[patch 02/20] write_file: make transaction-safe

[Denis Kenzior]

Hi Inaky,

On 07/23/2010 03:59 PM, Inaky Perez-Gonzalez wrote:
> From: Inaky Perez-Gonzalez <inaky.perez-gonzalez at intel.com>
> 
> write_file(), as written wasn't transaction-safe; a crash bewtween a
> file being open and the buffer being written before a safe close would
> leave the file with a set of undetermined contents.
> 
> Modified to the file is written to a temporary file name; once
> completed, it is renamed to the final name. This way, a crash in the
> middle doesn't leave half-baked files.
> ---
>  src/storage.c |   42 +++++++++++++++++++++++++++++++-----------
>  1 files changed, 31 insertions(+), 11 deletions(-)
> 
> diff --git a/src/storage.c b/src/storage.c
> index cac5835..c88a8c8 100644
> --- a/src/storage.c
> +++ b/src/storage.c
> @@ -98,11 +98,21 @@ ssize_t read_file(unsigned char *buffer, size_t len,
>  	return r;
>  }
>  
> +/*
> + * Write a buffer to a file in a transactionally safe form
> + *
> + * Given a buffer, write it to a file named after
> + * @path_fmt+args. However, to make sure the file contents are
> + * consistent (ie: a crash right after opening or during write()
> + * doesn't leave a file half baked), the contents are written to a
> + * file with a temporary name and when closed, it is renamed to the
> + * specified name (@path_fmt+args).
> + */
>  ssize_t write_file(const unsigned char *buffer, size_t len, mode_t mode,
>  			const char *path_fmt, ...)
>  {
>  	va_list ap;
> -	char *path;
> +	char *tmp_path, *path;
>  	ssize_t r;
>  	int fd;
>  
> @@ -110,26 +120,36 @@ ssize_t write_file(const unsigned char *buffer, size_t len, mode_t mode,
>  	path = g_strdup_vprintf(path_fmt, ap);
>  	va_end(ap);
>  
> -	if (create_dirs(path, mode | S_IXUSR) != 0) {
> -		g_free(path);
> -		return -1;
> -	}
> +	tmp_path = g_strdup_printf("%s.XXXXXX.tmp", path);
>  
> -	fd = TFR(open(path, O_WRONLY | O_CREAT | O_TRUNC, mode));
> -	if (fd == -1) {
> -		g_free(path);
> -		return -1;
> -	}
> +	r = -1;
> +	if (create_dirs(path, mode | S_IXUSR) != 0)
> +		goto error_create_dirs;

Please do me a favor and add an empty line here.

> +	fd = TFR(g_mkstemp_full(tmp_path, O_WRONLY | O_CREAT | O_TRUNC, mode));
> +	if (fd == -1)
> +		goto error_mkstemp_full;
>  
>  	r = TFR(write(fd, buffer, len));
>  
>  	TFR(close(fd));
>  
>  	if (r != (ssize_t) len) {
> -		unlink(path);
>  		r = -1;
> +		goto error_write;
>  	}
>  
> +	/* Now that the file contents are written, rename to the real
> +	 * file name; this way we are uniquely sure that the whole
> +	 * thing is there. */

Please follow comment conventions per doc/coding-style.txt Section M2.

> +	unlink(path);

There should be an empty line here per doc/coding-style.txt Section M1.

> +	/* conserve @r's value from 'write' */
> +	if (link(tmp_path, path) == -1)
> +		r = -1;

Another empty line here (before and after if/while/do/for blocks)

> +error_write:
> +	unlink(tmp_path);
> +error_mkstemp_full:
> +error_create_dirs:
> +	g_free(tmp_path);
>  	g_free(path);
>  	return r;
>  }

Regards,
-Denis