Re: [PATCH v2 0/4] [RFC] repack: add --filter=

[Taylor Blau <me@ttaylorr.com>]

On Mon, Feb 21, 2022 at 10:10:15PM +0100, Christian Couder wrote:
> > > Also, to have more protection we can either
> > >
> > > 1. add a config value that needs to be set to true for repack to remove
> > > objects (repack.allowDestroyFilter).
>
> I don't think it's of much value. We don't have such config values for
> other possibly destructive operations.
>
> > > 2. --filter is dry-run by default and prints out objects that would have been removed,
> > > and it has to be combined with another flag --destroy in order for it to actually remove
> > > objects from the odb.
>
> I am not sure it's of much value either compared to naming it
> --filter-destroy. It's likely to just make things more difficult for
> users to understand.

On this and the above, I agree with Christian.

> > I share the same concern as Robert and Stolee do. But I think this issue
> > goes deeper than just naming.
> >
> > Even if we called this `git repack --delete-filter` and only ran it with
> > `--i-know-what-im-doing` flag, we would still be leaving repository
> > corruption on the table, just making it marginally more difficult to
> > achieve.
>
> My opinion on this is that the promisor object mechanism assumes by
> design that some objects are outside a repo, and that this repo
> shouldn't care much about these objects possibly being corrupted.

For what it's worth, I am fine having a mode of repack which allows us
to remove objects that we know are stored by a promisor remote. But this
series doesn't do that, so users could easily run `git repack -d
--filter=...` and find that they have irrecoverably corrupted their
repository.

I think that there are some other reasonable directions, though. One
which Robert and I discussed was making it possible to split a
repository into two packs, one which holds objects that match some
`--filter` criteria, and one which holds the objects that don't match
that filter.

Another option would be to prune the repository according to objects
that are already made available by a promisor remote.

An appealing quality about the above two directions is that the first
doesn't actually remove any objects, just makes it easier to push a
whole pack of unwanted objects off to a promsior remote. The second
prunes the repository according to objects that are already made
available by the promisor remote. (Yes, there is a TOCTOU race there,
too, but it's the same prune-while-pushing race that Git already has
today).

> I am not against a name and some docs that strongly state that users
> should be very careful when using such a command, but otherwise I
> think such a command is perfectly ok. We have other commands that by
> design could lead to some objects or data being lost.

I can think of a handful of ways to remove objects which are unreachable
from a repository, but I am not sure we have any ways to remove objects
which are reachable.

> > But as it stands right now, I worry that this feature is too easily
> > misused and could result in unintended repository corruption.
>
> Are you worrying about the UI or about what it does?
>
> I am ok with improving the UI, but I think what it does is reasonable.

I am more worried about the proposal's functionality than its UI,
hopefully my concerns there are summarized above.

Thanks,
Taylor