fix: deleteVersions causing xl.meta to have empty Versions[] slice (#14483)

This is a side-affect of the optimization done in PR #13544 which
causes a certain type of delete operations on given object versions
can cause lastVersion indication to be skipped, which leads to
an `xl.meta` where Versions[] slice is empty while the entire
file is intact by itself.

This PR tries to ensure that such files are visible and deletable
by regular means of listing as null 'delete-marker' and also
avoid the situation where this potential issue might arise.

cmd/erasure-object_test.go

@@ -128,6 +128,86 @@ func TestErasureDeleteObjectBasic(t *testing.T) {
 	removeRoots(fsDirs)
 }
 
+func TestDeleteObjectsVersioned(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	obj, fsDirs, err := prepareErasure(ctx, 16)
+	if err != nil {
+		t.Fatal("Unable to initialize 'Erasure' object layer.", err)
+	}
+	// Remove all dirs.
+	for _, dir := range fsDirs {
+		defer os.RemoveAll(dir)
+	}
+
+	type testCaseType struct {
+		bucket string
+		object string
+	}
+
+	bucketName := "bucket"
+	testCases := []testCaseType{
+		{bucketName, "dir/obj1"},
+		{bucketName, "dir/obj1"},
+	}
+
+	err = obj.MakeBucketWithLocation(ctx, bucketName, BucketOptions{
+		VersioningEnabled: true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	names := make([]ObjectToDelete, len(testCases))
+	for i, testCase := range testCases {
+		objInfo, err := obj.PutObject(ctx, testCase.bucket, testCase.object,
+			mustGetPutObjReader(t, bytes.NewReader([]byte("abcd")), int64(len("abcd")), "", ""), ObjectOptions{
+				Versioned: true,
+			})
+		if err != nil {
+			t.Fatalf("Erasure Object upload failed: <ERROR> %s", err)
+		}
+		names[i] = ObjectToDelete{
+			ObjectV: ObjectV{
+				ObjectName: objInfo.Name,
+				VersionID:  objInfo.VersionID,
+			},
+		}
+
+	}
+	names = append(names, ObjectToDelete{
+		ObjectV: ObjectV{
+			ObjectName: "dir/obj1",
+			VersionID:  mustGetUUID(), // add a non-existent UUID.
+		},
+	})
+
+	_, delErrs := obj.DeleteObjects(ctx, bucketName, names, ObjectOptions{
+		Versioned: true,
+	})
+	for i := range delErrs {
+		if delErrs[i] != nil {
+			t.Errorf("Failed to remove object `%v` with the error: `%v`", names[i], delErrs[i])
+		}
+	}
+
+	for i, test := range testCases {
+		_, statErr := obj.GetObjectInfo(ctx, test.bucket, test.object, ObjectOptions{
+			VersionID: names[i].ObjectV.VersionID,
+		})
+		switch statErr.(type) {
+		case VersionNotFound:
+		default:
+			t.Fatalf("Object %s is not removed", test.bucket+SlashSeparator+test.object)
+		}
+	}
+
+	if _, err = ioutil.ReadFile(pathJoin(fsDirs[0], bucketName, "dir/obj1", "xl.meta")); err == nil {
+		t.Fatalf("xl.meta still present after removal")
+	}
+}
+
 func TestErasureDeleteObjectsErasureSet(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()