/* Emulation of copy_file_range.

   Copyright (C) 2017-2018 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

   <http://www.gnu.org/licenses/>.  */

/* The following macros should be defined before including this

   file:

   COPY_FILE_RANGE_DECL   Declaration specifiers for the function below.

   COPY_FILE_RANGE        Name of the function to define.  */

#include <errno.h>

#include <fcntl.h>

#include <inttypes.h>

#include <limits.h>

#include <sys/stat.h>

#include <sys/types.h>

#include <unistd.h>

COPY_FILE_RANGE_DECL

ssize_t

COPY_FILE_RANGE (int infd, __off64_t *pinoff,

                 int outfd, __off64_t *poutoff,

                 size_t length, unsigned int flags)

{

  if (flags != 0)

    {

      __set_errno (EINVAL);

      return -1;

    }

  {

    struct stat64 instat;

    struct stat64 outstat;

    if (fstat64 (infd, &instat) != 0 || fstat64 (outfd, &outstat) != 0)

      return -1;

    if (S_ISDIR (instat.st_mode) || S_ISDIR (outstat.st_mode))

      {

        __set_errno (EISDIR);

        return -1;

      }

    if (!S_ISREG (instat.st_mode) || !S_ISREG (outstat.st_mode))

      {

        /* We need a regular input file so that the we can seek

           backwards in case of a write failure.  */

        __set_errno (EINVAL);

        return -1;

      }

    if (instat.st_dev != outstat.st_dev)

      {

        /* Cross-device copies are not supported.  */

        __set_errno (EXDEV);

        return -1;

      }

  }

  /* The output descriptor must not have O_APPEND set.  */

  {

    int flags = __fcntl (outfd, F_GETFL);

    if (flags & O_APPEND)

      {

        __set_errno (EBADF);

        return -1;

      }

  }

  /* Avoid an overflow in the result.  */

  if (length > SSIZE_MAX)

    length = SSIZE_MAX;

  /* Main copying loop.  The buffer size is arbitrary and is a

     trade-off between stack size consumption, cache usage, and

     amortization of system call overhead.  */

  size_t copied = 0;

  char buf[8192];

  while (length > 0)

    {

      size_t to_read = length;

      if (to_read > sizeof (buf))

        to_read = sizeof (buf);

      /* Fill the buffer.  */

      ssize_t read_count;

      if (pinoff == NULL)

        read_count = read (infd, buf, to_read);

      else

        read_count = __libc_pread64 (infd, buf, to_read, *pinoff);

      if (read_count == 0)

        /* End of file reached prematurely.  */

        return copied;

      if (read_count < 0)

        {

          if (copied > 0)

            /* Report the number of bytes copied so far.  */

            return copied;

          return -1;

        }

      if (pinoff != NULL)

        *pinoff += read_count;

      /* Write the buffer part which was read to the destination.  */

      char *end = buf + read_count;

      for (char *p = buf; p < end; )

        {

          ssize_t write_count;

          if (poutoff == NULL)

            write_count = write (outfd, p, end - p);

          else

            write_count = __libc_pwrite64 (outfd, p, end - p, *poutoff);

          if (write_count < 0)

            {

              /* Adjust the input read position to match what we have

                 written, so that the caller can pick up after the

                 error.  */

              size_t written = p - buf;

              /* NB: This needs to be signed so that we can form the

                 negative value below.  */

              ssize_t overread = read_count - written;

              if (pinoff == NULL)

                {

                  if (overread > 0)

                    {

                      /* We are on an error recovery path, so we

                         cannot deal with failure here.  */

                      int save_errno = errno;

                      (void) __libc_lseek64 (infd, -overread, SEEK_CUR);

                      __set_errno (save_errno);

                    }

                }

              else /* pinoff != NULL */

                *pinoff -= overread;

              if (copied + written > 0)

                /* Report the number of bytes copied so far.  */

                return copied + written;

              return -1;

            }

          p += write_count;

          if (poutoff != NULL)

            *poutoff += write_count;

        } /* Write loop.  */

      copied += read_count;

      length -= read_count;

    }

  return copied;

}