Add support for separate attestation storage repo (#62)

* Add support for separate attestation storage repo
* Move mapping file types and parsing to config package
* Change signature of Verify to take image/platform
* Separate Attestation Resolvers to their own files (registry, layout and referrers)
* Add support configuring referrers resolution style in mapping.yaml
* Add registry test

pkg/oci/referrers.go

@@ -0,0 +1,121 @@
+package oci
+
+import (
+	"context"
+	"fmt"
+
+	att "github.com/docker/attest/pkg/attestation"
+	"github.com/google/go-containerregistry/pkg/name"
+	"github.com/google/go-containerregistry/pkg/v1/remote"
+	"github.com/pkg/errors"
+)
+
+type ReferrersResolver struct {
+	digest        string
+	referrersRepo string
+	manifests     []*AttestationManifest
+	*RegistryImageDetailsResolver
+}
+
+func NewReferrersAttestationResolver(src *RegistryImageDetailsResolver, options ...func(*ReferrersResolver) error) (*ReferrersResolver, error) {
+	res := &ReferrersResolver{
+		RegistryImageDetailsResolver: src,
+	}
+	for _, opt := range options {
+		err := opt(res)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return res, nil
+}
+
+func WithReferrersRepo(repo string) func(*ReferrersResolver) error {
+	return func(r *ReferrersResolver) error {
+		r.referrersRepo = repo
+		return nil
+	}
+}
+
+func (r *ReferrersResolver) resolveAttestations(ctx context.Context) error {
+	if r.manifests == nil {
+		subjectRef, err := name.ParseReference(r.Identifier)
+		if err != nil {
+			return fmt.Errorf("failed to parse reference: %w", err)
+		}
+		subjectDigest, err := r.ImageDigest(ctx)
+		if err != nil {
+			return fmt.Errorf("failed to get digest: %w", err)
+		}
+		var referrersSubjectRef name.Digest
+		if r.referrersRepo != "" {
+			referrersSubjectRef, err = name.NewDigest(fmt.Sprintf("%s@%s", r.referrersRepo, subjectDigest))
+			if err != nil {
+				return fmt.Errorf("failed to create referrers reference: %w", err)
+			}
+		} else {
+			referrersSubjectRef = subjectRef.Context().Digest(subjectDigest)
+		}
+		referrersIndex, err := remote.Referrers(referrersSubjectRef)
+		if err != nil {
+			return fmt.Errorf("failed to get referrers: %w", err)
+		}
+		referrersIndexManifest, err := referrersIndex.IndexManifest()
+		if err != nil {
+			return fmt.Errorf("failed to get index manifest: %w", err)
+		}
+		if len(referrersIndexManifest.Manifests) == 0 {
+			return errors.New("no referrers found")
+		}
+		aManifests := make([]*AttestationManifest, 0)
+		for _, m := range referrersIndexManifest.Manifests {
+
+			remoteRef := referrersSubjectRef.Context().Digest(m.Digest.String())
+			attestationImage, err := remote.Image(remoteRef)
+			if err != nil {
+				return fmt.Errorf("failed to get referred image: %w", err)
+			}
+			manifest, err := attestationImage.Manifest()
+			if err != nil {
+				return fmt.Errorf("failed to get manifest: %w", err)
+			}
+			if manifest.Annotations[att.DockerReferenceType] != AttestationManifestType {
+				continue
+			}
+			if manifest.Annotations[att.DockerReferenceDigest] != subjectDigest {
+				continue
+			}
+			attest := &AttestationManifest{
+				Name:       r.Identifier,
+				Image:      attestationImage,
+				Manifest:   manifest,
+				Descriptor: &m,
+				Digest:     subjectDigest,
+				Platform:   r.Platform,
+			}
+			aManifests = append(aManifests, attest)
+		}
+
+		if len(aManifests) == 0 {
+			return errors.New("no attestation manifests found")
+		}
+		r.manifests = aManifests
+	}
+	return nil
+}
+
+func (r *ReferrersResolver) Attestations(ctx context.Context, predicateType string) ([]*att.Envelope, error) {
+	err := r.resolveAttestations(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("failed to resolve attestations: %w", err)
+	}
+	var envs []*att.Envelope
+	for _, attest := range r.manifests {
+		es, err := ExtractEnvelopes(attest, predicateType)
+		if err != nil {
+			return nil, fmt.Errorf("failed to extract envelopes: %w", err)
+		}
+		envs = append(envs, es...)
+	}
+	return envs, nil
+}