testlib: Add an assertion that a fd has really been closed

We can't easily assert this without triggering warnings from tools like
valgrind by doing an invalid operation on a closed fd, so we only check
this when under `-m undefined`.

Originally contributed to GLib 2.76 in GNOME/glib@b3934133
"gstdio: Add g_clear_fd() and g_autofd". The implementation in GLib used
g_fsync() as a portable thing that we can do with a fd, but that
function is newer than our minimum GLib version, and libglnx isn't
portable to non-Unix anyway, so use fnctl() instead.

Signed-off-by: Simon McVittie <smcv@collabora.com>

tests/libglnx-testlib.c

@@ -23,6 +23,7 @@
 #include "libglnx-testlib.h"
 
 #include <errno.h>
+#include <fcntl.h>
 
 #include <glib/gstdio.h>
 
@@ -72,3 +73,21 @@ _glnx_test_auto_temp_dir_leave (_GLnxTestAutoTempDir *dir)
   g_free (dir->old_cwd);
   g_free (dir);
 }
+
+void
+_glnx_test_assert_fd_was_closed (int fd)
+{
+  /* We can't tell a fd was really closed without behaving as though it
+   * was still valid */
+  if (g_test_undefined ())
+    {
+      int result;
+      int errsv;
+
+      errno = 0;
+      result = fcntl (fd, F_GETFD, 0);
+      errsv = errno;
+      g_assert_cmpint (result, <, 0);
+      g_assert_cmpint (errsv, ==, EBADF);
+    }
+}

tests/libglnx-testlib.h

@@ -1,7 +1,7 @@
 /* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
  *
  * Copyright (C) 2017 Red Hat, Inc.
- * Copyright 2019 Collabora Ltd.
+ * Copyright 2019-2022 Collabora Ltd.
  * SPDX-License-Identifier: LGPL-2.0-or-later
  *
  * This library is free software; you can redistribute it and/or
@@ -23,6 +23,7 @@
 #pragma once
 
 #include <glib.h>
+#include <glib/gstdio.h>
 
 #include "glnx-backport-autoptr.h"
 
@@ -47,3 +48,5 @@ G_DEFINE_AUTOPTR_CLEANUP_FUNC(_GLnxTestAutoTempDir, _glnx_test_auto_temp_dir_lea
 
 #define _GLNX_TEST_SCOPED_TEMP_DIR \
   G_GNUC_UNUSED g_autoptr(_GLnxTestAutoTempDir) temp_dir = _glnx_test_auto_temp_dir_enter ()
+
+void _glnx_test_assert_fd_was_closed (int fd);