From 9d2cc941923f2a0124e713940926b9e14a44278d Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sat, 28 Jan 2023 11:29:17 +0800
Subject: [PATCH 01/19] feat: support discover

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 cmd/oras/discover.go | 71 +++++++++++++++++++++++++++++++-------------
 1 file changed, 51 insertions(+), 20 deletions(-)

diff --git a/cmd/oras/discover.go b/cmd/oras/discover.go
index e34af855f..537afcc83 100644
--- a/cmd/oras/discover.go
+++ b/cmd/oras/discover.go
@@ -24,8 +24,8 @@ import (
 
 	"gopkg.in/yaml.v3"
 	"oras.land/oras-go/v2"
+	"oras.land/oras-go/v2/content"
 	"oras.land/oras-go/v2/registry/remote"
-	"oras.land/oras/cmd/oras/internal/errors"
 	"oras.land/oras/cmd/oras/internal/option"
 
 	"github.com/need-being/go-tree"
@@ -36,10 +36,9 @@ import (
 
 type discoverOptions struct {
 	option.Common
-	option.Remote
 	option.Platform
+	option.Target
 
-	targetRef    string
 	artifactType string
 	outputType   string
 }
@@ -73,10 +72,10 @@ Example - Discover referrers with type 'test-artifact' of manifest 'hello:v1' in
 `,
 		Args: cobra.ExactArgs(1),
 		PreRunE: func(cmd *cobra.Command, args []string) error {
+			opts.RawReference = args[0]
 			return option.Parse(&opts)
 		},
 		RunE: func(cmd *cobra.Command, args []string) error {
-			opts.targetRef = args[0]
 			return runDiscover(opts)
 		},
 	}
@@ -90,24 +89,24 @@ Example - Discover referrers with type 'test-artifact' of manifest 'hello:v1' in
 
 func runDiscover(opts discoverOptions) error {
 	ctx, _ := opts.SetLoggerLevel()
-	repo, err := opts.NewRepository(opts.targetRef, opts.Common)
+	repo, err := opts.NewReadonlyTarget(ctx, opts.Common)
 	if err != nil {
 		return err
 	}
-	if repo.Reference.Reference == "" {
-		return errors.NewErrInvalidReference(repo.Reference)
+	if err := opts.EnsureReferenceNotEmpty(); err != nil {
+		return err
 	}
 
 	// discover artifacts
 	resolveOpts := oras.DefaultResolveOptions
 	resolveOpts.TargetPlatform = opts.Platform.Platform
-	desc, err := oras.Resolve(ctx, repo, repo.Reference.Reference, resolveOpts)
+	desc, err := oras.Resolve(ctx, repo, opts.Reference, resolveOpts)
 	if err != nil {
 		return err
 	}
 
 	if opts.outputType == "tree" {
-		root := tree.New(repo.Reference.String())
+		root := tree.New(opts.Reference)
 		err = fetchAllReferrers(ctx, repo, desc, opts.artifactType, root, &opts)
 		if err != nil {
 			return err
@@ -124,9 +123,9 @@ func runDiscover(opts discoverOptions) error {
 	}
 
 	if n := len(refs); n > 1 {
-		fmt.Println("Discovered", n, "artifacts referencing", repo.Reference)
+		fmt.Println("Discovered", n, "artifacts referencing", opts.Reference)
 	} else {
-		fmt.Println("Discovered", n, "artifact referencing", repo.Reference)
+		fmt.Println("Discovered", n, "artifact referencing", opts.Reference)
 	}
 	fmt.Println("Digest:", desc.Digest)
 	if len(refs) > 0 {
@@ -136,19 +135,51 @@ func runDiscover(opts discoverOptions) error {
 	return nil
 }
 
-func fetchReferrers(ctx context.Context, repo *remote.Repository, desc ocispec.Descriptor, artifactType string) ([]ocispec.Descriptor, error) {
-	results := []ocispec.Descriptor{}
-	err := repo.Referrers(ctx, desc, artifactType, func(referrers []ocispec.Descriptor) error {
-		results = append(results, referrers...)
-		return nil
-	})
-	if err != nil {
-		return nil, err
+func fetchReferrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc ocispec.Descriptor, artifactType string) ([]ocispec.Descriptor, error) {
+	var results []ocispec.Descriptor
+	if repo, ok := target.(*remote.Repository); ok {
+		err := repo.Referrers(ctx, desc, artifactType, func(referrers []ocispec.Descriptor) error {
+			results = append(results, referrers...)
+			return nil
+		})
+		if err != nil {
+			return nil, err
+		}
+	} else {
+		// fill in artifact type and filter
+		predecessors, err := target.Predecessors(ctx, desc)
+		if err != nil {
+			return nil, err
+		}
+		for _, r := range predecessors {
+			var fetched []byte
+			if rc, err := target.Fetch(ctx, r); err != nil {
+				return nil, err
+			} else {
+				fetched, err = content.ReadAll(rc, r)
+				if err != nil {
+					return nil, err
+				}
+			}
+			switch r.MediaType {
+			case ocispec.MediaTypeArtifactManifest:
+				var artifact ocispec.Artifact
+				json.Unmarshal(fetched, &artifact)
+				r.ArtifactType = artifact.ArtifactType
+			case ocispec.MediaTypeImageManifest:
+				var image ocispec.Manifest
+				json.Unmarshal(fetched, &image)
+				r.ArtifactType = image.Config.MediaType
+			}
+			if artifactType == "" || artifactType == r.ArtifactType {
+				results = append(results, r)
+			}
+		}
 	}
 	return results, nil
 }
 
-func fetchAllReferrers(ctx context.Context, repo *remote.Repository, desc ocispec.Descriptor, artifactType string, node *tree.Node, opts *discoverOptions) error {
+func fetchAllReferrers(ctx context.Context, repo oras.ReadOnlyGraphTarget, desc ocispec.Descriptor, artifactType string, node *tree.Node, opts *discoverOptions) error {
 	results, err := fetchReferrers(ctx, repo, desc, artifactType)
 	if err != nil {
 		return err

From e67be053eb2d86c3117e84a5f0f87a3b897d5108 Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sat, 28 Jan 2023 12:31:44 +0800
Subject: [PATCH 02/19] add index support in OCI image layout

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 cmd/oras/discover.go | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/cmd/oras/discover.go b/cmd/oras/discover.go
index 537afcc83..f7ca14cfc 100644
--- a/cmd/oras/discover.go
+++ b/cmd/oras/discover.go
@@ -170,6 +170,8 @@ func fetchReferrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc o
 				var image ocispec.Manifest
 				json.Unmarshal(fetched, &image)
 				r.ArtifactType = image.Config.MediaType
+			case ocispec.MediaTypeImageIndex:
+				r.ArtifactType = "index (manifest list)"
 			}
 			if artifactType == "" || artifactType == r.ArtifactType {
 				results = append(results, r)

From 14abb31f07be76613148d9033588ce39285ca069 Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sat, 28 Jan 2023 15:22:15 +0800
Subject: [PATCH 03/19] align discover experience to remote registry

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 cmd/oras/discover.go | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/cmd/oras/discover.go b/cmd/oras/discover.go
index f7ca14cfc..ac69b7077 100644
--- a/cmd/oras/discover.go
+++ b/cmd/oras/discover.go
@@ -138,6 +138,7 @@ func runDiscover(opts discoverOptions) error {
 func fetchReferrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc ocispec.Descriptor, artifactType string) ([]ocispec.Descriptor, error) {
 	var results []ocispec.Descriptor
 	if repo, ok := target.(*remote.Repository); ok {
+		// get referrers directly
 		err := repo.Referrers(ctx, desc, artifactType, func(referrers []ocispec.Descriptor) error {
 			results = append(results, referrers...)
 			return nil
@@ -146,35 +147,38 @@ func fetchReferrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc o
 			return nil, err
 		}
 	} else {
-		// fill in artifact type and filter
+		// find matched referrers in all predecessors
 		predecessors, err := target.Predecessors(ctx, desc)
 		if err != nil {
 			return nil, err
 		}
-		for _, r := range predecessors {
+		for _, node := range predecessors {
 			var fetched []byte
-			if rc, err := target.Fetch(ctx, r); err != nil {
+			if rc, err := target.Fetch(ctx, node); err != nil {
 				return nil, err
 			} else {
-				fetched, err = content.ReadAll(rc, r)
+				fetched, err = content.ReadAll(rc, node)
 				if err != nil {
 					return nil, err
 				}
 			}
-			switch r.MediaType {
+			var setArtifactType = func(node *ocispec.Descriptor, subject *ocispec.Descriptor, artifactType string) {
+				if subject != nil && content.Equal(*subject, *node) {
+					node.ArtifactType = artifactType
+				}
+			}
+			switch node.MediaType {
 			case ocispec.MediaTypeArtifactManifest:
 				var artifact ocispec.Artifact
 				json.Unmarshal(fetched, &artifact)
-				r.ArtifactType = artifact.ArtifactType
+				setArtifactType(&node, artifact.Subject, artifact.ArtifactType)
 			case ocispec.MediaTypeImageManifest:
 				var image ocispec.Manifest
 				json.Unmarshal(fetched, &image)
-				r.ArtifactType = image.Config.MediaType
-			case ocispec.MediaTypeImageIndex:
-				r.ArtifactType = "index (manifest list)"
+				setArtifactType(&node, image.Subject, image.Config.ArtifactType)
 			}
-			if artifactType == "" || artifactType == r.ArtifactType {
-				results = append(results, r)
+			if node.ArtifactType != "" && (artifactType == "" || artifactType == node.ArtifactType) {
+				results = append(results, node)
 			}
 		}
 	}

From 6c103529db8e3f3ddbc6501155b8f421563dd075 Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sat, 28 Jan 2023 17:20:44 +0800
Subject: [PATCH 04/19] bug fix

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 cmd/oras/discover.go | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/cmd/oras/discover.go b/cmd/oras/discover.go
index ac69b7077..9fa67556a 100644
--- a/cmd/oras/discover.go
+++ b/cmd/oras/discover.go
@@ -162,8 +162,8 @@ func fetchReferrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc o
 					return nil, err
 				}
 			}
-			var setArtifactType = func(node *ocispec.Descriptor, subject *ocispec.Descriptor, artifactType string) {
-				if subject != nil && content.Equal(*subject, *node) {
+			var matchSubject = func(got *ocispec.Descriptor, want ocispec.Descriptor, node *ocispec.Descriptor, artifactType string) {
+				if got != nil && content.Equal(*got, want) {
 					node.ArtifactType = artifactType
 				}
 			}
@@ -171,11 +171,11 @@ func fetchReferrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc o
 			case ocispec.MediaTypeArtifactManifest:
 				var artifact ocispec.Artifact
 				json.Unmarshal(fetched, &artifact)
-				setArtifactType(&node, artifact.Subject, artifact.ArtifactType)
+				matchSubject(artifact.Subject, desc, &node, artifact.ArtifactType)
 			case ocispec.MediaTypeImageManifest:
 				var image ocispec.Manifest
 				json.Unmarshal(fetched, &image)
-				setArtifactType(&node, image.Subject, image.Config.ArtifactType)
+				matchSubject(image.Subject, desc, &node, image.Config.ArtifactType)
 			}
 			if node.ArtifactType != "" && (artifactType == "" || artifactType == node.ArtifactType) {
 				results = append(results, node)

From 19014c42e5f8cbb44df3bd27fda756c985741642 Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sat, 28 Jan 2023 17:25:31 +0800
Subject: [PATCH 05/19] bug fix

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 cmd/oras/discover.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/cmd/oras/discover.go b/cmd/oras/discover.go
index 9fa67556a..f3e74d7ea 100644
--- a/cmd/oras/discover.go
+++ b/cmd/oras/discover.go
@@ -175,7 +175,7 @@ func fetchReferrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc o
 			case ocispec.MediaTypeImageManifest:
 				var image ocispec.Manifest
 				json.Unmarshal(fetched, &image)
-				matchSubject(image.Subject, desc, &node, image.Config.ArtifactType)
+				matchSubject(image.Subject, desc, &node, image.Config.MediaType)
 			}
 			if node.ArtifactType != "" && (artifactType == "" || artifactType == node.ArtifactType) {
 				results = append(results, node)

From b1d4c029a744f1dd4048cbedeeaa645681743b8e Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sat, 28 Jan 2023 17:35:08 +0800
Subject: [PATCH 06/19] add example

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 cmd/oras/discover.go | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/cmd/oras/discover.go b/cmd/oras/discover.go
index f3e74d7ea..aac7e4ffb 100644
--- a/cmd/oras/discover.go
+++ b/cmd/oras/discover.go
@@ -69,6 +69,10 @@ Example - Discover all the referrers of manifest with annotations, displayed in
 
 Example - Discover referrers with type 'test-artifact' of manifest 'hello:v1' in registry 'localhost:5000':
   oras discover --artifact-type test-artifact localhost:5000/hello:v1
+
+Example - Discover referrers of the manifest tagged 'v1' in an OCI layout folder 'layout-dir':
+  oras discover layout-dir:v1
+  oras discover -v -o tree layout-dir:v1
 `,
 		Args: cobra.ExactArgs(1),
 		PreRunE: func(cmd *cobra.Command, args []string) error {

From f0fc6b8a0308c97221048c05edbce1d902e0458e Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sat, 28 Jan 2023 17:40:34 +0800
Subject: [PATCH 07/19] add error case

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 cmd/oras/discover.go               |  4 ++++
 cmd/oras/internal/option/remote.go |  2 +-
 cmd/oras/internal/option/spec.go   | 18 +++++++++---------
 3 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/cmd/oras/discover.go b/cmd/oras/discover.go
index aac7e4ffb..886df49dc 100644
--- a/cmd/oras/discover.go
+++ b/cmd/oras/discover.go
@@ -18,6 +18,7 @@ package main
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"os"
 	"strings"
@@ -76,6 +77,9 @@ Example - Discover referrers of the manifest tagged 'v1' in an OCI layout folder
 `,
 		Args: cobra.ExactArgs(1),
 		PreRunE: func(cmd *cobra.Command, args []string) error {
+			if opts.ReferrersAPI != nil && opts.Type == option.TargetTypeOCILayout {
+				return errors.New("cannot specify --distribution-spec flag on an image layout target")
+			}
 			opts.RawReference = args[0]
 			return option.Parse(&opts)
 		},
diff --git a/cmd/oras/internal/option/remote.go b/cmd/oras/internal/option/remote.go
index caa810b7f..e474c21bc 100644
--- a/cmd/oras/internal/option/remote.go
+++ b/cmd/oras/internal/option/remote.go
@@ -46,11 +46,11 @@ type Remote struct {
 	Username          string
 	PasswordFromStdin bool
 	Password          string
+	DistributionSpec
 
 	resolveFlag           []string
 	resolveDialContext    func(dialer *net.Dialer) func(context.Context, string, string) (net.Conn, error)
 	applyDistributionSpec bool
-	distributionSpec      distributionSpec
 }
 
 // EnableDistributionSpecFlag set distribution specification flag as applicable.
diff --git a/cmd/oras/internal/option/spec.go b/cmd/oras/internal/option/spec.go
index b32ca6062..8a7a10a8e 100644
--- a/cmd/oras/internal/option/spec.go
+++ b/cmd/oras/internal/option/spec.go
@@ -51,27 +51,27 @@ func (opts *ImageSpec) ApplyFlags(fs *pflag.FlagSet) {
 	fs.StringVar(&opts.specFlag, "image-spec", "", "specify manifest type for building artifact. options: v1.1-image, v1.1-artifact")
 }
 
-// distributionSpec option struct.
-type distributionSpec struct {
-	// referrersAPI indicates the preference of the implementation of the Referrers API.
+// DistributionSpec option struct.
+type DistributionSpec struct {
+	// ReferrersAPI indicates the preference of the implementation of the Referrers API.
 	// Set to true for referrers API, false for referrers tag scheme, and nil for auto fallback.
-	referrersAPI *bool
+	ReferrersAPI *bool
 
 	// specFlag should be provided in form of`<version>-<api>-<option>`
 	specFlag string
 }
 
 // Parse parses flags into the option.
-func (opts *distributionSpec) Parse() error {
+func (opts *DistributionSpec) Parse() error {
 	switch opts.specFlag {
 	case "":
-		opts.referrersAPI = nil
+		opts.ReferrersAPI = nil
 	case "v1.1-referrers-tag":
 		isApi := false
-		opts.referrersAPI = &isApi
+		opts.ReferrersAPI = &isApi
 	case "v1.1-referrers-api":
 		isApi := true
-		opts.referrersAPI = &isApi
+		opts.ReferrersAPI = &isApi
 	default:
 		return fmt.Errorf("unknown image specification flag: %q", opts.specFlag)
 	}
@@ -79,7 +79,7 @@ func (opts *distributionSpec) Parse() error {
 }
 
 // ApplyFlagsWithPrefix applies flags to a command flag set with a prefix string.
-func (opts *distributionSpec) ApplyFlagsWithPrefix(fs *pflag.FlagSet, prefix, description string) {
+func (opts *DistributionSpec) ApplyFlagsWithPrefix(fs *pflag.FlagSet, prefix, description string) {
 	flagPrefix, notePrefix := applyPrefix(prefix, description)
 	fs.StringVar(&opts.specFlag, flagPrefix+"distribution-spec", "", "set OCI distribution spec version and API option for "+notePrefix+"target. options: v1.1-referrers-api, v1.1-referrers-tag")
 }

From bf77ecc4de7126f0557adfbd0ae4fcc11410a82b Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sat, 28 Jan 2023 17:44:08 +0800
Subject: [PATCH 08/19] fix build error

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 cmd/oras/internal/option/remote.go | 6 +++---
 cmd/oras/internal/option/target.go | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/cmd/oras/internal/option/remote.go b/cmd/oras/internal/option/remote.go
index e474c21bc..7e08f900e 100644
--- a/cmd/oras/internal/option/remote.go
+++ b/cmd/oras/internal/option/remote.go
@@ -86,7 +86,7 @@ func (opts *Remote) ApplyFlagsWithPrefix(fs *pflag.FlagSet, prefix, description
 	flagPrefix, notePrefix = applyPrefix(prefix, description)
 
 	if opts.applyDistributionSpec {
-		opts.distributionSpec.ApplyFlagsWithPrefix(fs, prefix, description)
+		opts.DistributionSpec.ApplyFlagsWithPrefix(fs, prefix, description)
 	}
 	fs.StringVarP(&opts.Username, flagPrefix+"username", shortUser, "", notePrefix+"registry username")
 	fs.StringVarP(&opts.Password, flagPrefix+"password", shortPassword, "", notePrefix+"registry password or identity token")
@@ -270,8 +270,8 @@ func (opts *Remote) NewRepository(reference string, common Common) (repo *remote
 	if repo.Client, err = opts.authClient(hostname, common.Debug); err != nil {
 		return nil, err
 	}
-	if opts.distributionSpec.referrersAPI != nil {
-		repo.SetReferrersCapability(*opts.distributionSpec.referrersAPI)
+	if opts.DistributionSpec.ReferrersAPI != nil {
+		repo.SetReferrersCapability(*opts.DistributionSpec.ReferrersAPI)
 	}
 	return
 }
diff --git a/cmd/oras/internal/option/target.go b/cmd/oras/internal/option/target.go
index ca67908d8..b4187a8a4 100644
--- a/cmd/oras/internal/option/target.go
+++ b/cmd/oras/internal/option/target.go
@@ -84,7 +84,7 @@ func (opts *Target) Parse() error {
 	switch {
 	case opts.isOCILayout:
 		opts.Type = TargetTypeOCILayout
-		if opts.Remote.distributionSpec.referrersAPI != nil {
+		if opts.Remote.DistributionSpec.ReferrersAPI != nil {
 			return errors.New("cannot enforce referrers API for image layout target")
 		}
 	default:

From 2454cddbafa3791b6817ba97a9a6e10b71556b37 Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sat, 28 Jan 2023 17:48:24 +0800
Subject: [PATCH 09/19] Revert "fix build error"

This reverts commit bf77ecc4de7126f0557adfbd0ae4fcc11410a82b.

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 cmd/oras/internal/option/remote.go | 6 +++---
 cmd/oras/internal/option/target.go | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/cmd/oras/internal/option/remote.go b/cmd/oras/internal/option/remote.go
index 7e08f900e..e474c21bc 100644
--- a/cmd/oras/internal/option/remote.go
+++ b/cmd/oras/internal/option/remote.go
@@ -86,7 +86,7 @@ func (opts *Remote) ApplyFlagsWithPrefix(fs *pflag.FlagSet, prefix, description
 	flagPrefix, notePrefix = applyPrefix(prefix, description)
 
 	if opts.applyDistributionSpec {
-		opts.DistributionSpec.ApplyFlagsWithPrefix(fs, prefix, description)
+		opts.distributionSpec.ApplyFlagsWithPrefix(fs, prefix, description)
 	}
 	fs.StringVarP(&opts.Username, flagPrefix+"username", shortUser, "", notePrefix+"registry username")
 	fs.StringVarP(&opts.Password, flagPrefix+"password", shortPassword, "", notePrefix+"registry password or identity token")
@@ -270,8 +270,8 @@ func (opts *Remote) NewRepository(reference string, common Common) (repo *remote
 	if repo.Client, err = opts.authClient(hostname, common.Debug); err != nil {
 		return nil, err
 	}
-	if opts.DistributionSpec.ReferrersAPI != nil {
-		repo.SetReferrersCapability(*opts.DistributionSpec.ReferrersAPI)
+	if opts.distributionSpec.referrersAPI != nil {
+		repo.SetReferrersCapability(*opts.distributionSpec.referrersAPI)
 	}
 	return
 }
diff --git a/cmd/oras/internal/option/target.go b/cmd/oras/internal/option/target.go
index b4187a8a4..ca67908d8 100644
--- a/cmd/oras/internal/option/target.go
+++ b/cmd/oras/internal/option/target.go
@@ -84,7 +84,7 @@ func (opts *Target) Parse() error {
 	switch {
 	case opts.isOCILayout:
 		opts.Type = TargetTypeOCILayout
-		if opts.Remote.DistributionSpec.ReferrersAPI != nil {
+		if opts.Remote.distributionSpec.referrersAPI != nil {
 			return errors.New("cannot enforce referrers API for image layout target")
 		}
 	default:

From 032e7aac1b3935a8e37b2bdbe1160dd8d0634c74 Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sat, 28 Jan 2023 17:49:01 +0800
Subject: [PATCH 10/19] Revert "add error case"

This reverts commit f0fc6b8a0308c97221048c05edbce1d902e0458e.

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 cmd/oras/discover.go               |  4 ----
 cmd/oras/internal/option/remote.go |  2 +-
 cmd/oras/internal/option/spec.go   | 18 +++++++++---------
 3 files changed, 10 insertions(+), 14 deletions(-)

diff --git a/cmd/oras/discover.go b/cmd/oras/discover.go
index 886df49dc..aac7e4ffb 100644
--- a/cmd/oras/discover.go
+++ b/cmd/oras/discover.go
@@ -18,7 +18,6 @@ package main
 import (
 	"context"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"os"
 	"strings"
@@ -77,9 +76,6 @@ Example - Discover referrers of the manifest tagged 'v1' in an OCI layout folder
 `,
 		Args: cobra.ExactArgs(1),
 		PreRunE: func(cmd *cobra.Command, args []string) error {
-			if opts.ReferrersAPI != nil && opts.Type == option.TargetTypeOCILayout {
-				return errors.New("cannot specify --distribution-spec flag on an image layout target")
-			}
 			opts.RawReference = args[0]
 			return option.Parse(&opts)
 		},
diff --git a/cmd/oras/internal/option/remote.go b/cmd/oras/internal/option/remote.go
index e474c21bc..caa810b7f 100644
--- a/cmd/oras/internal/option/remote.go
+++ b/cmd/oras/internal/option/remote.go
@@ -46,11 +46,11 @@ type Remote struct {
 	Username          string
 	PasswordFromStdin bool
 	Password          string
-	DistributionSpec
 
 	resolveFlag           []string
 	resolveDialContext    func(dialer *net.Dialer) func(context.Context, string, string) (net.Conn, error)
 	applyDistributionSpec bool
+	distributionSpec      distributionSpec
 }
 
 // EnableDistributionSpecFlag set distribution specification flag as applicable.
diff --git a/cmd/oras/internal/option/spec.go b/cmd/oras/internal/option/spec.go
index 8a7a10a8e..b32ca6062 100644
--- a/cmd/oras/internal/option/spec.go
+++ b/cmd/oras/internal/option/spec.go
@@ -51,27 +51,27 @@ func (opts *ImageSpec) ApplyFlags(fs *pflag.FlagSet) {
 	fs.StringVar(&opts.specFlag, "image-spec", "", "specify manifest type for building artifact. options: v1.1-image, v1.1-artifact")
 }
 
-// DistributionSpec option struct.
-type DistributionSpec struct {
-	// ReferrersAPI indicates the preference of the implementation of the Referrers API.
+// distributionSpec option struct.
+type distributionSpec struct {
+	// referrersAPI indicates the preference of the implementation of the Referrers API.
 	// Set to true for referrers API, false for referrers tag scheme, and nil for auto fallback.
-	ReferrersAPI *bool
+	referrersAPI *bool
 
 	// specFlag should be provided in form of`<version>-<api>-<option>`
 	specFlag string
 }
 
 // Parse parses flags into the option.
-func (opts *DistributionSpec) Parse() error {
+func (opts *distributionSpec) Parse() error {
 	switch opts.specFlag {
 	case "":
-		opts.ReferrersAPI = nil
+		opts.referrersAPI = nil
 	case "v1.1-referrers-tag":
 		isApi := false
-		opts.ReferrersAPI = &isApi
+		opts.referrersAPI = &isApi
 	case "v1.1-referrers-api":
 		isApi := true
-		opts.ReferrersAPI = &isApi
+		opts.referrersAPI = &isApi
 	default:
 		return fmt.Errorf("unknown image specification flag: %q", opts.specFlag)
 	}
@@ -79,7 +79,7 @@ func (opts *DistributionSpec) Parse() error {
 }
 
 // ApplyFlagsWithPrefix applies flags to a command flag set with a prefix string.
-func (opts *DistributionSpec) ApplyFlagsWithPrefix(fs *pflag.FlagSet, prefix, description string) {
+func (opts *distributionSpec) ApplyFlagsWithPrefix(fs *pflag.FlagSet, prefix, description string) {
 	flagPrefix, notePrefix := applyPrefix(prefix, description)
 	fs.StringVar(&opts.specFlag, flagPrefix+"distribution-spec", "", "set OCI distribution spec version and API option for "+notePrefix+"target. options: v1.1-referrers-api, v1.1-referrers-tag")
 }

From 875f179abe486d3984586e74367700f3efc55650 Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sat, 28 Jan 2023 17:52:39 +0800
Subject: [PATCH 11/19] code clean

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 cmd/oras/discover.go | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/cmd/oras/discover.go b/cmd/oras/discover.go
index aac7e4ffb..a56393c84 100644
--- a/cmd/oras/discover.go
+++ b/cmd/oras/discover.go
@@ -166,20 +166,22 @@ func fetchReferrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc o
 					return nil, err
 				}
 			}
-			var matchSubject = func(got *ocispec.Descriptor, want ocispec.Descriptor, node *ocispec.Descriptor, artifactType string) {
-				if got != nil && content.Equal(*got, want) {
-					node.ArtifactType = artifactType
-				}
+			var isMatch = func(got *ocispec.Descriptor, want ocispec.Descriptor) bool {
+				return got != nil && content.Equal(*got, want)
 			}
 			switch node.MediaType {
 			case ocispec.MediaTypeArtifactManifest:
 				var artifact ocispec.Artifact
 				json.Unmarshal(fetched, &artifact)
-				matchSubject(artifact.Subject, desc, &node, artifact.ArtifactType)
+				if isMatch(artifact.Subject, desc) {
+					node.ArtifactType = artifact.ArtifactType
+				}
 			case ocispec.MediaTypeImageManifest:
 				var image ocispec.Manifest
 				json.Unmarshal(fetched, &image)
-				matchSubject(image.Subject, desc, &node, image.Config.MediaType)
+				if isMatch(image.Subject, desc) {
+					node.ArtifactType = image.Config.MediaType
+				}
 			}
 			if node.ArtifactType != "" && (artifactType == "" || artifactType == node.ArtifactType) {
 				results = append(results, node)

From 01d1f05289384d3645cecec14f8691c5e3a24ee7 Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sun, 29 Jan 2023 14:25:20 +0800
Subject: [PATCH 12/19] resolve comments

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 cmd/oras/discover.go | 72 +++++++++++++++++++++++---------------------
 1 file changed, 37 insertions(+), 35 deletions(-)

diff --git a/cmd/oras/discover.go b/cmd/oras/discover.go
index a56393c84..1909cb815 100644
--- a/cmd/oras/discover.go
+++ b/cmd/oras/discover.go
@@ -25,7 +25,7 @@ import (
 	"gopkg.in/yaml.v3"
 	"oras.land/oras-go/v2"
 	"oras.land/oras-go/v2/content"
-	"oras.land/oras-go/v2/registry/remote"
+	"oras.land/oras-go/v2/registry"
 	"oras.land/oras/cmd/oras/internal/option"
 
 	"github.com/need-being/go-tree"
@@ -71,8 +71,8 @@ Example - Discover referrers with type 'test-artifact' of manifest 'hello:v1' in
   oras discover --artifact-type test-artifact localhost:5000/hello:v1
 
 Example - Discover referrers of the manifest tagged 'v1' in an OCI layout folder 'layout-dir':
-  oras discover layout-dir:v1
-  oras discover -v -o tree layout-dir:v1
+  oras discover --oci-layout layout-dir:v1
+  oras discover --oci-layout -v -o tree layout-dir:v1
 `,
 		Args: cobra.ExactArgs(1),
 		PreRunE: func(cmd *cobra.Command, args []string) error {
@@ -141,7 +141,7 @@ func runDiscover(opts discoverOptions) error {
 
 func fetchReferrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc ocispec.Descriptor, artifactType string) ([]ocispec.Descriptor, error) {
 	var results []ocispec.Descriptor
-	if repo, ok := target.(*remote.Repository); ok {
+	if repo, ok := target.(registry.ReferrerLister); ok {
 		// get referrers directly
 		err := repo.Referrers(ctx, desc, artifactType, func(referrers []ocispec.Descriptor) error {
 			results = append(results, referrers...)
@@ -150,43 +150,45 @@ func fetchReferrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc o
 		if err != nil {
 			return nil, err
 		}
-	} else {
-		// find matched referrers in all predecessors
-		predecessors, err := target.Predecessors(ctx, desc)
-		if err != nil {
-			return nil, err
-		}
-		for _, node := range predecessors {
-			var fetched []byte
-			if rc, err := target.Fetch(ctx, node); err != nil {
+		return results, nil
+	}
+	// find matched referrers in all predecessors
+	predecessors, err := target.Predecessors(ctx, desc)
+	if err != nil {
+		return nil, err
+	}
+	for _, node := range predecessors {
+		switch node.MediaType {
+		case ocispec.MediaTypeArtifactManifest:
+			_, fetched, err := oras.FetchBytes(ctx, target, node.Digest.String(), oras.DefaultFetchBytesOptions)
+			if err != nil {
 				return nil, err
-			} else {
-				fetched, err = content.ReadAll(rc, node)
-				if err != nil {
-					return nil, err
-				}
 			}
-			var isMatch = func(got *ocispec.Descriptor, want ocispec.Descriptor) bool {
-				return got != nil && content.Equal(*got, want)
+			var artifact ocispec.Artifact
+			if err := json.Unmarshal(fetched, &artifact); err != nil {
+				return nil, err
 			}
-			switch node.MediaType {
-			case ocispec.MediaTypeArtifactManifest:
-				var artifact ocispec.Artifact
-				json.Unmarshal(fetched, &artifact)
-				if isMatch(artifact.Subject, desc) {
-					node.ArtifactType = artifact.ArtifactType
-				}
-			case ocispec.MediaTypeImageManifest:
-				var image ocispec.Manifest
-				json.Unmarshal(fetched, &image)
-				if isMatch(image.Subject, desc) {
-					node.ArtifactType = image.Config.MediaType
-				}
+			if artifact.Subject != nil && content.Equal(*artifact.Subject, desc) {
+				node.ArtifactType = artifact.ArtifactType
+				node.Annotations = artifact.Annotations
+			}
+		case ocispec.MediaTypeImageManifest:
+			_, fetched, err := oras.FetchBytes(ctx, target, node.Digest.String(), oras.DefaultFetchBytesOptions)
+			if err != nil {
+				return nil, err
+			}
+			var image ocispec.Manifest
+			if err := json.Unmarshal(fetched, &image); err != nil {
+				return nil, err
 			}
-			if node.ArtifactType != "" && (artifactType == "" || artifactType == node.ArtifactType) {
-				results = append(results, node)
+			if image.Subject != nil && content.Equal(*image.Subject, desc) {
+				node.ArtifactType = image.Config.MediaType
+				node.Annotations = image.Annotations
 			}
 		}
+		if node.ArtifactType != "" && (artifactType == "" || artifactType == node.ArtifactType) {
+			results = append(results, node)
+		}
 	}
 	return results, nil
 }

From eef812a8d8e634517e1591ce5803f5e233cd7192 Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sun, 29 Jan 2023 15:55:23 +0800
Subject: [PATCH 13/19] move referrers func to a dedicated package

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 cmd/oras/discover.go    | 61 ++---------------------------------------
 internal/graph/graph.go | 57 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 60 insertions(+), 58 deletions(-)

diff --git a/cmd/oras/discover.go b/cmd/oras/discover.go
index 1909cb815..e3c254d04 100644
--- a/cmd/oras/discover.go
+++ b/cmd/oras/discover.go
@@ -24,9 +24,8 @@ import (
 
 	"gopkg.in/yaml.v3"
 	"oras.land/oras-go/v2"
-	"oras.land/oras-go/v2/content"
-	"oras.land/oras-go/v2/registry"
 	"oras.land/oras/cmd/oras/internal/option"
+	"oras.land/oras/internal/graph"
 
 	"github.com/need-being/go-tree"
 	"github.com/opencontainers/image-spec/specs-go"
@@ -118,7 +117,7 @@ func runDiscover(opts discoverOptions) error {
 		return tree.Print(root)
 	}
 
-	refs, err := fetchReferrers(ctx, repo, desc, opts.artifactType)
+	refs, err := graph.Referrers(ctx, repo, desc, opts.artifactType)
 	if err != nil {
 		return err
 	}
@@ -139,62 +138,8 @@ func runDiscover(opts discoverOptions) error {
 	return nil
 }
 
-func fetchReferrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc ocispec.Descriptor, artifactType string) ([]ocispec.Descriptor, error) {
-	var results []ocispec.Descriptor
-	if repo, ok := target.(registry.ReferrerLister); ok {
-		// get referrers directly
-		err := repo.Referrers(ctx, desc, artifactType, func(referrers []ocispec.Descriptor) error {
-			results = append(results, referrers...)
-			return nil
-		})
-		if err != nil {
-			return nil, err
-		}
-		return results, nil
-	}
-	// find matched referrers in all predecessors
-	predecessors, err := target.Predecessors(ctx, desc)
-	if err != nil {
-		return nil, err
-	}
-	for _, node := range predecessors {
-		switch node.MediaType {
-		case ocispec.MediaTypeArtifactManifest:
-			_, fetched, err := oras.FetchBytes(ctx, target, node.Digest.String(), oras.DefaultFetchBytesOptions)
-			if err != nil {
-				return nil, err
-			}
-			var artifact ocispec.Artifact
-			if err := json.Unmarshal(fetched, &artifact); err != nil {
-				return nil, err
-			}
-			if artifact.Subject != nil && content.Equal(*artifact.Subject, desc) {
-				node.ArtifactType = artifact.ArtifactType
-				node.Annotations = artifact.Annotations
-			}
-		case ocispec.MediaTypeImageManifest:
-			_, fetched, err := oras.FetchBytes(ctx, target, node.Digest.String(), oras.DefaultFetchBytesOptions)
-			if err != nil {
-				return nil, err
-			}
-			var image ocispec.Manifest
-			if err := json.Unmarshal(fetched, &image); err != nil {
-				return nil, err
-			}
-			if image.Subject != nil && content.Equal(*image.Subject, desc) {
-				node.ArtifactType = image.Config.MediaType
-				node.Annotations = image.Annotations
-			}
-		}
-		if node.ArtifactType != "" && (artifactType == "" || artifactType == node.ArtifactType) {
-			results = append(results, node)
-		}
-	}
-	return results, nil
-}
-
 func fetchAllReferrers(ctx context.Context, repo oras.ReadOnlyGraphTarget, desc ocispec.Descriptor, artifactType string, node *tree.Node, opts *discoverOptions) error {
-	results, err := fetchReferrers(ctx, repo, desc, artifactType)
+	results, err := graph.Referrers(ctx, repo, desc, artifactType)
 	if err != nil {
 		return err
 	}
diff --git a/internal/graph/graph.go b/internal/graph/graph.go
index 41b02c9b9..5077d0394 100644
--- a/internal/graph/graph.go
+++ b/internal/graph/graph.go
@@ -20,7 +20,9 @@ import (
 	"encoding/json"
 
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+	"oras.land/oras-go/v2"
 	"oras.land/oras-go/v2/content"
+	"oras.land/oras-go/v2/registry"
 	"oras.land/oras/internal/docker"
 )
 
@@ -59,3 +61,58 @@ func Successors(ctx context.Context, fetcher content.Fetcher, node ocispec.Descr
 	}
 	return
 }
+
+// Referrers returns referrer nodes of desc.
+func Referrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc ocispec.Descriptor, artifactType string) ([]ocispec.Descriptor, error) {
+	var results []ocispec.Descriptor
+	if repo, ok := target.(registry.ReferrerLister); ok {
+		// get referrers directly
+		err := repo.Referrers(ctx, desc, artifactType, func(referrers []ocispec.Descriptor) error {
+			results = append(results, referrers...)
+			return nil
+		})
+		if err != nil {
+			return nil, err
+		}
+		return results, nil
+	}
+	// find matched referrers in all predecessors
+	predecessors, err := target.Predecessors(ctx, desc)
+	if err != nil {
+		return nil, err
+	}
+	for _, node := range predecessors {
+		switch node.MediaType {
+		case ocispec.MediaTypeArtifactManifest:
+			_, fetched, err := oras.FetchBytes(ctx, target, node.Digest.String(), oras.DefaultFetchBytesOptions)
+			if err != nil {
+				return nil, err
+			}
+			var artifact ocispec.Artifact
+			if err := json.Unmarshal(fetched, &artifact); err != nil {
+				return nil, err
+			}
+			if artifact.Subject != nil && content.Equal(*artifact.Subject, desc) {
+				node.ArtifactType = artifact.ArtifactType
+				node.Annotations = artifact.Annotations
+			}
+		case ocispec.MediaTypeImageManifest:
+			_, fetched, err := oras.FetchBytes(ctx, target, node.Digest.String(), oras.DefaultFetchBytesOptions)
+			if err != nil {
+				return nil, err
+			}
+			var image ocispec.Manifest
+			if err := json.Unmarshal(fetched, &image); err != nil {
+				return nil, err
+			}
+			if image.Subject != nil && content.Equal(*image.Subject, desc) {
+				node.ArtifactType = image.Config.MediaType
+				node.Annotations = image.Annotations
+			}
+		}
+		if node.ArtifactType != "" && (artifactType == "" || artifactType == node.ArtifactType) {
+			results = append(results, node)
+		}
+	}
+	return results, nil
+}

From 560c3909712a29e0da61c3d489a9931dd16042c6 Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sun, 29 Jan 2023 16:40:24 +0800
Subject: [PATCH 14/19] add unit tests

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 internal/graph/graph.go      |  20 ++++---
 internal/graph/graph_test.go | 109 +++++++++++++++++++++++++++++++++++
 2 files changed, 120 insertions(+), 9 deletions(-)
 create mode 100644 internal/graph/graph_test.go

diff --git a/internal/graph/graph.go b/internal/graph/graph.go
index 5077d0394..c38dbd19d 100644
--- a/internal/graph/graph.go
+++ b/internal/graph/graph.go
@@ -23,6 +23,7 @@ import (
 	"oras.land/oras-go/v2"
 	"oras.land/oras-go/v2/content"
 	"oras.land/oras-go/v2/registry"
+	"oras.land/oras/internal/descriptor"
 	"oras.land/oras/internal/docker"
 )
 
@@ -62,7 +63,7 @@ func Successors(ctx context.Context, fetcher content.Fetcher, node ocispec.Descr
 	return
 }
 
-// Referrers returns referrer nodes of desc.
+// Referrers returns referrer nodes of desc in target.
 func Referrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc ocispec.Descriptor, artifactType string) ([]ocispec.Descriptor, error) {
 	var results []ocispec.Descriptor
 	if repo, ok := target.(registry.ReferrerLister); ok {
@@ -76,18 +77,23 @@ func Referrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc ocispe
 		}
 		return results, nil
 	}
+
+	if !descriptor.IsImageManifest(desc) && desc.MediaType != ocispec.MediaTypeArtifactManifest {
+		return nil, nil
+	}
+
 	// find matched referrers in all predecessors
 	predecessors, err := target.Predecessors(ctx, desc)
 	if err != nil {
 		return nil, err
 	}
 	for _, node := range predecessors {
+		_, fetched, err := oras.FetchBytes(ctx, target, node.Digest.String(), oras.DefaultFetchBytesOptions)
+		if err != nil {
+			return nil, err
+		}
 		switch node.MediaType {
 		case ocispec.MediaTypeArtifactManifest:
-			_, fetched, err := oras.FetchBytes(ctx, target, node.Digest.String(), oras.DefaultFetchBytesOptions)
-			if err != nil {
-				return nil, err
-			}
 			var artifact ocispec.Artifact
 			if err := json.Unmarshal(fetched, &artifact); err != nil {
 				return nil, err
@@ -97,10 +103,6 @@ func Referrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc ocispe
 				node.Annotations = artifact.Annotations
 			}
 		case ocispec.MediaTypeImageManifest:
-			_, fetched, err := oras.FetchBytes(ctx, target, node.Digest.String(), oras.DefaultFetchBytesOptions)
-			if err != nil {
-				return nil, err
-			}
 			var image ocispec.Manifest
 			if err := json.Unmarshal(fetched, &image); err != nil {
 				return nil, err
diff --git a/internal/graph/graph_test.go b/internal/graph/graph_test.go
new file mode 100644
index 000000000..67bb547d4
--- /dev/null
+++ b/internal/graph/graph_test.go
@@ -0,0 +1,109 @@
+/*
+Copyright The ORAS Authors.
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package graph
+
+import (
+	"context"
+	"errors"
+	"reflect"
+	"testing"
+
+	"github.com/opencontainers/go-digest"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+	"oras.land/oras-go/v2"
+)
+
+var (
+	artifactReferrerDesc = ocispec.Descriptor{
+		MediaType: ocispec.MediaTypeArtifactManifest,
+		Size:      123,
+		Digest:    digest.FromBytes([]byte{}),
+	}
+	imageReferrerDesc = ocispec.Descriptor{
+		MediaType: ocispec.MediaTypeImageManifest,
+		Size:      123,
+		Digest:    digest.FromBytes([]byte{}),
+	}
+	indexDesc = ocispec.Descriptor{
+		MediaType: ocispec.MediaTypeImageIndex,
+		Size:      123,
+		Digest:    digest.FromBytes([]byte{}),
+	}
+	subject = ocispec.Descriptor{
+		MediaType: ocispec.MediaTypeArtifactManifest,
+		Size:      123,
+		Digest:    digest.FromBytes([]byte{}),
+	}
+	referrers    = []ocispec.Descriptor{artifactReferrerDesc, imageReferrerDesc}
+	predecessors = []ocispec.Descriptor{artifactReferrerDesc}
+)
+
+type errLister struct {
+	oras.ReadOnlyGraphTarget
+}
+
+func (e *errLister) Referrers(ctx context.Context, desc ocispec.Descriptor, artifactType string, fn func(referrers []ocispec.Descriptor) error) error {
+	return errors.New("")
+}
+
+type refLister struct {
+	oras.ReadOnlyGraphTarget
+}
+
+func (m *refLister) Referrers(ctx context.Context, desc ocispec.Descriptor, artifactType string, fn func(referrers []ocispec.Descriptor) error) error {
+	return fn(referrers)
+}
+
+type predecessorFinder struct {
+	oras.ReadOnlyGraphTarget
+}
+
+func (m *predecessorFinder) Predecessors(ctx context.Context, node ocispec.Descriptor) ([]ocispec.Descriptor, error) {
+	return predecessors, nil
+}
+
+func TestReferrers(t *testing.T) {
+	type args struct {
+		ctx          context.Context
+		target       oras.ReadOnlyGraphTarget
+		desc         ocispec.Descriptor
+		artifactType string
+	}
+	ctx := context.Background()
+	tests := []struct {
+		name    string
+		args    args
+		want    []ocispec.Descriptor
+		wantErr bool
+	}{
+		// TODO: Add test cases.
+		{"should fail when a referrer lister failed to get referrers", args{ctx, &errLister{}, ocispec.Descriptor{}, ""}, nil, true},
+		{"should return referrers when target is a referrer lister", args{ctx, &refLister{}, ocispec.Descriptor{}, ""}, referrers, false},
+		{"should return nil for non-manifest node", args{ctx, &predecessorFinder{}, indexDesc, ""}, nil, false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := Referrers(tt.args.ctx, tt.args.target, tt.args.desc, tt.args.artifactType)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("Referrers() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("Referrers() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}

From da809668a037efa494b63fa87678ace1e25933fb Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sun, 29 Jan 2023 17:42:39 +0800
Subject: [PATCH 15/19] add unit test

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 internal/graph/graph.go      |  12 +++--
 internal/graph/graph_test.go | 102 +++++++++++++++++++++++++----------
 2 files changed, 81 insertions(+), 33 deletions(-)

diff --git a/internal/graph/graph.go b/internal/graph/graph.go
index c38dbd19d..68606ff74 100644
--- a/internal/graph/graph.go
+++ b/internal/graph/graph.go
@@ -88,12 +88,12 @@ func Referrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc ocispe
 		return nil, err
 	}
 	for _, node := range predecessors {
-		_, fetched, err := oras.FetchBytes(ctx, target, node.Digest.String(), oras.DefaultFetchBytesOptions)
-		if err != nil {
-			return nil, err
-		}
 		switch node.MediaType {
 		case ocispec.MediaTypeArtifactManifest:
+			_, fetched, err := oras.FetchBytes(ctx, target, node.Digest.String(), oras.DefaultFetchBytesOptions)
+			if err != nil {
+				return nil, err
+			}
 			var artifact ocispec.Artifact
 			if err := json.Unmarshal(fetched, &artifact); err != nil {
 				return nil, err
@@ -103,6 +103,10 @@ func Referrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc ocispe
 				node.Annotations = artifact.Annotations
 			}
 		case ocispec.MediaTypeImageManifest:
+			_, fetched, err := oras.FetchBytes(ctx, target, node.Digest.String(), oras.DefaultFetchBytesOptions)
+			if err != nil {
+				return nil, err
+			}
 			var image ocispec.Manifest
 			if err := json.Unmarshal(fetched, &image); err != nil {
 				return nil, err
diff --git a/internal/graph/graph_test.go b/internal/graph/graph_test.go
index 67bb547d4..49b521c33 100644
--- a/internal/graph/graph_test.go
+++ b/internal/graph/graph_test.go
@@ -17,6 +17,7 @@ package graph
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"reflect"
 	"testing"
@@ -26,31 +27,6 @@ import (
 	"oras.land/oras-go/v2"
 )
 
-var (
-	artifactReferrerDesc = ocispec.Descriptor{
-		MediaType: ocispec.MediaTypeArtifactManifest,
-		Size:      123,
-		Digest:    digest.FromBytes([]byte{}),
-	}
-	imageReferrerDesc = ocispec.Descriptor{
-		MediaType: ocispec.MediaTypeImageManifest,
-		Size:      123,
-		Digest:    digest.FromBytes([]byte{}),
-	}
-	indexDesc = ocispec.Descriptor{
-		MediaType: ocispec.MediaTypeImageIndex,
-		Size:      123,
-		Digest:    digest.FromBytes([]byte{}),
-	}
-	subject = ocispec.Descriptor{
-		MediaType: ocispec.MediaTypeArtifactManifest,
-		Size:      123,
-		Digest:    digest.FromBytes([]byte{}),
-	}
-	referrers    = []ocispec.Descriptor{artifactReferrerDesc, imageReferrerDesc}
-	predecessors = []ocispec.Descriptor{artifactReferrerDesc}
-)
-
 type errLister struct {
 	oras.ReadOnlyGraphTarget
 }
@@ -60,22 +36,88 @@ func (e *errLister) Referrers(ctx context.Context, desc ocispec.Descriptor, arti
 }
 
 type refLister struct {
+	referrers []ocispec.Descriptor
 	oras.ReadOnlyGraphTarget
 }
 
 func (m *refLister) Referrers(ctx context.Context, desc ocispec.Descriptor, artifactType string, fn func(referrers []ocispec.Descriptor) error) error {
-	return fn(referrers)
+	return fn(m.referrers)
 }
 
 type predecessorFinder struct {
+	predecessors []ocispec.Descriptor
 	oras.ReadOnlyGraphTarget
 }
 
 func (m *predecessorFinder) Predecessors(ctx context.Context, node ocispec.Descriptor) ([]ocispec.Descriptor, error) {
-	return predecessors, nil
+	return m.predecessors, nil
 }
 
 func TestReferrers(t *testing.T) {
+	var blobs [][]byte
+	var descs []ocispec.Descriptor
+	appendBlob := func(mediaType string, blob []byte) {
+		blobs = append(blobs, blob)
+		descs = append(descs, ocispec.Descriptor{
+			MediaType: mediaType,
+			Digest:    digest.FromBytes(blob),
+			Size:      int64(len(blob)),
+		})
+	}
+	generateImage := func(subject *ocispec.Descriptor, annotations map[string]string, config ocispec.Descriptor, layers ...ocispec.Descriptor) {
+		manifest := ocispec.Manifest{
+			Subject:     subject,
+			Config:      config,
+			Layers:      layers,
+			Annotations: annotations,
+		}
+		manifestJSON, err := json.Marshal(manifest)
+		if err != nil {
+			t.Fatal(err)
+		}
+		appendBlob(ocispec.MediaTypeImageManifest, manifestJSON)
+	}
+	generateArtifact := func(artifactType string, subject *ocispec.Descriptor, annotations map[string]string, blobs ...ocispec.Descriptor) {
+		manifest := ocispec.Artifact{
+			Subject:     subject,
+			Blobs:       blobs,
+			Annotations: annotations,
+		}
+		manifestJSON, err := json.Marshal(manifest)
+		if err != nil {
+			t.Fatal(err)
+		}
+		appendBlob(ocispec.MediaTypeImageManifest, manifestJSON)
+	}
+	generateIndex := func(manifests ...ocispec.Descriptor) {
+		index := ocispec.Index{
+			Manifests: manifests,
+		}
+		manifestJSON, err := json.Marshal(index)
+		if err != nil {
+			t.Fatal(err)
+		}
+		appendBlob(ocispec.MediaTypeImageManifest, manifestJSON)
+	}
+	const (
+		subject = iota
+		imgConfig
+		imgReferrer
+		artifactReferrer
+		index
+	)
+	anno := map[string]string{"test": "foo"}
+	appendBlob(ocispec.MediaTypeArtifactManifest, []byte("subject content"))
+	imageArtifactType := "test.image"
+	appendBlob(imageArtifactType, []byte("config content"))
+	generateImage(&descs[subject], anno, descs[imgConfig])
+	artifactType := "test.artifact"
+	generateArtifact(artifactType, &descs[subject], anno)
+	generateIndex(descs[subject])
+
+	referrers := []ocispec.Descriptor{descs[imgReferrer], descs[imgReferrer]}
+	predecessors := []ocispec.Descriptor{descs[imgReferrer], descs[imgReferrer], descs[index]}
+
 	type args struct {
 		ctx          context.Context
 		target       oras.ReadOnlyGraphTarget
@@ -91,8 +133,10 @@ func TestReferrers(t *testing.T) {
 	}{
 		// TODO: Add test cases.
 		{"should fail when a referrer lister failed to get referrers", args{ctx, &errLister{}, ocispec.Descriptor{}, ""}, nil, true},
-		{"should return referrers when target is a referrer lister", args{ctx, &refLister{}, ocispec.Descriptor{}, ""}, referrers, false},
-		{"should return nil for non-manifest node", args{ctx, &predecessorFinder{}, indexDesc, ""}, nil, false},
+		{"should return referrers when target is a referrer lister", args{ctx, &refLister{referrers: referrers}, ocispec.Descriptor{}, ""}, referrers, false},
+		{"should return nil for non-manifest node", args{ctx, &predecessorFinder{}, descs[index], ""}, nil, false},
+		{"should return nil for non-manifest node", args{ctx, &predecessorFinder{}, descs[imgConfig], ""}, nil, false},
+		{"should return referrers in predecessor", args{ctx, &predecessorFinder{predecessors: predecessors}, descs[subject], ""}, referrers, false},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

From b5a3112c63162565afb1920f78faa0c9ad51499e Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sun, 29 Jan 2023 17:45:57 +0800
Subject: [PATCH 16/19] add unit test

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 internal/graph/graph_test.go | 1 -
 1 file changed, 1 deletion(-)

diff --git a/internal/graph/graph_test.go b/internal/graph/graph_test.go
index 49b521c33..4fbfbecb7 100644
--- a/internal/graph/graph_test.go
+++ b/internal/graph/graph_test.go
@@ -136,7 +136,6 @@ func TestReferrers(t *testing.T) {
 		{"should return referrers when target is a referrer lister", args{ctx, &refLister{referrers: referrers}, ocispec.Descriptor{}, ""}, referrers, false},
 		{"should return nil for non-manifest node", args{ctx, &predecessorFinder{}, descs[index], ""}, nil, false},
 		{"should return nil for non-manifest node", args{ctx, &predecessorFinder{}, descs[imgConfig], ""}, nil, false},
-		{"should return referrers in predecessor", args{ctx, &predecessorFinder{predecessors: predecessors}, descs[subject], ""}, referrers, false},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

From 243fa5bd6150395fa6f4e8e56f0e58828281a911 Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sun, 29 Jan 2023 18:48:01 +0800
Subject: [PATCH 17/19] add unit test with memory store

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 internal/graph/graph.go      | 13 ++++++--
 internal/graph/graph_test.go | 63 ++++++++++++++++++++++++------------
 2 files changed, 54 insertions(+), 22 deletions(-)

diff --git a/internal/graph/graph.go b/internal/graph/graph.go
index 68606ff74..ba4357e9f 100644
--- a/internal/graph/graph.go
+++ b/internal/graph/graph.go
@@ -90,7 +90,7 @@ func Referrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc ocispe
 	for _, node := range predecessors {
 		switch node.MediaType {
 		case ocispec.MediaTypeArtifactManifest:
-			_, fetched, err := oras.FetchBytes(ctx, target, node.Digest.String(), oras.DefaultFetchBytesOptions)
+			fetched, err := fetchBytes(ctx, target, node)
 			if err != nil {
 				return nil, err
 			}
@@ -103,7 +103,7 @@ func Referrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc ocispe
 				node.Annotations = artifact.Annotations
 			}
 		case ocispec.MediaTypeImageManifest:
-			_, fetched, err := oras.FetchBytes(ctx, target, node.Digest.String(), oras.DefaultFetchBytesOptions)
+			fetched, err := fetchBytes(ctx, target, node)
 			if err != nil {
 				return nil, err
 			}
@@ -122,3 +122,12 @@ func Referrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc ocispe
 	}
 	return results, nil
 }
+
+func fetchBytes(ctx context.Context, fetcher content.Fetcher, desc ocispec.Descriptor) ([]byte, error) {
+	rc, err := fetcher.Fetch(ctx, desc)
+	if err != nil {
+		return nil, err
+	}
+	defer rc.Close()
+	return content.ReadAll(rc, desc)
+}
diff --git a/internal/graph/graph_test.go b/internal/graph/graph_test.go
index 4fbfbecb7..5e811a531 100644
--- a/internal/graph/graph_test.go
+++ b/internal/graph/graph_test.go
@@ -16,6 +16,7 @@ limitations under the License.
 package graph
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"errors"
@@ -25,6 +26,7 @@ import (
 	"github.com/opencontainers/go-digest"
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 	"oras.land/oras-go/v2"
+	"oras.land/oras-go/v2/content/memory"
 )
 
 type errLister struct {
@@ -45,15 +47,11 @@ func (m *refLister) Referrers(ctx context.Context, desc ocispec.Descriptor, arti
 }
 
 type predecessorFinder struct {
-	predecessors []ocispec.Descriptor
-	oras.ReadOnlyGraphTarget
-}
-
-func (m *predecessorFinder) Predecessors(ctx context.Context, node ocispec.Descriptor) ([]ocispec.Descriptor, error) {
-	return m.predecessors, nil
+	*memory.Store
 }
 
 func TestReferrers(t *testing.T) {
+	ctx := context.Background()
 	var blobs [][]byte
 	var descs []ocispec.Descriptor
 	appendBlob := func(mediaType string, blob []byte) {
@@ -79,15 +77,16 @@ func TestReferrers(t *testing.T) {
 	}
 	generateArtifact := func(artifactType string, subject *ocispec.Descriptor, annotations map[string]string, blobs ...ocispec.Descriptor) {
 		manifest := ocispec.Artifact{
-			Subject:     subject,
-			Blobs:       blobs,
-			Annotations: annotations,
+			Subject:      subject,
+			Blobs:        blobs,
+			Annotations:  annotations,
+			ArtifactType: artifactType,
 		}
 		manifestJSON, err := json.Marshal(manifest)
 		if err != nil {
 			t.Fatal(err)
 		}
-		appendBlob(ocispec.MediaTypeImageManifest, manifestJSON)
+		appendBlob(ocispec.MediaTypeArtifactManifest, manifestJSON)
 	}
 	generateIndex := func(manifests ...ocispec.Descriptor) {
 		index := ocispec.Index{
@@ -97,26 +96,36 @@ func TestReferrers(t *testing.T) {
 		if err != nil {
 			t.Fatal(err)
 		}
-		appendBlob(ocispec.MediaTypeImageManifest, manifestJSON)
+		appendBlob(ocispec.MediaTypeImageIndex, manifestJSON)
 	}
 	const (
 		subject = iota
 		imgConfig
-		imgReferrer
-		artifactReferrer
+		image
+		artifact
 		index
 	)
 	anno := map[string]string{"test": "foo"}
 	appendBlob(ocispec.MediaTypeArtifactManifest, []byte("subject content"))
-	imageArtifactType := "test.image"
-	appendBlob(imageArtifactType, []byte("config content"))
+	imageType := "test.image"
+	appendBlob(imageType, []byte("config content"))
 	generateImage(&descs[subject], anno, descs[imgConfig])
+	imageDesc := descs[image]
+	imageDesc.Annotations = anno
+	imageDesc.ArtifactType = imageType
 	artifactType := "test.artifact"
 	generateArtifact(artifactType, &descs[subject], anno)
 	generateIndex(descs[subject])
+	artifactDesc := descs[artifact]
+	artifactDesc.Annotations = anno
+	artifactDesc.ArtifactType = artifactType
 
-	referrers := []ocispec.Descriptor{descs[imgReferrer], descs[imgReferrer]}
-	predecessors := []ocispec.Descriptor{descs[imgReferrer], descs[imgReferrer], descs[index]}
+	referrers := []ocispec.Descriptor{descs[image], descs[image]}
+	memory := memory.New()
+	for i := range descs {
+		memory.Push(ctx, descs[i], bytes.NewReader(blobs[i]))
+	}
+	finder := &predecessorFinder{Store: memory}
 
 	type args struct {
 		ctx          context.Context
@@ -124,7 +133,6 @@ func TestReferrers(t *testing.T) {
 		desc         ocispec.Descriptor
 		artifactType string
 	}
-	ctx := context.Background()
 	tests := []struct {
 		name    string
 		args    args
@@ -134,8 +142,10 @@ func TestReferrers(t *testing.T) {
 		// TODO: Add test cases.
 		{"should fail when a referrer lister failed to get referrers", args{ctx, &errLister{}, ocispec.Descriptor{}, ""}, nil, true},
 		{"should return referrers when target is a referrer lister", args{ctx, &refLister{referrers: referrers}, ocispec.Descriptor{}, ""}, referrers, false},
-		{"should return nil for non-manifest node", args{ctx, &predecessorFinder{}, descs[index], ""}, nil, false},
-		{"should return nil for non-manifest node", args{ctx, &predecessorFinder{}, descs[imgConfig], ""}, nil, false},
+		{"should return nil for index node", args{ctx, finder, descs[index], ""}, nil, false},
+		{"should return nil for config node", args{ctx, finder, descs[imgConfig], ""}, nil, false},
+		{"should find filtered image referrer", args{ctx, finder, descs[subject], imageType}, []ocispec.Descriptor{imageDesc}, false},
+		{"should find filtered artifact referrer", args{ctx, finder, descs[subject], artifactType}, []ocispec.Descriptor{artifactDesc}, false},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -149,4 +159,17 @@ func TestReferrers(t *testing.T) {
 			}
 		})
 	}
+
+	t.Run("should find referrers in predecessors", func(t *testing.T) {
+		want1 := []ocispec.Descriptor{artifactDesc, imageDesc}
+		want2 := []ocispec.Descriptor{imageDesc, artifactDesc}
+		got, err := Referrers(ctx, finder, descs[subject], "")
+		if err != nil {
+			t.Errorf("Referrers() error = %v", err)
+			return
+		}
+		if !reflect.DeepEqual(got, want1) && !reflect.DeepEqual(got, want2) {
+			t.Errorf("Referrers() = %v, want %v", got, want1)
+		}
+	})
 }

From 764310a87974433fb141a7b6d1462b9c9ecaf6b5 Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sun, 29 Jan 2023 18:52:45 +0800
Subject: [PATCH 18/19] code clean

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 internal/graph/graph_test.go | 1 -
 1 file changed, 1 deletion(-)

diff --git a/internal/graph/graph_test.go b/internal/graph/graph_test.go
index 5e811a531..be2b4ffdf 100644
--- a/internal/graph/graph_test.go
+++ b/internal/graph/graph_test.go
@@ -139,7 +139,6 @@ func TestReferrers(t *testing.T) {
 		want    []ocispec.Descriptor
 		wantErr bool
 	}{
-		// TODO: Add test cases.
 		{"should fail when a referrer lister failed to get referrers", args{ctx, &errLister{}, ocispec.Descriptor{}, ""}, nil, true},
 		{"should return referrers when target is a referrer lister", args{ctx, &refLister{referrers: referrers}, ocispec.Descriptor{}, ""}, referrers, false},
 		{"should return nil for index node", args{ctx, finder, descs[index], ""}, nil, false},

From 3271b20b204bf7669132552389c56f9f883b2162 Mon Sep 17 00:00:00 2001
From: Billy Zha <jinzha1@microsoft.com>
Date: Sun, 29 Jan 2023 19:31:46 +0800
Subject: [PATCH 19/19] resolve comments

Signed-off-by: Billy Zha <jinzha1@microsoft.com>
---
 internal/graph/graph.go | 21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/internal/graph/graph.go b/internal/graph/graph.go
index ba4357e9f..84d5c9620 100644
--- a/internal/graph/graph.go
+++ b/internal/graph/graph.go
@@ -23,7 +23,6 @@ import (
 	"oras.land/oras-go/v2"
 	"oras.land/oras-go/v2/content"
 	"oras.land/oras-go/v2/registry"
-	"oras.land/oras/internal/descriptor"
 	"oras.land/oras/internal/docker"
 )
 
@@ -78,10 +77,6 @@ func Referrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc ocispe
 		return results, nil
 	}
 
-	if !descriptor.IsImageManifest(desc) && desc.MediaType != ocispec.MediaTypeArtifactManifest {
-		return nil, nil
-	}
-
 	// find matched referrers in all predecessors
 	predecessors, err := target.Predecessors(ctx, desc)
 	if err != nil {
@@ -98,10 +93,11 @@ func Referrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc ocispe
 			if err := json.Unmarshal(fetched, &artifact); err != nil {
 				return nil, err
 			}
-			if artifact.Subject != nil && content.Equal(*artifact.Subject, desc) {
-				node.ArtifactType = artifact.ArtifactType
-				node.Annotations = artifact.Annotations
+			if artifact.Subject == nil || !content.Equal(*artifact.Subject, desc) {
+				continue
 			}
+			node.ArtifactType = artifact.ArtifactType
+			node.Annotations = artifact.Annotations
 		case ocispec.MediaTypeImageManifest:
 			fetched, err := fetchBytes(ctx, target, node)
 			if err != nil {
@@ -111,10 +107,13 @@ func Referrers(ctx context.Context, target oras.ReadOnlyGraphTarget, desc ocispe
 			if err := json.Unmarshal(fetched, &image); err != nil {
 				return nil, err
 			}
-			if image.Subject != nil && content.Equal(*image.Subject, desc) {
-				node.ArtifactType = image.Config.MediaType
-				node.Annotations = image.Annotations
+			if image.Subject == nil || !content.Equal(*image.Subject, desc) {
+				continue
 			}
+			node.ArtifactType = image.Config.MediaType
+			node.Annotations = image.Annotations
+		default:
+			continue
 		}
 		if node.ArtifactType != "" && (artifactType == "" || artifactType == node.ArtifactType) {
 			results = append(results, node)