test: add unit test for error scenario in `internal/credential/store.…

…go` (oras-project#982)

Signed-off-by: Akanksha Gahalot <akankshagahlot0307@gmail.com>
Signed-off-by: Billy Zha <jinzha1@microsoft.com>

internal/credential/store_test.go

@@ -0,0 +1,48 @@
+/*
+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 credential
+
+import (
+	"os"
+	"path"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func TestNewStoreError(t *testing.T) {
+	tmpDir := t.TempDir()
+	filename := path.Join(tmpDir, "testfile.txt")
+	_, err := os.Create(filename)
+	if err != nil {
+		t.Errorf("error: cannot create file : %v", err)
+	}
+	err = os.Chmod(filename, 000)
+	if err != nil {
+		t.Errorf("error: cannot change file permissions: %v", err)
+	}
+	credStore, err := NewStore(filename)
+	if credStore != nil {
+		t.Errorf("Expected NewStore to return nil but actually returned %v ", credStore)
+	}
+	if err != nil {
+		ok := strings.Contains(err.Error(), "failed to open config file")
+		reflect.DeepEqual(ok, true)
+
+	} else {
+		t.Errorf("Expected err to be not nil")
+	}
+}