Re: [PATCH 1/2] ref-filter: hacky "streaming" mode

From: ZheNing Hu <adlternative@gmail.com>
Date: Sep 7, 2021, 05:28 UTC
Message-ID: <CAOLTT8QbdNBSY95bCa+UNJBqsJEEHbnaKfZLzvN2Qzd-Np8Lqg@mail.gmail.com>
In-Reply-To: <YTTARcEvpXWSDfYW@coredump.intra.peff.net>

Jeff King <peff@peff.net> 于2021年9月5日周日 下午9:04写道：

Show 36 quoted lines

>
> On Sun, Sep 05, 2021 at 04:20:02PM +0800, ZheNing Hu wrote:
>
> > But here is a terrible fact: we did not use ref_array_sort() for sorting here.
> > So in fact, for_each_fullref_in() does the sorting work () for us by
> > default sort (%(refname)),
> > This may be due to the file system or some implementation of ref_iterator.
> > But this limit the application of this optimization when we use other
> > atoms to sort.
>
> Right. This technique does not help at all if you use --sort. I do think
> it's reasonable to rely on the sorted output of the for_each_ref()
> functions; other parts of Git likely do as well, so I think we'd try
> pretty hard to maintain that (and no, it's not a filesystem thing; we do
> make sure to sort it ourselves; see the calls to sort_ref_dir()).
>
> > > +       /*
> > > +        * the %(symref) placeholder is broken with pretty_print_ref(),
> > > +        * which our streaming code uses. I suspect this is a sign of breakage
> > > +        * in other callers like verify_tag(), which should be fixed. But for
> > > +        * now just disable streaming.
> > > +        *
> > > +        * Note that this implies we've parsed the format already with
> > > +        * verify_ref_format().
> > > +        */
> > > +       if (need_symref)
> > > +               return;
> > > +
> >
> > I haven’t taken a closer look at why pretty_print_ref() does not
> > support %(symref),
> > we can skip it first.
>
> I think it's just because pretty_print_ref() does not take a "flag"
> parameter for the caller. So it never sees that REF_ISSYMREF is set.
>

yeah, pretty_print_ref() does not set the flag, this is a defect of pretty_print_ref(), maybe we need to find a way to set this flag. I also deliberately avoided %(symref) when refactoring git cat-file --batch, perhaps the improvements here can also be applied to git cat-file --batch

Show 21 quoted lines

> There aren't many callers of that function, so I think nobody ever
> really noticed. But you can see the bug like this:
>
>   git init repo
>   cd repo
>
>   git commit --allow-empty -m foo
>   git tag -s -m bar annotated &&
>   git symbolic-ref refs/tags/symref refs/tags/annotated
>
>   # this is ok; ref-filter handles the flags
>   git tag --list --format='list: %(refname) %(symref)'
>
>   # not ok; we do not tell the formatter about the flags, so it does
>   # not notice that "symref" is a symref
>   git tag --verify --format='verify: %(refname) %(symref)' annotated symref
>
> I notice that the --verify output also shows the short refname, which
> makes me wonder if %(symref) would have other bugs there (because it
> has to re-resolve the ref to come up with the symref destination).
>

This may be easy to fix:

diff --git a/builtin/tag.c b/builtin/tag.c
index 452558ec95..4be5d36366 100644
--- a/builtin/tag.c
+++ b/builtin/tag.c
@@ -152,11 +152,11 @@ static int verify_tag(const char *name, const char *ref,
        if (format->format)
                flags = GPG_VERIFY_OMIT_STATUS;

-       if (gpg_verify_tag(oid, name, flags))
+       if (gpg_verify_tag(oid, ref, flags))
                return -1;

        if (format->format)
-               pretty_print_ref(name, oid, format);
+               pretty_print_ref(ref, oid, format);

        return 0;
 }

> > Unfortunately, this optimization may not be helpful for git cat-file --batch,
> > see [1], batch_object_write() directly constructs a ref_array_item and call
> > format_ref_array_item() to grab data, It does not use ref_array. So it also
> > does not have this malloc() | free() overhead.
>
> Right. It would only be helped by the "quick" formats in the second
> patch (and by skipping the malloc/free of the individual
> ref_array_items).
>

Yes, your test is indeed worth thinking about: how to avoid
intermediate data to reduce overhead.

> -Peff

Thanks.
--
ZheNing Hu