls
+`
+	ref, argv, ok = DetectSyntax(bytes.NewBuffer([]byte(dt)))
+	require.False(t, ok)
+	require.Equal(t, ref, "")
+	require.Equal(t, argv, "")
+
+}