Re: [boost] Git: maintaining super-project

Subject: Re: [boost] Git: maintaining super-project
From: Alexander Lamaison (awl03_at_[hidden])
Date: 2013-12-05 08:54:13

Andrey Semashev <andrey.semashev_at_[hidden]> writes:

> On Wed, Dec 4, 2013 at 12:06 PM, Vladimir Prus <ghost_at_[hidden]> wrote:
>> On 04.12.2013 11:32, Andrey Semashev wrote:
>>
>>> I just checked out the super-project:
>>>
>>>    git clone --recursive http://github.com/boostorg/boost.git
>>> modular-boost
>>>
>>> It actually said:
>>>
>>>    Submodule path 'libs/program_options': checked out
>>> 'fb4f36f3eecbef1e3b4ace5b263e01d56e42d5c7'
>>>
>>> But after these commands I saw the newer revisions:
>>>
>>>    cd modular-boost/libs/program_options
>>>    git checkout master
>>>    Switched to branch 'master'
>>>
>>>    git log -1
>>>    commit 9d7c9875265ec7fec03aabf18e3d53c3e5e1a46c
>>>    Author: Vladimir Prus <vladimir_at_[hidden]>
>>>    Date:   Wed Dec 4 09:17:17 2013 +0400
>>>
>>>      Remove tabs.
>>>
>>> I'm not sure what the "Submodule" line means, but it seems the
>>> repository is cloned with the newer revisions.
>
> man git-submodule says that the submodule involves something called
> "gitlink", which refers to the particular revision. I don't know what
> this is but is it possible to specify HEAD instead of the specific
> revisions in gitlinks?

It's not possible.  Git submodules were purposely designed to only let
you link to a specific revision.  This is A Good Thing and different
from how svn:externals work.

The reason for fixed revision links is that a particular commit in the
super project is then a snapshot of the entire source tree, submodules
and everything.  If you could link to a submodule HEAD it would never be
possible to get a stable snapshot of superproject commit
blah023blah034blah093blah because the source of the submodule may have
changed since blah023blah034blah093blah was committed.  In short, a
dependency nightmare.

Git submodules make it very explicit what was intended to link to what.
If you want to change the linkage you have to do it _explicitly_ by
committing an updated submodule link.  That way noone is left confused
about what the commit author really intended.

Yes, this confused me horribly when I started using git too.  Now I
can't imagine why I ever wanted it otherwise.

Alex

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