Limit value size of additional annotation for avoiding unpack failure

In containerd, there is a size limit for label size (4096 chars).
Currently if an image has many layers (> (4096-39)/72 > 56),
`containerd.io/snapshot/cri.image-layers` will hit the limit of label size and
the unpack will fail.
This commit fixes this by limiting the size of the annotation.

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

pkg/server/image_pull_test.go

@@ -17,9 +17,14 @@
 package server
 
 import (
+	"context"
 	"encoding/base64"
+	"fmt"
+	"strings"
 	"testing"
 
+	digest "github.com/opencontainers/go-digest"
+	imagespec "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/stretchr/testify/assert"
 	runtime "k8s.io/cri-api/pkg/apis/runtime/v1alpha2"
 
@@ -327,3 +332,48 @@ func TestEncryptedImagePullOpts(t *testing.T) {
 		assert.Equal(t, test.expectedOpts, got)
 	}
 }
+
+func TestImageLayersLabel(t *testing.T) {
+	sampleKey := "sampleKey"
+	sampleDigest, err := digest.Parse("sha256:aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa")
+	assert.NoError(t, err)
+	sampleMaxSize := 300
+	sampleValidate := func(k, v string) error {
+		if (len(k) + len(v)) > sampleMaxSize {
+			return fmt.Errorf("invalid: %q: %q", k, v)
+		}
+		return nil
+	}
+
+	tests := []struct {
+		name      string
+		layersNum int
+		wantNum   int
+	}{
+		{
+			name:      "valid number of layers",
+			layersNum: 2,
+			wantNum:   2,
+		},
+		{
+			name:      "many layers",
+			layersNum: 5, // hits sampleMaxSize (300 chars).
+			wantNum:   4, // layers should be ommitted for avoiding invalid label.
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var sampleLayers []imagespec.Descriptor
+			for i := 0; i < tt.layersNum; i++ {
+				sampleLayers = append(sampleLayers, imagespec.Descriptor{
+					MediaType: imagespec.MediaTypeImageLayerGzip,
+					Digest:    sampleDigest,
+				})
+			}
+			gotS := getLayers(context.Background(), sampleKey, sampleLayers, sampleValidate)
+			got := len(strings.Split(gotS, ","))
+			assert.Equal(t, tt.wantNum, got)
+		})
+	}
+}