Re: [PATCH] git clone depth of 0 not possible.

From: Junio C Hamano <gitster@pobox.com>
Date: Jan 8, 2013, 07:36 UTC
Message-ID: <7vd2xggm8a.fsf@alter.siamese.dyndns.org>
In-Reply-To: <7vip78go6b.fsf@alter.siamese.dyndns.org>

Junio C Hamano <gitster@pobox.com> writes:

Show 25 quoted lines

> Jonathan Nieder <jrnieder@gmail.com> writes:
>
>> Stefan Beller wrote:
>>
>>> Currently it is not possible to have a shallow depth of
>>> just 0, i.e. only one commit in that repository after cloning.
>>> The minimum number of commits is 2, caused by depth=1.
>>
>> Sounds buggy.  Would anything break if we were to make --depth=1 mean
>> "1 deep, including the tip commit"?
>
> As long as we do not change the meaning of the "shallow" count going
> over the wire (i.e. the number we receive from the user will be
> fudged, so that user's "depth 1" that used to mean "the tip and one
> behind it" is expressed as "depth 2" at the end-user level, and we
> send over the wire the number that corresponded to the old "depth
> 1"), I do not think anything will break, and then --depth=0 may
> magically start meaning "only the tip; its immediate parents will
> not be transferred and recorded as the shallow boundary in the
> receiving repository".
>
> I do not mind carrying such a (technially) backward incompatible
> change in jn/clone-2.0-depth-off-by-one branch, keep it cooking in
> 'next' for a while and push it out together with other "2.0" topics
> in a future release ;-).

Speaking of --depth, I think in Git 2.0 we should fix the semantics of "deepening" done with "git fetch".

Its "--depth" parameter is used to specify the new depth of the history that you can tangle from the updated tip of remote tracking branches, and it has a rather unpleasant ramifications.

Suppose you start from "git clone --depth=1 $there". You have the today's snapshot, and one parent behind it. You keep working happily with the code and then realize that you want to know a bit more history behind the snapshot you started from.

 (upstream)
  ---o---o---o---A---B

 (you)
                 A---B

So you do:

    $ git fetch --depth=3

 (upstream)
  ---o---o---o---A---B---C---D---E---F---...---W---X---Y---Z

 (you)
                 A---B                         W---X---Y---Z

But in the meantime, if the upstream accumulated 20+ commits, you end up getting the commit at the updated tip of the upstream, and 3 generations of parents behind it. There will be a 10+ commit worth of gap between the bottom of the new shallow history and the old tip you have been working on, and the history becomes disjoint.

I think we need a protocol update to fix this; instead of sending "Now I want your tips and N commits behind it, please update my shallow bottom accordingly", which creates the above by giving you Z and 3 generations back and updates your cut-off point to W, the receiving end should be able to ask "I have a shallow history that cuts off at these commits. I want to get the history leading up to your tips, and also deepen the history further back from my current cut-off points by N commits", so that you would instead end up with something like this:

 (you)
     o---o---o---A---B---C---D---E---F---...---W---X---Y---Z

That is, truly "deepen my history by 3". We could call that "git fetch --deepen=3" or something.