Re: [PATCH] parse-options-cb.c: use string_list_append_nodup in OPT_STRING_LIST()

[Jeff King <peff@peff.net>]

On Fri, Jun 10, 2016 at 06:57:26PM +0700, Nguyễn Thái Ngọc Duy wrote:

> If the given string list has strdup_strings set (*), the string will be
> duplicated again. Pointless and leak memory. Ignore that flag.
> 
> (*) only interpret-trailers.c does it at the moment
> 
> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
> ---
>  parse-options-cb.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/parse-options-cb.c b/parse-options-cb.c
> index 239898d..8a1b6e6 100644
> --- a/parse-options-cb.c
> +++ b/parse-options-cb.c
> @@ -144,7 +144,7 @@ int parse_opt_string_list(const struct option *opt, const char *arg, int unset)
>  	if (!arg)
>  		return -1;
>  
> -	string_list_append(v, xstrdup(arg));
> +	string_list_append_nodup(v, xstrdup(arg));

Hmm. So I agree this is an improvement, in the sense that we are
double-allocating when v->strdup_strings is set.  But I think there's a
deeper issue here. Why are we always allocating in the first place?

If the memory we are getting in "arg" is not stable, then we _do_ need
to make a copy of it. But in that case, we want "strdup_strings" to be
set; without it any time we later run string_list_clear(), we leak the
allocated memory, because the struct has no idea that it is the owner of
the memory (and we do call string_list_clear() when we see "--no-foo").

If the memory _is_ stable, then we are fine to add a direct reference to
it, and can lose the extra xstrdup() here. Only the caller knows for
sure, so we should be respecting their value of strdup_strings (so lose
the xstrdup, but keep calling string_list_append()).

In practice, I suspect the memory _is_ stable, because we are generally
parsing command-line arguments. But it does not hurt to stay on the
conservative side, and always make a copy (in case we are parsing
something besides the global argv array) . Apparently I am the original
author of this code, in c8ba163 (parse-options: add OPT_STRING_LIST
helper, 2011-06-09), but there's no mention of this point there, in the
list archives, or in my brain.

So if we are doing the conservative thing, then I think the resulting
code should either look like:

  if (!v->strdup_strings)
	die("BUG: OPT_STRING_LIST should always use strdup_strings");
  string_list_append(v, arg);

or:

  /* silently enable for convenience */
  v->strdup_strings = 1;
  string_list_append(v, arg);

Of the two, I like the top one as it is less magical, but it would
require adjusting the initialization of the string-list for most of the
callers.

-Peff