Re: [PATCH v3] doc: describe Git bundle format

[Masaya Suzuki <masayasuzuki@google.com>]

On Fri, Feb 7, 2020 at 5:49 PM Junio C Hamano <gitster@pobox.com> wrote:
>
> Masaya Suzuki <masayasuzuki@google.com> writes:
>
> > Yes. The reason that I've been trying to check the semantics of the
> > prerequisites is that I DO recognize that this is possible
> > format-wise. I'm not sure if this Git implementation can create such
> > bundles, but format-wise such bundles can be created.
>
> Yeah, now I get it.
>
> The problem is *not* that v2 format "cannot represent a shallow
> clone repository", but is that there is nothing that prevents a
> bundle in v2 format from depending on objects behind (not just at)
> the shallow boundary, making it impossible for a reader to guarantee
> that a bundle with prereqs can be used to create an equivalent
> shallow repository with shallow boundary at the same place as
> prereqs.  IOW, bundle with prereqs in the v2 format allows more
> objects to be omitted than an equivalent shallow repository omits,
> because prereqs and shallow cutoff points mean different things.

Yes. So, I think it's better to say prereqs and shallow boundaries are
different.

> While we are at it, I suspect that with reachability bitmap, a "git
> fetch" that updates a history up to commit A to a new history up to
> commit B can omit more objects than what is directly reachable from
> the commit A.  That is, if A's direct child (call it C) is a commit
> that reverts A, a blob in A's tree won't be in the bundle (because A
> is a prereq), but the blob at the same path in C is the same blob as
> the blob at the same path in A's parent (that is what it means for
> that A's direct child to be a revert of A).  In the normal
> enumeration based on object-walk to decide which objects to send,
> such a blob in C will be included in the pack,

That's interesting. I have never looked CGit's implementation, but I
think JGit would omit those objects. (At least that was my
understanding. Not confirmed with the code.)

Anyway. Is it OK with adding this small note on "prereq is not a
shallow boundary"? In practice, there are not many Git implementations
that handle Git bundles, so it's not that big deal as long those few
implementers recognize this, but this document is meant for those
implementers.