/* Detecting file changes using modification times.

   Copyright (C) 2017-2020 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 <stdbool.h>

#include <stddef.h>

#include <stdio.h>

#include <sys/stat.h>

#include <sys/types.h>

/* Items for identifying a particular file version.  Excerpt from

   struct stat64.  */

struct file_change_detection

{

  /* Special values: 0 if file does not exist.  -1 to force mismatch

     with the next comparison.  */

  off64_t size;

  ino64_t ino;

  struct timespec mtime;

  struct timespec ctime;

};

/* Returns true if *LEFT and *RIGHT describe the same version of the

   same file.  */

static bool __attribute__ ((unused))

file_is_unchanged (const struct file_change_detection *left,

                   const struct file_change_detection *right)

{

  if (left->size < 0 || right->size < 0)

    /* Negative sizes are used as markers and never match.  */

    return false;

  else if (left->size == 0 && right->size == 0)

    /* Both files are empty or do not exist, so they have the same

       content, no matter what the other fields indicate.  */

    return true;

  else

    return left->size == right->size

      && left->ino == right->ino

      && left->mtime.tv_sec == right->mtime.tv_sec

      && left->mtime.tv_nsec == right->mtime.tv_nsec

      && left->ctime.tv_sec == right->ctime.tv_sec

      && left->ctime.tv_nsec == right->ctime.tv_nsec;

}

/* Extract file change information to *FILE from the stat buffer

   *ST.  */

static void __attribute__ ((unused))

file_change_detection_for_stat (struct file_change_detection *file,

                                const struct stat64 *st)

{

  if (S_ISDIR (st->st_mode))

    /* Treat as empty file.  */

    file->size = 0;

  else if (!S_ISREG (st->st_mode))

    /* Non-regular files cannot be cached.  */

    file->size = -1;

  else

    {

      file->size = st->st_size;

      file->ino = st->st_ino;

      file->mtime = st->st_mtim;

      file->ctime = st->st_ctim;

    }

}

/* Writes file change information for PATH to *FILE.  Returns true on

   success.  For benign errors, *FILE is cleared, and true is

   returned.  For errors indicating resource outages and the like,

   false is returned.  */

static bool __attribute__ ((unused))

file_change_detection_for_path (struct file_change_detection *file,

                                const char *path)

{

  struct stat64 st;

  if (stat64 (path, &st) != 0)

    switch (errno)

      {

      case EACCES:

      case EISDIR:

      case ELOOP:

      case ENOENT:

      case ENOTDIR:

      case EPERM:

        /* Ignore errors due to file system contents.  Instead, treat

           the file as empty.  */

        file->size = 0;

        return true;

      default:

        /* Other errors are fatal.  */

        return false;

      }

  else /* stat64 was successfull.  */

    {

      file_change_detection_for_stat (file, &st);

      return true;

    }

}

/* Writes file change information for the stream FP to *FILE.  Returns

   ture on success, false on failure.  If FP is NULL, treat the file

   as non-existing.  */

static bool __attribute__ ((unused))

file_change_detection_for_fp (struct file_change_detection *file,

                              FILE *fp)

{

  if (fp == NULL)

    {

      /* The file does not exist.  */

      file->size = 0;

      return true;

    }

  else

    {

      struct stat64 st;

      if (fstat64 (__fileno (fp), &st) != 0)

        /* If we already have a file descriptor, all errors are fatal.  */

        return false;

      else

        {

          file_change_detection_for_stat (file, &st);

          return true;

        }

    }

}