Add config for allowing GC to clean unpacked layers up

This commit adds a flag through Pull API for allowing GC to clean layer contents
up after unpacking these contents completed.

This patch takes an approach to directly delete GC labels pointing to layers
from the manifest blob. This will result in other snapshotters cannot reuse
these contents on the next pull. But this patch mainly focuses on CRI use-cases
where single snapshotter is usually used throughout the node lifecycle so this
shouldn't be a matter.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

client_test.go

@@ -29,13 +29,17 @@ import (
 	"time"
 
 	"github.com/containerd/containerd/defaults"
+	"github.com/containerd/containerd/errdefs"
 	"github.com/containerd/containerd/images"
+	"github.com/containerd/containerd/leases"
 	"github.com/containerd/containerd/log"
 	"github.com/containerd/containerd/log/logtest"
 	"github.com/containerd/containerd/namespaces"
 	"github.com/containerd/containerd/pkg/testutil"
 	"github.com/containerd/containerd/platforms"
 	"github.com/containerd/containerd/sys"
+	"github.com/opencontainers/go-digest"
+	"github.com/opencontainers/image-spec/identity"
 	"github.com/sirupsen/logrus"
 )
 
@@ -215,6 +219,80 @@ func TestImagePull(t *testing.T) {
 	}
 }
 
+func TestImagePullWithDiscardContent(t *testing.T) {
+	client, err := newClient(t, address)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer client.Close()
+
+	ctx, cancel := testContext(t)
+	defer cancel()
+
+	ls := client.LeasesService()
+	l, err := ls.Create(ctx, leases.WithRandomID(), leases.WithExpiration(24*time.Hour))
+	if err != nil {
+		t.Fatal(err)
+	}
+	ctx = leases.WithLease(ctx, l.ID)
+	img, err := client.Pull(ctx, testImage,
+		WithPlatformMatcher(platforms.Default()),
+		WithPullUnpack,
+		WithDiscardContent,
+	)
+	// Synchronously garbage collect contents
+	if errL := ls.Delete(ctx, l, leases.SynchronousDelete); errL != nil {
+		t.Fatal(errL)
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Check if all layer contents have been unpacked and aren't preserved
+	var (
+		diffIDs []digest.Digest
+		layers  []digest.Digest
+	)
+	cs := client.ContentStore()
+	manifest, err := images.Manifest(ctx, cs, img.Target(), platforms.Default())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(manifest.Layers) == 0 {
+		t.Fatalf("failed to get children from %v", img.Target())
+	}
+	for _, l := range manifest.Layers {
+		layers = append(layers, l.Digest)
+	}
+	config, err := images.Config(ctx, cs, img.Target(), platforms.Default())
+	if err != nil {
+		t.Fatal(err)
+	}
+	diffIDs, err = images.RootFS(ctx, cs, config)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(layers) != len(diffIDs) {
+		t.Fatalf("number of layers and diffIDs don't match: %d != %d", len(layers), len(diffIDs))
+	} else if len(layers) == 0 {
+		t.Fatalf("there is no layers in the target image(parent: %v)", img.Target())
+	}
+	var (
+		sn    = client.SnapshotService("")
+		chain []digest.Digest
+	)
+	for i, dgst := range layers {
+		chain = append(chain, diffIDs[i])
+		chainID := identity.ChainID(chain).String()
+		if _, err := sn.Stat(ctx, chainID); err != nil {
+			t.Errorf("snapshot %v must exist: %v", chainID, err)
+		}
+		if _, err := cs.Info(ctx, dgst); err == nil || !errdefs.IsNotFound(err) {
+			t.Errorf("content %v must be garbage collected: %v", dgst, err)
+		}
+	}
+}
+
 func TestImagePullAllPlatforms(t *testing.T) {
 	client, err := newClient(t, address)
 	if err != nil {