Re: [PATCH 2/8] builtin/multi-pack-index.c: support --stdin-packs mode

From: Jeff King <peff@peff.net>
To: Taylor Blau <me@ttaylorr.com>
Cc: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>, git@vger.kernel.org
Subject: Re: [PATCH 2/8] builtin/multi-pack-index.c: support --stdin-packs mode
Date: Tue, 14 Sep 2021 15:02:38 -0400	[thread overview]
Message-ID: <YUDxzhAnzI5Anm6F@coredump.intra.peff.net> (raw)
In-Reply-To: <YTzYYfBGqG87O5mN@nand.local>

On Sat, Sep 11, 2021 at 12:25:05PM -0400, Taylor Blau wrote:

> > Before calling string_list_clear(). I.e. we didn't strdup(), but during
> > free() we pretend that we did, because we did, just not in
> > string_list_append().
> 
> Good catch. It's kind of gross, but the result is:
> 
> --- 8< ---
> 
> diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
> index 77488b6b7b..e6cab975e3 100644
> --- a/builtin/multi-pack-index.c
> +++ b/builtin/multi-pack-index.c
> @@ -64,7 +64,7 @@ static struct option *add_common_options(struct option *prev)
> 
>  static void read_packs_from_stdin(struct string_list *to)
>  {
> -	struct strbuf buf = STRBUF_INIT;
> +	struct strbuf buf = STRBUF_INIT_NODUP;

Did you mean to use STRING_LIST_INIT_NODUP on the string-list
declaration?

>  	while (strbuf_getline(&buf, stdin) != EOF) {
>  		string_list_append(to, strbuf_detach(&buf, NULL));
>  	}
> @@ -107,6 +107,11 @@ static int cmd_multi_pack_index_write(int argc, const char **argv)
>  		ret = write_midx_file_only(opts.object_dir, &packs,
>  					   opts.preferred_pack, opts.flags);
> 
> +		/*
> +		 * pretend strings are duplicated to free the memory allocated
> +		 * by read_packs_from_stdin()
> +		 */
> +		packs.strdup_strings = 1;
>  		string_list_clear(&packs, 0);
> 
>  		return ret;

I think the root of the problem here is the non-idiomatic use of
strbuf_getline(). The usual thing (and in fact the thing done by the
quite-similar code in read_packs_list_from_stdin() in pack-objects.c)
is not to detach, because strbuf_getline() will reset the buffer each
time. I.e.:

  struct string_list to = STRING_LIST_INIT_DUP;
  ...
  struct strbuf buf = STRBUF
  while (strbuf_getline(&buf, stdin) != EOF)
	string_list_append(to, &buf);
  strbuf_release(&buf);

That avoids any clever string-list allocation games. The number of heap
allocations is about the same (one strbuf and N list items, versus N
strbufs and 0 list items). There's a little extra copying (from the
strbuf into the list items), but the strings themselves are more
efficiently allocated (strbuf may over-allocate, and we lock in
that choice forever by handing over the string).

Not that efficiency probably matters either way for this spot. I'd do it
this way because it's simple and idiomatic for our code base.

-Peff