If no tag or digest specified when pulling, return error (#440)

Signed-off-by: Billy Zha <[email protected]>

cmd/oras/errors.go

@@ -0,0 +1,27 @@
+/*
+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 main
+
+import (
+	"fmt"
+
+	"oras.land/oras-go/v2/registry"
+)
+
+// newErrInvalidReference creates a new error based on the reference string.
+func newErrInvalidReference(ref registry.Reference) error {
+	return fmt.Errorf("%s: invalid image reference, expecting <name:tag|name@digest>", ref)
+}

cmd/oras/pull.go

@@ -87,6 +87,9 @@ func runPull(opts pullOptions) error {
 	if err != nil {
 		return err
 	}
+	if repo.Reference.Reference == "" {
+		return newErrInvalidReference(repo.Reference)
+	}
 	var src oras.Target = repo
 	if opts.cacheRoot != "" {
 		ociStore, err := oci.New(opts.cacheRoot)