Linux: Add io/tst-o_path-locks test

The O_PATH-based fchmodat emulation will rely on the fact that closing
an O_PATH descriptor never releases POSIX advisory locks, so this
commit adds a test case for this behavior.

sysdeps/unix/sysv/linux/Makefile

@@ -274,7 +274,7 @@ sysdep_routines += xstatconv internal_statvfs internal_statvfs64 \
 
 sysdep_headers += bits/fcntl-linux.h
 
-tests += tst-fallocate tst-fallocate64
+tests += tst-fallocate tst-fallocate64 tst-o_path-locks
 endif
 
 ifeq ($(subdir),elf)

sysdeps/unix/sysv/linux/tst-o_path-locks.c

@@ -0,0 +1,100 @@
+/* Test that closing O_PATH descriptors does not release POSIX advisory locks.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/namespace.h>
+#include <support/support.h>
+#include <support/temp_file.h>
+#include <support/xunistd.h>
+
+/* The subprocess writes the errno value of the lock operation
+   here.  */
+static int *shared_errno;
+
+/* The path of the temporary file which is locked.  */
+static char *path;
+
+/* Try to obtain an exclusive lock on the file at path.  */
+static void
+subprocess (void *closure)
+{
+  int fd = xopen (path, O_RDWR, 0);
+  struct flock64 lock = { .l_type = F_WRLCK, };
+  int ret = fcntl64 (fd, F_SETLK, &lock);
+  if (ret == 0)
+    *shared_errno = 0;
+  else
+    *shared_errno = errno;
+  xclose (fd);
+}
+
+/* Return true if the file at path is currently locked, false if
+   not.  */
+static bool
+probe_lock (void)
+{
+  *shared_errno = -1;
+  support_isolate_in_subprocess (subprocess, NULL);
+  if (*shared_errno == 0)
+    /* Lock was aquired by the subprocess, so this process has not
+       locked it.  */
+    return false;
+  else
+    {
+      /* POSIX allows both EACCES and EAGAIN.  Linux use EACCES.  */
+      TEST_COMPARE (*shared_errno, EAGAIN);
+      return true;
+    }
+}
+
+static int
+do_test (void)
+{
+  shared_errno = support_shared_allocate (sizeof (*shared_errno));
+  int fd = create_temp_file ("tst-o_path-locks-", &path);
+
+  /* The file is not locked initially.  */
+  TEST_VERIFY (!probe_lock ());
+
+  struct flock64 lock = { .l_type = F_WRLCK, };
+  TEST_COMPARE (fcntl64 (fd, F_SETLK, &lock), 0);
+
+  /* The lock has been acquired.  */
+  TEST_VERIFY (probe_lock ());
+
+  /* Closing the same file via a different descriptor releases the
+     lock.  */
+  xclose (xopen (path, O_RDONLY, 0));
+  TEST_VERIFY (!probe_lock ());
+
+  /* But not if it is an O_PATH descriptor.  */
+  TEST_COMPARE (fcntl64 (fd, F_SETLK, &lock), 0);
+  xclose (xopen (path, O_PATH, 0));
+  TEST_VERIFY (probe_lock ());
+
+  xclose (fd);
+  free (path);
+  support_shared_free (shared_errno);
+  return 0;
+}
+
+#include <support/test-driver.c>