nfs: add missing DirListTest

Summary:
This should have been added in D27243075 (5a150e125a) but I forgot to run `hg add` and it
was thus not added...

Reviewed By: fanzeyi

Differential Revision: D27279169

fbshipit-source-id: 69807cc05fef33f51b2a491b66c2e8aeb7136deb

eden/fs/nfs/test/DirListTest.cpp

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This software may be used and distributed according to the terms of the
+ * GNU General Public License version 2.
+ */
+
+#ifndef _WIN32
+
+#include <gtest/gtest.h>
+#include "eden/fs/nfs/NfsdRpc.h"
+#include "eden/fs/nfs/testharness/XdrTestUtils.h"
+
+namespace facebook::eden {
+
+namespace {
+size_t computeInitialOverhead() {
+  return XdrTrait<post_op_attr>::serializedSize(post_op_attr{fattr3{}}) +
+      XdrTrait<uint64_t>::serializedSize(0) +
+      2 * XdrTrait<bool>::serializedSize(false);
+}
+} // namespace
+
+TEST(DirListTest, size) {
+  // Verify that the computeInitialOverhead in DirList.cpp is correct.
+  // If this fails, do not modify the value below! It means that the XDR
+  // datastructures have changed and no longer have the correct size.
+  EXPECT_EQ(computeInitialOverhead(), 104);
+}
+
+} // namespace facebook::eden
+
+#endif