test: add a new case for UnwrapNopCloser (#725)

Add a new unit test case by the change to `UnwrapNopCloser` introduced
by #715

---------

Signed-off-by: Shiwei Zhang <shizh@microsoft.com>

internal/ioutil/io.go

@@ -44,23 +44,23 @@ func CopyBuffer(dst io.Writer, src io.Reader, buf []byte, desc ocispec.Descripto
 	return vr.Verify()
 }
 
-// nopCloserType is the type of `io.NopCloser()`.
-var nopCloserType = reflect.TypeOf(io.NopCloser(nil))
-//  nopCloserWriterToType is the type of `io.nopCloserWriterTo`
-var nopCloserWriterToType = reflect.TypeOf(io.NopCloser(struct {
-	io.Reader
-	io.WriterTo
-}{}))
+// Types returned by `io.NopCloser()`.
+var (
+	nopCloserType         = reflect.TypeOf(io.NopCloser(nil))
+	nopCloserWriterToType = reflect.TypeOf(io.NopCloser(struct {
+		io.Reader
+		io.WriterTo
+	}{}))
+)
 
-// UnwrapNopCloser returns the underlying reader if rc is a NopCloser
-// else it simply returns rc.
+// UnwrapNopCloser unwraps the reader wrapped by `io.NopCloser()`.
 // Similar implementation can be found in the built-in package `net/http`.
 // Reference: https://github.com/golang/go/blob/go1.22.1/src/net/http/transfer.go#L1090-L1105
-func UnwrapNopCloser(rc io.Reader) io.Reader {
-	switch reflect.TypeOf(rc) {
+func UnwrapNopCloser(r io.Reader) io.Reader {
+	switch reflect.TypeOf(r) {
 	case nopCloserType, nopCloserWriterToType:
-		return reflect.ValueOf(rc).Field(0).Interface().(io.Reader)
+		return reflect.ValueOf(r).Field(0).Interface().(io.Reader)
 	default:
-		return rc
+		return r
 	}
 }

internal/ioutil/io_test.go

@@ -28,6 +28,14 @@ import (
 )
 
 func TestUnwrapNopCloser(t *testing.T) {
+	var reader struct {
+		io.Reader
+	}
+	var readerWithWriterTo struct {
+		io.Reader
+		io.WriterTo
+	}
+
 	tests := []struct {
 		name string
 		rc   io.Reader
@@ -37,9 +45,14 @@ func TestUnwrapNopCloser(t *testing.T) {
 			name: "nil",
 		},
 		{
-			name: "no-op closer",
-			rc:   io.NopCloser(os.Stdin),
-			want: os.Stdin,
+			name: "no-op closer with plain io.Reader",
+			rc:   io.NopCloser(reader),
+			want: reader,
+		},
+		{
+			name: "no-op closer with io.WriteTo",
+			rc:   io.NopCloser(readerWithWriterTo),
+			want: readerWithWriterTo,
 		},
 		{
 			name: "any ReadCloser",