Re: [PATCH 1/1] delete multiple tags in a single transaction

[Phil Hord <phil.hord@gmail.com>]

On Thu, Aug 8, 2019 at 12:39 PM Junio C Hamano <gitster@pobox.com> wrote:
>
> Phil Hord <phil.hord@gmail.com> writes:
>
> > From: Phil Hord <phil.hord@gmail.com>
> >
> > 'git tag -d' accepts one or more tag refs to delete, but each deletion
> > is done by calling `delete_ref` on each argv. This is painfully slow
> > when removing from packed refs. Use delete_refs instead so all the
> > removals can be done inside a single transaction with a single write.
> >
> > I have a repo with 24,000 tags, most of which are not useful to any
> > developers. Having this many refs slows down many operations that
> > would otherwise be very fast. Removing these tags when they've been
> > accidentally fetched again takes about 30 minutes using delete_ref.
> >
> >     git tag -l feature/* | xargs git tag -d
> >
> > Removing the same tags using delete_refs takes less than 5 seconds.
>
> Makes sense.  As mentioned elsewhere in the thread already,
> a batched update-ref would open the packed-refs ony once because
> everything is done in a single transaction, so presumably a pipeline
> like this
>
>         git tag -l feature/* |
>         sed -e 's|^|delete refs/tags/|' |
>         git update-ref --stdin
>
> may work well, and "git tag -d" that gets these refs on the command
> line should be capable of doing the same.
>
> > -static int delete_tag(const char *name, const char *ref,
> > -                   const struct object_id *oid, const void *cb_data)
> > +struct tag_args {
> > +     char *oid_abbrev;
> > +     char *refname;
> > +};
> > +
> > +static int make_string_list(const char *name, const char *ref,
> > +                         const struct object_id *oid, void *cb_data)
>
> Please think about a few more minutes before naming a function like
> this, and make it a habit for your future patches.
>
> We can see that the callback is used to insert more strings into a
> string list, but the type (i.e. string_list) used to represent the
> set is not all that important.  What is more important is why you
> are building that set for, and saying what is in the set (as opposed
> to saying that the container happens to be a string_list) would be a
> good first step.
>
> I presume that you are enumerating the tags to be deleted, together
> with the data necessary for you to report the deletion of the tags?

Hm.  collect_tags?  collect_tags_to_delete?

It's true I didn't put enought thought into that.  I was experimenting
a bit here and was surprised how little code I ended up needing.

> >  {
> > -     if (delete_ref(NULL, ref, oid, 0))
> > -             return 1;
> > -     printf(_("Deleted tag '%s' (was %s)\n"), name,
> > -            find_unique_abbrev(oid, DEFAULT_ABBREV));
> > +     struct string_list *ref_list = cb_data;
> > +     struct tag_args *info = xmalloc(sizeof(struct tag_args));
> > +
> > +     string_list_append(ref_list, ref);
> > +
> > +     info->oid_abbrev = xstrdup(find_unique_abbrev(oid, DEFAULT_ABBREV));
> > +     info->refname = xstrdup(name);
> > +     ref_list->items[ref_list->nr - 1].util = info;
> >       return 0;
> >  }
> >
> > +static int delete_tags(const char **argv)
> > +{
> > +     int result;
> > +     struct string_list ref_list = STRING_LIST_INIT_DUP;
> > +     struct string_list_item *ref_list_item;
> > +
> > +     result = for_each_tag_name(argv, make_string_list, (void *) &ref_list);
> > +     if (!result)
> > +             result = delete_refs(NULL, &ref_list, REF_NO_DEREF);
> > +
> > +     for_each_string_list_item(ref_list_item, &ref_list) {
> > +             struct tag_args * info = ref_list_item->util;
> > +             if (!result)
> > +                     printf(_("Deleted tag '%s' (was %s)\n"), info->refname,
> > +                             info->oid_abbrev);
> > +             free(info->oid_abbrev);
> > +             free(info->refname);
> > +             free(info);
>
> It is not performance critical, but info->refname is computable from
> ref_list_item->string, isn't it?

Oh, I guess it is.  It's a fixed offset into the string, after all.
Thanks.  I did look for a way to avoid the struct noise. Just not
well.

> I am just wondering if we can do
> this without having to allocate the .util field for each of 20,000
> tags.  We still need to remember oid (or oid_abbrev, but if I were
> writing this, I'd record the full oid in .util and make the code
> that prints call find_unique_abbrev() on it), so I guess we cannot
> really leave .util NULL.

My original patch did this (.util = oid).  But then I needed a name.
I'll go back to keeping the oid.  Much cleaner.

>
> > +     }
> > +     string_list_clear(&ref_list, 0);
> > +     return result;
>
> We used to return the returned value from for_each_tag_name() that
> repeatedly called delete_tag().
>
> Now we return value from delete_refs().  Are our caller(s) OK with
> the values that may come back from that function?  Can delete_refs()
> return a value that is not appropriate to be returned from
> cmd_tag(), for example a negative value?

Yes it does.  Will fix.

>
> > +}
> > +
> >  static int verify_tag(const char *name, const char *ref,
> > -                   const struct object_id *oid, const void *cb_data)
> > +                   const struct object_id *oid, void *cb_data)
> >  {
> >       int flags;
> >       const struct ref_format *format = cb_data;
> > @@ -511,7 +543,7 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
> >       if (filter.merge_commit)
> >               die(_("--merged and --no-merged options are only allowed in list mode"));
> >       if (cmdmode == 'd')
> > -             return for_each_tag_name(argv, delete_tag, NULL);
> > +             return delete_tags(argv);
>
> Thanks.