On Thu, Jun 20, 2013 at 12:36:21PM -0700, Junio C Hamano wrote:

> > I like the latter option. It takes a non-trivial amount of time to load
> > the commits from disk, and now we are potentially doing it 2 or 3 times
> > for a run (once to parse, once to get the author info for topo-sort, and
> > possibly later to show it if --pretty is given; though I did not check
> > and maybe we turn off save_commit_buffer with --pretty). It would be
> > nice to have an extended parse_object that handled that. I'm not sure of
> > the interface. Maybe variadic with pairs of type/slab, like:
> >
> >   parse_commit_extended(commit,
> >                         PARSE_COMMIT_AUTHORDATE, &authordate_slab,
> >                         PARSE_COMMIT_DONE);
> >
> > ?
> What I had in mind actually was a custom slab tailored for each
> caller that is an array of struct.  If the caller is interested in
> authordate and authorname, instead of populating two separate
> authordate_slab and authorname_slab, the caller declares a
>       struct {
>               unsigned long date;
>                 char name[FLEX_ARRAY];
>       } author_info;
> prepares author_info_slab, and use your commit_parser API to fill
> them.

Yes, I think it is nicer to stay in one slab if you have multiple
values, but it means more custom code for the caller. If the
commit_parser API is nice, it should not be that much code, though.

It does make it harder to support arbitrary combinations directly in
parse_commit. If a caller wants to also parse_commit and use the same
buffer to pick out its custom information, I think we'd need to do one

  1. Give parse_commit a callback, so that the callback can pick out the
     data it wants while parse_commit has the commit buffer in memory.

       void grab_author_info(const char *buf, unsigned long len, void *data)
              struct author_info *ai = data;
              /* fill fields from buffer */

       parse_commit_extra(commit, grab_author_info,
                          slab_at(&author_slab, commit));

  2. Teach parse_commit to operate not only on a raw commit object, but
     also on the commit_parser API. Like:

       struct commit_parser parser = {0};

       /* actually open the object and start our incremental parser */
       init_commit_parser(&parser, commit);

       /* fill in parents, date, etc, as parse_commit does now */
       parse_commit_from_parser(commit, &parser);

       /* fill in whatever extra data we are interested in */
       *slab_at(&slab, commit) = get_author_date(&parser);

       /* done, drop the buffer */

The latter would need to handle transferring ownership of the buffer to
"struct commit" from "struct commit_parser" when save_commit_buffer is
turned off.

I think we're a bit high-level now to be making such decisions, though,
as we do not even have such a commit_parser API.

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