dockerfile: allow automatic platform variables

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

frontend/dockerfile/dockerfile2llb/convert.go

@@ -58,6 +58,11 @@ func Dockerfile2LLB(ctx context.Context, dt []byte, opt ConvertOpt) (*llb.State,
 
 	platformOpt := buildPlatformOpt(&opt)
 
+	optMetaArgs := getPlatformArgs(platformOpt)
+	for i, arg := range optMetaArgs {
+		optMetaArgs[i] = setKVValue(arg, opt.BuildArgs)
+	}
+
 	dockerfile, err := parser.Parse(bytes.NewReader(dt))
 	if err != nil {
 		return nil, nil, err
@@ -70,7 +75,6 @@ func Dockerfile2LLB(ctx context.Context, dt []byte, opt ConvertOpt) (*llb.State,
 		return nil, nil, err
 	}
 
-	optMetaArgs := []instructions.KeyValuePairOptional{}
 	for _, metaArg := range metaArgs {
 		optMetaArgs = append(optMetaArgs, setKVValue(metaArg.KeyValuePairOptional, opt.BuildArgs))
 	}

frontend/dockerfile/dockerfile2llb/platform.go

@@ -2,6 +2,7 @@ package dockerfile2llb
 
 import (
 	"github.com/containerd/containerd/platforms"
+	"github.com/moby/buildkit/frontend/dockerfile/instructions"
 	specs "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
@@ -34,3 +35,24 @@ func buildPlatformOpt(opt *ConvertOpt) *platformOpt {
 		implicitTarget: implicitTargetPlatform,
 	}
 }
+
+func getPlatformArgs(po *platformOpt) []instructions.KeyValuePairOptional {
+	bp := po.buildPlatforms[0]
+	tp := po.targetPlatform
+	m := map[string]string{
+		"BUILDPLATFORM":  platforms.Format(bp),
+		"BUILDOS":        bp.OS,
+		"BUILDARCH":      bp.Architecture,
+		"BUILDVARIANT":   bp.Variant,
+		"TARGETPLATFORM": platforms.Format(tp),
+		"TARGETOS":       tp.OS,
+		"TARGETARCH":     tp.Architecture,
+		"TARGETVARIANT":  tp.Variant,
+	}
+	opts := make([]instructions.KeyValuePairOptional, 0, len(m))
+	for k, v := range m {
+		s := v
+		opts = append(opts, instructions.KeyValuePairOptional{Key: k, Value: &s})
+	}
+	return opts
+}

frontend/dockerfile/dockerfile_test.go

@@ -13,6 +13,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"sort"
 	"strings"
 	"testing"
@@ -69,6 +70,8 @@ func TestIntegration(t *testing.T) {
 		testNoSnapshotLeak,
 		testCopySymlinks,
 		testContextChangeDirToFile,
+		testPlatformArgsImplicit,
+		testPlatformArgsExplicit,
 	})
 }
 
@@ -2281,6 +2284,106 @@ COPY --from=s1 unique2 /
 	require.NotEqual(t, string(unique2Dir1), string(unique2Dir3))
 }
 
+func testPlatformArgsImplicit(t *testing.T, sb integration.Sandbox) {
+	t.Parallel()
+
+	dockerfile := []byte(fmt.Sprintf(`
+FROM scratch AS build-%s
+COPY foo bar
+FROM build-${TARGETOS}
+COPY foo2 bar2
+`, runtime.GOOS))
+
+	dir, err := tmpdir(
+		fstest.CreateFile("Dockerfile", dockerfile, 0600),
+		fstest.CreateFile("foo", []byte("d0"), 0600),
+		fstest.CreateFile("foo2", []byte("d1"), 0600),
+	)
+	require.NoError(t, err)
+	defer os.RemoveAll(dir)
+
+	c, err := client.New(context.TODO(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	destDir, err := ioutil.TempDir("", "buildkit")
+	require.NoError(t, err)
+	defer os.RemoveAll(destDir)
+
+	opt := client.SolveOpt{
+		Frontend:          "dockerfile.v0",
+		Exporter:          client.ExporterLocal,
+		ExporterOutputDir: destDir,
+		LocalDirs: map[string]string{
+			builder.LocalNameDockerfile: dir,
+			builder.LocalNameContext:    dir,
+		},
+	}
+
+	_, err = c.Solve(context.TODO(), nil, opt, nil)
+	require.NoError(t, err)
+
+	dt, err := ioutil.ReadFile(filepath.Join(destDir, "bar"))
+	require.NoError(t, err)
+	require.Equal(t, "d0", string(dt))
+
+	dt, err = ioutil.ReadFile(filepath.Join(destDir, "bar2"))
+	require.NoError(t, err)
+	require.Equal(t, "d1", string(dt))
+}
+
+func testPlatformArgsExplicit(t *testing.T, sb integration.Sandbox) {
+	t.Parallel()
+
+	dockerfile := []byte(`
+FROM --platform=$BUILDPLATFORM busybox AS build
+ARG TARGETPLATFORM
+ARG TARGETOS
+RUN mkdir /out && echo -n $TARGETPLATFORM > /out/platform && echo -n $TARGETOS > /out/os
+FROM scratch
+COPY --from=build out .
+`)
+
+	dir, err := tmpdir(
+		fstest.CreateFile("Dockerfile", dockerfile, 0600),
+	)
+	require.NoError(t, err)
+	defer os.RemoveAll(dir)
+
+	c, err := client.New(context.TODO(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	destDir, err := ioutil.TempDir("", "buildkit")
+	require.NoError(t, err)
+	defer os.RemoveAll(destDir)
+
+	opt := client.SolveOpt{
+		Frontend: "dockerfile.v0",
+		Exporter: client.ExporterLocal,
+		FrontendAttrs: map[string]string{
+			"platform":           "darwin/ppc64le",
+			"build-arg:TARGETOS": "freebsd",
+		},
+		ExporterOutputDir: destDir,
+		LocalDirs: map[string]string{
+			builder.LocalNameDockerfile: dir,
+			builder.LocalNameContext:    dir,
+		},
+	}
+
+	_, err = c.Solve(context.TODO(), nil, opt, nil)
+	require.NoError(t, err)
+
+	dt, err := ioutil.ReadFile(filepath.Join(destDir, "platform"))
+	require.NoError(t, err)
+	require.Equal(t, "darwin/ppc64le", string(dt))
+
+	dt, err = ioutil.ReadFile(filepath.Join(destDir, "os"))
+	require.NoError(t, err)
+	require.Equal(t, "freebsd", string(dt))
+}
+
 func testBuiltinArgs(t *testing.T, sb integration.Sandbox) {
 	t.Parallel()