[PATCH v2 1/3] support: Add support_fcntl_support_ofd_locks ()

[Sergey Bugaev <bugaevc@gmail.com>]

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
---

support_xxx_support_yyy () seems to be the naming pattern for this
kind of functions?

 support/Makefile                          |  1 +
 support/support.h                         |  3 ++
 support/support_fcntl_support_ofd_locks.c | 44 +++++++++++++++++++++++
 3 files changed, 48 insertions(+)
 create mode 100644 support/support_fcntl_support_ofd_locks.c

diff --git a/support/Makefile b/support/Makefile
index 92f1a246..e9a00b2d 100644
--- a/support/Makefile
+++ b/support/Makefile
@@ -58,6 +58,7 @@ libsupport-routines = \
   support_descriptors \
   support_enter_mount_namespace \
   support_enter_network_namespace \
+  support_fcntl_support_ofd_locks \
   support_format_address_family \
   support_format_addrinfo \
   support_format_dns_packet \
diff --git a/support/support.h b/support/support.h
index b7f76bf0..e20d2ce7 100644
--- a/support/support.h
+++ b/support/support.h
@@ -178,6 +178,9 @@ static __inline bool support_itimer_support_time64 (void)
 #endif
 }
 
+/* Return true if the kernel/file supports open file description locks.  */
+extern bool support_fcntl_support_ofd_locks (int fd);
+
 /* Return true if stat supports nanoseconds resolution.  PATH is used
    for tests and its ctime may change.  */
 extern bool support_stat_nanoseconds (const char *path);
diff --git a/support/support_fcntl_support_ofd_locks.c b/support/support_fcntl_support_ofd_locks.c
new file mode 100644
index 00000000..fb197a70
--- /dev/null
+++ b/support/support_fcntl_support_ofd_locks.c
@@ -0,0 +1,44 @@
+/* Return whether the kernel/file supports OFD locks.
+   Copyright (C) 2023 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 <support/support.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <string.h>
+
+bool
+support_fcntl_support_ofd_locks (int fd)
+{
+#ifdef F_OFD_GETLK
+  int res;
+  struct flock flock;
+  memset (&flock, 0, sizeof (flock));
+
+  flock.l_type = F_WRLCK;
+  flock.l_whence = SEEK_SET;
+  flock.l_start = 0;
+  flock.l_len = INT32_MAX;
+  flock.l_pid = 0;
+
+  res = fcntl (fd, F_OFD_GETLK, &flock);
+  return res != -1 || errno != EINVAL;
+#else
+  (void) fd;
+  return false;
+#endif
+}
-- 
2.40.1