export(local): wrap opt

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

README.md

@@ -295,6 +295,54 @@ buildctl build ... --output type=tar,dest=out.tar
 buildctl build ... --output type=tar > out.tar
 ```
 
+With a [multi-platform build](docs/multi-platform.md), a subfolder matching
+each target platform will be created in the destination directory:
+
+```dockerfile
+FROM busybox AS build
+ARG TARGETOS
+ARG TARGETARCH
+RUN mkdir /out && echo foo > /out/hello-$TARGETOS-$TARGETARCH
+
+FROM scratch
+COPY --from=build /out /
+```
+
+```bash
+$ buildctl build \
+  --frontend dockerfile.v0 \
+  --opt platform=linux/amd64,linux/arm64 \
+  --output type=local,dest=./bin/release
+
+$ tree ./bin
+./bin/
+└── release
+    ├── linux_amd64
+    │   └── hello-linux-amd64
+    └── linux_arm64
+        └── hello-linux-arm64
+```
+
+You can use the `wrap` option to wrap content in destination directory:
+
+```bash
+$ buildctl build \
+  --frontend dockerfile.v0 \
+  --opt platform=linux/amd64,linux/arm64 \
+  --output type=local,dest=./bin/release,wrap=true
+
+$ tree ./bin
+./bin/
+└── release
+    ├── hello-linux-amd64
+    └── hello-linux-arm64
+```
+
+> **Warning**
+>
+> Wrapping content may overwrite existing files from build result of other
+> platforms.
+
 #### Docker tarball
 
 ```bash

exporter/local/export.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"os"
+	"strconv"
 	"strings"
 	"time"
 
@@ -23,6 +24,12 @@ import (
 	"golang.org/x/time/rate"
 )
 
+const (
+	// wrap is an exporter option which can be used to merge result in the
+	// output directory when multiple references are exported.
+	attrWrap = "wrap"
+)
+
 type Opt struct {
 	SessionManager *session.Manager
 }
@@ -38,16 +45,28 @@ func New(opt Opt) (exporter.Exporter, error) {
 }
 
 func (e *localExporter) Resolve(ctx context.Context, opt map[string]string) (exporter.ExporterInstance, error) {
+	ei := &localExporterInstance{localExporter: e}
+
+	if v, ok := opt[attrWrap]; ok {
+		b, err := strconv.ParseBool(v)
+		if err != nil {
+			return nil, errors.Wrapf(err, "non-bool value for %s: %s", attrWrap, v)
+		}
+		ei.wrap = b
+	}
+
 	tm, _, err := epoch.ParseAttr(opt)
 	if err != nil {
 		return nil, err
 	}
+	ei.epoch = tm
 
-	return &localExporterInstance{localExporter: e, epoch: tm}, nil
+	return ei, nil
 }
 
 type localExporterInstance struct {
 	*localExporter
+	wrap  bool
 	epoch *time.Time
 }
 
@@ -152,16 +171,18 @@ func (e *localExporterInstance) Export(ctx context.Context, inp *exporter.Source
 			lbl := "copying files"
 			if isMap {
 				lbl += " " + k
-				st := fstypes.Stat{
-					Mode: uint32(os.ModeDir | 0755),
-					Path: strings.Replace(k, "/", "_", -1),
-				}
-				if e.epoch != nil {
-					st.ModTime = e.epoch.UnixNano()
-				}
-				fs, err = fsutil.SubDirFS([]fsutil.Dir{{FS: fs, Stat: st}})
-				if err != nil {
-					return err
+				if !e.wrap {
+					st := fstypes.Stat{
+						Mode: uint32(os.ModeDir | 0755),
+						Path: strings.Replace(k, "/", "_", -1),
+					}
+					if e.epoch != nil {
+						st.ModTime = e.epoch.UnixNano()
+					}
+					fs, err = fsutil.SubDirFS([]fsutil.Dir{{FS: fs, Stat: st}})
+					if err != nil {
+						return err
+					}
 				}
 			}
 

frontend/dockerfile/dockerfile_test.go

@@ -86,6 +86,7 @@ var allTests = integration.TestFuncs(
 	testPlatformArgsImplicit,
 	testPlatformArgsExplicit,
 	testExportMultiPlatform,
+	testExportLocalWrapMultiPlatform,
 	testQuotedMetaArgs,
 	testIgnoreEntrypoint,
 	testSymlinkedDockerfile,
@@ -1475,6 +1476,67 @@ COPY arch-$TARGETARCH whoami
 	}
 }
 
+func testExportLocalWrapMultiPlatform(t *testing.T, sb integration.Sandbox) {
+	integration.SkipIfDockerd(t, sb, "oci exporter", "multi-platform")
+	f := getFrontend(t, sb)
+
+	dockerfile := []byte(`
+FROM scratch
+ARG TARGETOS
+ARG TARGETARCH
+COPY hello-$TARGETOS-$TARGETARCH .
+`)
+
+	dir, err := integration.Tmpdir(
+		t,
+		fstest.CreateFile("Dockerfile", dockerfile, 0600),
+		fstest.CreateFile("hello-linux-arm", []byte(`hello linux/arm`), 0600),
+		fstest.CreateFile("hello-linux-amd64", []byte(`hello linux/amd64`), 0600),
+		fstest.CreateFile("hello-linux-s390x", []byte(`hello linux/s390x`), 0600),
+		fstest.CreateFile("hello-linux-ppc64le", []byte(`hello linux/ppc64le`), 0600),
+		fstest.CreateFile("hello-windows-amd64", []byte(`hello windows/amd64`), 0600),
+	)
+	require.NoError(t, err)
+
+	c, err := client.New(sb.Context(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	destDir := t.TempDir()
+
+	_, err = f.Solve(sb.Context(), c, client.SolveOpt{
+		LocalDirs: map[string]string{
+			builder.DefaultLocalNameDockerfile: dir,
+			builder.DefaultLocalNameContext:    dir,
+		},
+		FrontendAttrs: map[string]string{
+			"platform": "windows/amd64,linux/arm,linux/s390x",
+		},
+		Exports: []client.ExportEntry{
+			{
+				Type:      client.ExporterLocal,
+				OutputDir: destDir,
+				Attrs: map[string]string{
+					"wrap": "true",
+				},
+			},
+		},
+	}, nil)
+	require.NoError(t, err)
+
+	dt, err := os.ReadFile(filepath.Join(destDir, "hello-windows-amd64"))
+	require.NoError(t, err)
+	require.Equal(t, "hello windows/amd64", string(dt))
+
+	dt, err = os.ReadFile(filepath.Join(destDir, "hello-linux-arm"))
+	require.NoError(t, err)
+	require.Equal(t, "hello linux/arm", string(dt))
+
+	dt, err = os.ReadFile(filepath.Join(destDir, "hello-linux-s390x"))
+	require.NoError(t, err)
+	require.Equal(t, "hello linux/s390x", string(dt))
+}
+
 // tonistiigi/fsutil#46
 func testContextChangeDirToFile(t *testing.T, sb integration.Sandbox) {
 	f := getFrontend(t, sb)