/* Emulate flock on platforms that lack it, primarily Windows and MinGW.

   This is derived from sqlite3 sources.
   https://www.sqlite.org/src/finfo?name=src/os_win.c
   https://www.sqlite.org/copyright.html

   Written by Richard W.M. Jones <rjones.at.redhat.com>

   Copyright (C) 2008-2025 Free Software Foundation, Inc.

   This 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.

   This 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 this program.  If not, see <https://www.gnu.org/licenses/>.  */

#include "libbb.h"
#include <sys/file.h>

/* LockFileEx */

# include <errno.h>

/* _get_osfhandle */
#  include <io.h>

/* Determine the current size of a file.  Because the other braindead
 * APIs we'll call need lower/upper 32 bit pairs, keep the file size
 * like that too.
 */
static BOOL
file_size (HANDLE h, DWORD * lower, DWORD * upper)
{
  *lower = GetFileSize (h, upper);
  return 1;
}

/* LOCKFILE_FAIL_IMMEDIATELY is undefined on some Windows systems. */
# ifndef LOCKFILE_FAIL_IMMEDIATELY
#  define LOCKFILE_FAIL_IMMEDIATELY 1
# endif

/* Acquire a lock. */
static BOOL
do_lock (HANDLE h, int non_blocking, int exclusive)
{
  BOOL res;
  DWORD size_lower, size_upper;
  OVERLAPPED ovlp;
  int flags = 0;

  /* We're going to lock the whole file, so get the file size. */
  res = file_size (h, &size_lower, &size_upper);
  if (!res)
    return 0;

  /* Start offset is 0, and also zero the remaining members of this struct. */
  memset (&ovlp, 0, sizeof ovlp);

  if (non_blocking)
    flags |= LOCKFILE_FAIL_IMMEDIATELY;
  if (exclusive)
    flags |= LOCKFILE_EXCLUSIVE_LOCK;

  return LockFileEx (h, flags, 0, size_lower, size_upper, &ovlp);
}

/* Unlock reader or exclusive lock. */
static BOOL
do_unlock (HANDLE h)
{
  int res;
  DWORD size_lower, size_upper;

  res = file_size (h, &size_lower, &size_upper);
  if (!res)
    return 0;

  return UnlockFile (h, 0, 0, size_lower, size_upper);
}

/* Now our BSD-like flock operation. */
int
flock (int fd, int operation)
{
  HANDLE h = (HANDLE) _get_osfhandle (fd);
  DWORD res;
  int non_blocking;

  if (h == INVALID_HANDLE_VALUE)
    {
      errno = EBADF;
      return -1;
    }

  non_blocking = operation & LOCK_NB;
  operation &= ~LOCK_NB;

  switch (operation)
    {
    case LOCK_SH:
      res = do_lock (h, non_blocking, 0);
      break;
    case LOCK_EX:
      res = do_lock (h, non_blocking, 1);
      break;
    case LOCK_UN:
      res = do_unlock (h);
      break;
    default:
      errno = EINVAL;
      return -1;
    }

  /* Map Windows errors into Unix errnos.  As usual MSDN fails to
   * document the permissible error codes.
   */
  if (!res)
    {
      errno = err_win_to_posix();
      return -1;
    }

  return 0;
}