feat: make EntityNotFoundError a first-class error

Signed-off-by: Vishal Choudhary <[email protected]>

cmd/cosign/cli/sign/sign.go

@@ -179,7 +179,7 @@ func SignCmd(ro *options.RootOptions, ko options.KeyOpts, signOpts options.SignO
 
 		if digest, ok := ref.(name.Digest); ok && !signOpts.Recursive {
 			se, err := ociremote.SignedEntity(ref, opts...)
-			if ociremote.IsEntityNotFoundError(err) {
+			if _, isEntityNotFoundErr := err.(*ociremote.EntityNotFoundError); isEntityNotFoundErr {
 				se = ociremote.SignedUnknown(digest)
 			} else if err != nil {
 				return fmt.Errorf("accessing image: %w", err)

pkg/oci/remote/remote.go

@@ -20,7 +20,6 @@ import (
 	"fmt"
 	"io"
 	"net/http"
-	"strings"
 
 	"github.com/google/go-containerregistry/pkg/name"
 	v1 "github.com/google/go-containerregistry/pkg/v1"
@@ -37,18 +36,22 @@ var (
 	remoteIndex = remote.Index
 	remoteGet   = remote.Get
 	remoteWrite = remote.Write
-
-	// ErrEntityNotFound is the error that SignedEntity returns when the
-	// provided ref does not exist.
-	ErrEntityNotFound = "cosign remoteGet: entity not found in registry"
 )
 
-func NewEntityNotFoundError(err error) error {
-	return fmt.Errorf("%s error: %w", ErrEntityNotFound, err)
+// EntityNotFoundError is the error that SignedEntity returns when the
+// provided ref does not exist.
+type EntityNotFoundError struct {
+	baseErr error
+}
+
+func (e *EntityNotFoundError) Error() string {
+	return fmt.Sprintf("entity not found in registry, error: %v", e.baseErr)
 }
 
-func IsEntityNotFoundError(err error) bool {
-	return strings.Contains(err.Error(), ErrEntityNotFound)
+func NewEntityNotFoundError(err error) error {
+	return &EntityNotFoundError{
+		baseErr: err,
+	}
 }
 
 // SignedEntity provides access to a remote reference, and its signatures.