Re: Allowing weak references to blobs and strong references to commits

From: Mike Hommey <mh@glandium.org>
Date: Mar 31, 2015, 22:39 UTC
Message-ID: <20150331223919.GA24362@glandium.org>
In-Reply-To: <xmqq8uecrk5g.fsf@gitster.dls.corp.google.com>

On Tue, Mar 31, 2015 at 01:23:23PM -0700, Junio C Hamano wrote:

Show 17 quoted lines

> Mike Hommey <mh@glandium.org> writes:
> 
> > So I thought, since commits are already allowed in tree objects, for
> > submodules, why not add a bit to the mode that would tell git that
> > those commit object references are meant to always be there aka strong
> > reference, as opposed to the current weak references for submodules.
> 
> Unless you are recording the paths to these "commits" to be
> potentially checked out on the filesystem, do not put them in a
> "tree".  The entries in a tree object represent "This thing go to
> this path in the working tree".
> 
> It is not clear to me (and because you said "I won't bother you with
> all the whys and hows", I am guessing that it is OK for readers to
> be unclear), but I think you only want to make sure "git fetch" and
> "git push" transfers these objects, the graph formed by which is
> *not* any part of the main history of the project.

Transfer is not the main reason I have those refs, although it's a nice plus. I'm using the git database as a storage for metadata I need to keep track of the remote mercurial content that isn't part of the content history represented in the corresponding git history. Obviously, this isn't meant to be checked out, like a notes tree. The refs are only there so that a) git-cinnabar can find its data and b) git gc doesn't remove it.

> It is perfectly OK to represent these objects as a special purpose
> history and have a ref point at its tip.  The "notes" database is
> represented that way, for example.

Indeed, the notes database is in a similar situation. But the fact that in my use-case (cloning Mozilla mercurial repositories) _thousands_ of refs are involved is not really making it user-friendly. I'd rather hide those away from users.

> And I do not see anything wrong to use octopus merges in the history
> if you want to represent "here are the commit objects that I care
> about at this point in the 'special purpose' history (not the main
> history)".

Octopus merges are limited to 16 parents. That means to merge thousands of refs, I need to do that on at least 3 levels, involving hundreds of commit objects, many of which would become loose quickly, or become extra noise in the "metadata" "branches"... and while researching this further, I realize it doesn't seem there is such a limit to the number of parents for octopus merges. Where did I get that from? Was there such a limit in the past or was I high?

That being said, having names associated with those tips _is_ useful to me (it allows to know the corresponding mercurial sha1 and branch without having to do a note lookup for each), and a tree of commits would help, here, although I could put a tree of weak refs to commits as the content of an octopus merge...

So I guess I could live without the strong commit refs. Weak blob refs would still be useful, though.

Mike