fix: added a custom error type for getContent's use

Signed-off-by: Kyle M. Tarplee <[email protected]>

copy.go

@@ -144,7 +144,7 @@ func (opts *CopyGraphOptions) WithMount(sourceRepository string, mounter registr
 
 			// To avoid needing a content.Fetcher as an input argument we simply fall back to the default behavior
 			// as if getContent was nil
-			return nil, errdef.ErrUnsupported
+			return nil, registry.UseSourceRepository
 		}
 
 		// Mount or copy

copy_test.go

@@ -37,6 +37,7 @@ import (
 	"oras.land/oras-go/v2/internal/cas"
 	"oras.land/oras-go/v2/internal/docker"
 	"oras.land/oras-go/v2/internal/spec"
+	"oras.land/oras-go/v2/registry"
 )
 
 // storageTracker tracks storage API counts.
@@ -1549,8 +1550,8 @@ func TestCopyGraph_WithOptions(t *testing.T) {
 			}
 
 			_, err := getContent()
-			if !errors.Is(err, errdef.ErrUnsupported) {
-				t.Fatalf("Expected error %v", errdef.ErrUnsupported)
+			if !errors.Is(err, registry.UseSourceRepository) {
+				t.Fatalf("Expected error %v", registry.UseSourceRepository)
 			}
 			rc, err := src.Fetch(ctx, desc)
 			if err != nil {

registry/remote/repository.go

@@ -806,7 +806,7 @@ func (s *blobStore) Mount(ctx context.Context, desc ocispec.Descriptor, fromRepo
 	var r io.ReadCloser
 	if getContent != nil {
 		r, err = getContent()
-		if errors.Is(err, errdef.ErrUnsupported) {
+		if errors.Is(err, registry.UseSourceRepository) {
 			// getContent can return a ErrUnsupported to fallback to the default copy operation
 			r, err = s.sibling(fromRepo).Fetch(ctx, desc)
 		}

registry/remote/repository_test.go

@@ -436,11 +436,11 @@ func TestRepository_Mount_Fallback(t *testing.T) {
 		}
 	})
 
-	t.Run("getContent is ErrUnsupported", func(t *testing.T) {
+	t.Run("getContent is UseSourceRepository", func(t *testing.T) {
 		sequence = ""
 
 		err = repo.Mount(ctx, blobDesc, "test", func() (io.ReadCloser, error) {
-			return nil, errdef.ErrUnsupported
+			return nil, registry.UseSourceRepository
 		})
 		if err != nil {
 			t.Fatalf("Repository.Push() error = %v", err)

registry/repository.go

@@ -18,6 +18,7 @@ package registry
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 
@@ -128,6 +129,10 @@ type Mounter interface {
 	) error
 }
 
+// UseSourceRepository signals to Mount implementations that the content from the source repository should be used.  This may be returned from the getContent function of the Mounter.Mount method.
+// This is useful to avoid having to know that source repository in the definition of getContent.
+var UseSourceRepository = errors.New("use source repository")
+
 // Tags lists the tags available in the repository.
 func Tags(ctx context.Context, repo TagLister) ([]string, error) {
 	var res []string