Add --metadata-file flag to output build metadata

Signed-off-by: Jesse Rittner <rittneje@gmail.com>

README.md

@@ -374,6 +374,20 @@ consider client-side load balancing using consistent hashing.
 
 See [`./examples/kubernetes/consistenthash`](./examples/kubernetes/consistenthash).
 
+## Metadata
+
+To output build metadata such as the image digest, pass the `--metadata-file` flag.
+The metadata will be written as a JSON object to the specified file.
+The directory of the specified file must already exist and be writable.
+
+```
+buildctl build ... --metadata-file metadata.json
+```
+
+```
+{"containerimage.digest": "sha256:ea0cfb27fd41ea0405d3095880c1efa45710f5bcdddb7d7d5a7317ad4825ae14",...}
+```
+
 ## Systemd socket activation
 
 On Systemd based systems, you can communicate with the daemon via [Systemd socket activation](http://0pointer.de/blog/projects/socket-activation.html), use `buildkitd --addr fd://`.

cmd/buildctl/build.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"os"
 
+	"github.com/containerd/continuity"
 	"github.com/moby/buildkit/client"
 	"github.com/moby/buildkit/client/llb"
 	"github.com/moby/buildkit/cmd/buildctl/build"
@@ -101,6 +102,10 @@ var buildCommand = cli.Command{
 			Name:  "ssh",
 			Usage: "Allow forwarding SSH agent to the builder. Format default|<id>[=<socket>|<key>[,<key>]]",
 		},
+		cli.StringFlag{
+			Name:  "metadata-file",
+			Usage: "Output build metadata (e.g., image digest) to a file as JSON",
+		},
 	},
 }
 
@@ -290,7 +295,15 @@ func buildAction(clicontext *cli.Context) error {
 		for k, v := range resp.ExporterResponse {
 			logrus.Debugf("exporter response: %s=%s", k, v)
 		}
-		return err
+
+		metadataFile := clicontext.String("metadata-file")
+		if metadataFile != "" && resp.ExporterResponse != nil {
+			if err := writeMetadataFile(metadataFile, resp.ExporterResponse); err != nil {
+				return err
+			}
+		}
+
+		return nil
 	})
 
 	eg.Go(func() error {
@@ -300,3 +313,11 @@ func buildAction(clicontext *cli.Context) error {
 
 	return eg.Wait()
 }
+
+func writeMetadataFile(filename string, exporterResponse map[string]string) error {
+	b, err := json.Marshal(exporterResponse)
+	if err != nil {
+		return err
+	}
+	return continuity.AtomicWriteFile(filename, b, 0666)
+}

cmd/buildctl/build_test.go

@@ -3,6 +3,7 @@ package main
 import (
 	"bytes"
 	"context"
+	"encoding/json"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -111,6 +112,61 @@ func testBuildContainerdExporter(t *testing.T, sb integration.Sandbox) {
 	require.Equal(t, ok, true)
 }
 
+func testBuildMetadataFile(t *testing.T, sb integration.Sandbox) {
+	st := llb.Image("busybox").
+		Run(llb.Shlex("sh -c 'echo -n bar > /foo'"))
+
+	rdr, err := marshal(st.Root())
+	require.NoError(t, err)
+
+	tmpDir, err := ioutil.TempDir("", "buildkit-buildctl")
+	require.NoError(t, err)
+	defer os.RemoveAll(tmpDir)
+
+	imageName := "example.com/moby/metadata:test"
+	metadataFile := filepath.Join(tmpDir, "metadata.json")
+
+	buildCmd := []string{
+		"build", "--progress=plain",
+		"--output type=image,name=" + imageName + ",push=false",
+		"--metadata-file", metadataFile,
+	}
+
+	cmd := sb.Cmd(strings.Join(buildCmd, " "))
+	cmd.Stdin = rdr
+	err = cmd.Run()
+	require.NoError(t, err)
+
+	require.FileExists(t, metadataFile)
+	metadataBytes, err := ioutil.ReadFile(metadataFile)
+	require.NoError(t, err)
+
+	var metadata map[string]string
+	err = json.Unmarshal(metadataBytes, &metadata)
+	require.NoError(t, err)
+
+	require.Equal(t, imageName, metadata["image.name"])
+
+	digest := metadata["containerimage.digest"]
+	require.NotEmpty(t, digest)
+
+	cdAddress := sb.ContainerdAddress()
+	if cdAddress == "" {
+		t.Log("no containerd worker, skipping digest verification")
+	} else {
+		client, err := containerd.New(cdAddress, containerd.WithTimeout(60*time.Second))
+		require.NoError(t, err)
+		defer client.Close()
+
+		ctx := namespaces.WithNamespace(context.Background(), "buildkit")
+
+		img, err := client.GetImage(ctx, imageName)
+		require.NoError(t, err)
+
+		require.Equal(t, img.Metadata().Target.Digest.String(), digest)
+	}
+}
+
 func marshal(st llb.State) (io.Reader, error) {
 	def, err := st.Marshal(context.TODO())
 	if err != nil {

cmd/buildctl/buildctl_test.go

@@ -18,6 +18,7 @@ func TestCLIIntegration(t *testing.T) {
 		testBuildWithLocalFiles,
 		testBuildLocalExporter,
 		testBuildContainerdExporter,
+		testBuildMetadataFile,
 		testPrune,
 		testUsage,
 	},