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

[Taylor Blau <me@ttaylorr.com>]

On Tue, Sep 14, 2021 at 03:02:38PM -0400, Jeff King wrote:
> 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?

Yeah, and I thought that I even mentioned it in [1], but I apparently
sent the contents of my buffer before I saved it. So, I did notice it at
the time, but failed to tell anybody about it!

[1]: https://lore.kernel.org/git/YTzZPti%2FasQwZC%2FD@nand.local/

> >  	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);

(Assuming that you meant `s/&buf/buf.buf/`, but otherwise this makes
sense). There is no need to call strbuf_reset() inside the body of the
loop, since strbuf_getline() calls it via strbuf_getwholeline().

>   strbuf_release(&buf);

...But we should still be careful to release the memory used by the
strbuf at the end (which is safe to do, since the whole point is that we
copy each buffer linewise into the string_list).

> 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.

More than anything, I'm persuaded by that (though I was quite partial to
Eric's suggestion, which I found very clever).

Thanks,
Taylor