Re: [PATCH v2] diff --no-index: unleak paths[] elements

> "git diff --no-index" codepath starts with the two elements in
> argv[] and munges them into two paths to be compared, stored in a
> separate path[] arrays.  The munging is implemented in a rather
> haphazard way, sometimes overwriting old version with a new copy,
> and sometimes a constant string assigned to path[], making it
> impossible to release the resources properly:
>
>  * A single dash "-" from the command line is a special signal that
>    the standard input is used for the side to be compared, and is
>    internally replaced with a copy of string "-" at a known address.
>
>  * When run in a subdirectory, full paths to the two paths are
>    allocated and placed in path[].
>
>  * After the above happens, when comparing a file with a directory,
>    the directory side is replaced with the path to a file in the
>    directory with the same name as the file.
>
> This was perfectly fine for just two strings that are pathnames used
> during the lifetime of the program and cleaned up upon program exit,
> but it gets in the way when leak sanitizer is in effect.  The third
> step can be losing the full path that was allocated in the second
> step, but it is not easy to tell if its input is an allocated piece
> of memory to begin with.
>
> Loosen the earlier two steps a bit so that elements of the path[]
> array that come to the directory/file comparison code are either the
> singleton "-" or an allocated piece of memory.  Use that knowledge
> in the third step to release an allocated piece of memory when it
> replaces the path to a directory with the path to a file in that
> directory, and also at the end to release the two elements of the
> path[] array as needed.
>
> Signed-off-by: Junio C Hamano <gitster@pobox.com>
> ---
>
>  * The previous one allowed strbuf_release() to free replacement.buf
>    which may be used in path[0] or path[1] potentially leading to
>    double freeing.  The kosher way may be to use strbuf_detach() in
>    fixup_paths(), but this is a simpler fix, it is getting late in
>    the day, and I am getting sick of fighting the leak-checker, so...
>
>  diff-no-index.c | 24 ++++++++++++++++++++++--
>  1 file changed, 22 insertions(+), 2 deletions(-)
>
>
> diff --git a/diff-no-index.c b/diff-no-index.c
> index 9a8b09346b..77a126469b 100644
> --- a/diff-no-index.c
> +++ b/diff-no-index.c
> @@ -208,6 +208,14 @@ static void append_basename(struct strbuf *path, const char *dir, const char *fi
>  	strbuf_addstr(path, tail ? tail + 1 : file);
>  }
>
> +static void free_allocated_path(const char *path)
> +{
> +	if (!path ||

> +	    (path == file_from_standard_input))
> +		return;
> +	free((char *)path);
> +}
> +
>  /*
>   * DWIM "diff D F" into "diff D/F F" and "diff F D" into "diff F D/F"
>   * Note that we append the basename of F to D/, so "diff a/b/file D"
> @@ -226,9 +234,11 @@ static void fixup_paths(const char **path, struct strbuf *replacement)
>  		return;
>  	if (isdir0) {
>  		append_basename(replacement, path[0], path[1]);
> +		free_allocated_path(path[0]);
>  		path[0] = replacement->buf;
>  	} else {
>  		append_basename(replacement, path[1], path[0]);
> +		free_allocated_path(path[1]);
>  		path[1] = replacement->buf;
>  	}
>  }
> @@ -274,6 +284,8 @@ int diff_no_index(struct rev_info *revs,
>  			p = file_from_standard_input;
>  		else if (prefix)
>  			p = prefix_filename(prefix, p);
> +		else
> +			p = xstrdup(p);

>  		paths[i] = p;
>  	}
>
> @@ -294,13 +306,21 @@ int diff_no_index(struct rev_info *revs,
>  	setup_diff_pager(&revs->diffopt);
>  	revs->diffopt.flags.exit_with_status = 1;
>
> -	if (queue_diff(&revs->diffopt, paths[0], paths[1]))
> +	if (queue_diff(&revs->diffopt, paths[0], paths[1])) {
> +		free_allocated_path(paths[0]);
> +		free_allocated_path(paths[1]);
>  		return 1;
> +	}
>  	diff_set_mnemonic_prefix(&revs->diffopt, "1/", "2/");
>  	diffcore_std(&revs->diffopt);
>  	diff_flush(&revs->diffopt);
>
> -	strbuf_release(&replacement);
> +	/*
> +	 * do not strbuf_release(&replacement), as it is in paths[]
> +	 * when replacement was actually used.
> +	 */
> +	free_allocated_path(paths[0]);
> +	free_allocated_path(paths[1]);
>
>  	/*
>  	 * The return code for --no-index imitates diff(1):