publish: Build OCI compliant app manifest

Encapsulate the compose app manifest creation functionality in a
separate file and object.
Use the empty descriptor as a value of the app manifest config field
as the OCI image specification instructs.

Signed-off-by: Mike Sul <mike.sul@foundries.io>

internal/app_manifest_builder.go

@@ -0,0 +1,98 @@
+//go:build publish
+
+package internal
+
+import (
+	"context"
+	"encoding/json"
+	"github.com/docker/distribution"
+	"github.com/docker/distribution/manifest"
+	"github.com/docker/distribution/manifest/ocischema"
+	v1 "github.com/opencontainers/image-spec/specs-go/v1"
+)
+
+type (
+	AppManifest struct {
+		ocischema.Manifest
+		// ArtifactType is the IANA media type of the artifact this schema refers to.
+		ArtifactType string `json:"artifactType,omitempty"`
+		Manifests  []distribution.Descriptor `json:"manifests,omitempty"`
+	}
+	ManifestBuilder struct {
+		//*ocischema.Builder
+		bs distribution.BlobService
+		manifest AppManifest
+	}
+)
+
+var (
+	AppManifestTemplate = AppManifest{
+		Manifest:     ocischema.Manifest{
+			Versioned: manifest.Versioned{
+				SchemaVersion: 2,
+				MediaType:     v1.MediaTypeImageManifest,
+			},
+			// Set the empty descriptor for the config as the specification guides
+			// https://github.com/opencontainers/image-spec/blob/main/manifest.md#guidance-for-an-empty-descriptor
+			Config: distribution.Descriptor{
+				MediaType: v1.DescriptorEmptyJSON.MediaType,
+				Digest: v1.DescriptorEmptyJSON.Digest,
+				Size: v1.DescriptorEmptyJSON.Size,
+			},
+			Annotations: map[string]string{"compose-app": "v1"},
+		},
+		ArtifactType: "application/vnd.fio+compose-app",
+	}
+)
+
+func NewManifestBuilder(bs distribution.BlobService)  distribution.ManifestBuilder {
+	return &ManifestBuilder{
+		bs: bs,
+		manifest: AppManifestTemplate,
+	}
+}
+
+func (mb *ManifestBuilder) Build(ctx context.Context) (distribution.Manifest, error) {
+	_, err := mb.bs.Stat(ctx, mb.manifest.Config.Digest)
+	switch err {
+	case nil:
+		// Config blob is present in the blob store
+		return fromStruct(mb.manifest)
+	case distribution.ErrBlobUnknown:
+		// nop
+	default:
+		return nil, err
+	}
+	// Add config to the blob store
+	_, err = mb.bs.Put(ctx, mb.manifest.Config.MediaType, v1.DescriptorEmptyJSON.Data)
+	if err != nil {
+		return nil, err
+	}
+	return fromStruct(mb.manifest)
+}
+
+// AppendReference adds a reference to the current ManifestBuilder.
+func (mb *ManifestBuilder) AppendReference(d distribution.Describable) error {
+	mb.manifest.Layers = append(mb.manifest.Layers, d.Descriptor())
+	return nil
+}
+
+// References returns the current references added to this builder.
+func (mb *ManifestBuilder) References() []distribution.Descriptor {
+	return mb.manifest.Layers
+}
+
+func (mb *ManifestBuilder) SetLayerMetaManifests(manifests []distribution.Descriptor)  {
+	mb.manifest.Manifests = manifests
+}
+
+func fromStruct(m AppManifest) (*ocischema.DeserializedManifest, error) {
+	canonical, err := json.MarshalIndent(&m, "", "   ")
+
+	dm := ocischema.DeserializedManifest{}
+	err = dm.UnmarshalJSON(canonical)
+	if err != nil {
+		return nil, err
+	}
+	return &dm, err
+}

internal/publish.go

@@ -8,7 +8,6 @@ import (
 	"compress/gzip"
 	"context"
 	"crypto/sha256"
-	"encoding/json"
 	"errors"
 	"fmt"
 	"gopkg.in/yaml.v3"
@@ -274,7 +273,7 @@ func CreateApp(ctx context.Context, config map[string]interface{}, target string
 	}
 	fmt.Println("  |-> app blob: ", desc.Digest.String())
 
-	mb := ocischema.NewManifestBuilder(blobStore, []byte{}, map[string]string{"compose-app": "v1"})
+	mb := NewManifestBuilder(blobStore)
 	if err := mb.AppendReference(desc); err != nil {
 		return "", err
 	}
@@ -291,6 +290,10 @@ func CreateApp(ctx context.Context, config map[string]interface{}, target string
 		}
 	}
 
+	if layerManifests != nil {
+		mb.(*ManifestBuilder).SetLayerMetaManifests(layerManifests)
+	}
+
 	manifest, err := mb.Build(ctx)
 	if err != nil {
 		return "", err
@@ -300,29 +303,11 @@ func CreateApp(ctx context.Context, config map[string]interface{}, target string
 	if !ok {
 		return "", fmt.Errorf("invalid manifest type, expected *ocischema.DeserializedManifest, got: %T", manifest)
 	}
-	b, err := man.MarshalJSON()
+	_, b, err := man.Payload()
 	if err != nil {
 		return "", err
 	}
 
-	if layerManifests != nil {
-		manMap := make(map[string]interface{})
-		err = json.Unmarshal(b, &manMap)
-		if err != nil {
-			return "", err
-		}
-
-		manMap["manifests"] = layerManifests
-		b, err = json.MarshalIndent(manMap, "", "   ")
-		if err != nil {
-			return "", err
-		}
-		err = man.UnmarshalJSON(b)
-		if err != nil {
-			return "", err
-		}
-	}
-
 	fmt.Printf("  |-> manifest size: %d\n", len(b))
 	// TODO: this check is needed in order to overcome the aklite's check on the maximum manifest size (2048)
 	// Once the new version of aklite is deployed (max manifest size = 16K) then this check can be removed or MaxArchNumb increased