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Add internal <file_change_detection.h> header file

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The code started out with bits form resolv/resolv_conf.c, but it
was enhanced to deal with directories and FIFOs in a more predictable
manner.  A test case is included as well.

This will be used to implement the /etc/resolv.conf change detection.

This currently lives in a header file only.  Once there are multiple
users, the implementations should be moved into C files.
This commit is contained in:
Florian Weimer
2020-02-11 13:52:06 +01:00
parent ec44e1efcd
commit 6c80c6e876
3 changed files with 347 additions and 1 deletions
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140
include/file_change_detection.h Normal file
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/* 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;
}
}
}