Re: Notes on Subproject Support

[Junio C Hamano <junkio@cox.net>]

Junio C Hamano <junkio@cox.net> writes:

> In other words, I think the design so far does not require us to
> touch tree objects at all, and I'd be happy if we do not have to.
>
> One reason I started the bound commit approach was exactly
> because I only needed to muck with commit objects and did not
> have to touch trees and blobs; after trying to implement the
> core level for "gitlink", which I ended up touching quite a lot
> and have abandoned for now.

BTW, let's digress a bit.

I think recording "commit" in the tree objects is in line with
the logical organization described in README: "blob" and "tree"
represent a state, and have *nothing* to do with how we came
about to that state.  The historyh is described in "commit"
objects.  The bound commit approach keeps that property.

The "gitlink" approach, as I understand how Linus outlined in
his original suggestion, is a bit different.  The link objects
appear in tree objects, and when you "git cat-file link" one of
them, you would see something like this:

        commit	5b2bcc7b2d546c636f79490655b3347acc91d17f
        name	kernel

So in that sense, "gitlink" approach departs from the original
premise of "commit" being the only thing that ties things
together.  Tree objects with "gitlink" know where they are in
the history [*1*].

By this, I do not mean to say that "gitlink" approach is
inferior because it breaks that original premise.  I am just
pointing it out as a difference between two approaches.

Now, the current way index file is used is as a staging area to
create a new commit on top of the tip of the current branch.
However, it is interesting to note that logically, by itself
*alone*, it cannot be used that way.  The information the index
file records is something that can be used to write out a tree
object, and not a commit, because it does not know where the
current state sits in the history.  We have two separate files,
$GIT_DIR/HEAD that records which branch we are on, and the
branch head ref the HEAD points at, which records where the
current index came from, for that purpose.  The latter tells us
what commit we should use as the parent commit if we create such
a commit, and the former tells us which branch head to update
once we create one.  So in that sense, the index file is just a
staging area to create a new tree, not a new commit.

We could have done things differently.  I am not advocating to
do the following change, but offering a possibility as a thought
experiment.  It just felt interesting enough to point them out.

The index file could have recorded what commit the current state
recorded in the index came from.  By recording the commit the
index was read from in the index itself, independently from the
$GIT_DIR/refs/heads/$branch file, we could have been able to
allow fetching into the current branch.  When the $branch file
for the current branch was updated by a fast-forward fetch, we
would notice that the commit recorded there no longer match what
is recorded in the index.

Another interesting consequence is if the development is a
single repository and linear, we did not even need any file in
$GIT_DIR/refs/ ("branchless git").  The commit recorded as the
topmost in the index file itself would have served as the tip of
the development, and we would have been able to tangle the
history starting from the commit in the index file.

While we are doing a thought experiment, let's say we allow to
record more than one commits the current index is based upon.
'git merge' would record all the parent commits there, so that
writing out the merge result out of the index file as a tree and
then recording these commits as parents would have been the way
to create a merge commit.  We would not need the auxiliary file
$GIT_DIR/MERGE_HEAD if we did so.

In other words, if the index file recorded the commits its
contents were based upon, instead of being a staging area for a
new tree, it would have been a staging area for a new commit.

Now, the latest proposal, borrowing your idea, records the
subproject commits bound to subdirectories in the index itself.
This is halfway to make the index file a staging area for the
next commit.  If we were to do that, we also *could* record the
commits the current index is based upon, so that it can truly be
used as a staging area to create a new commit, not just a tree.

On the other hand, this could be a reason *not* to do the
`update-index --bind` to record the subproject information in
the index file.  An auxiliary file such as $GIT_DIR/bind might
be sufficient, just like $GIT_DIR/MERGE_HEAD has been good
enough for us so far.  One difference between MERGE_HEAD and
bind is that the former is very transient -- only exists during
a merge while the latter is persistent while the top commit is
checked out and being worked on.


[Footnote]

*1* One good property of "gitlink" approach is that we *could*
extend this blob-like object to store arbitrary human readable
information to represent a point-in-time from an arbitrary
foreign SCM system.  IOW, we do not necessarily have to require
`commit` line that name a git commit to be there.  It could say
"Please slurp http://www.kernel.org/pub/software/.../git.tar.gz
and extract it in git/ directory".

Of course, for such a toplevel project commit, the tool may not
be able to do a checkout automatically and require the user to
cat-file the link, download a tarball and extract the subtree
there manually.

The bound commit approach requires you to have git commit object
names on the `bind` lines, and it is fundamentally much harder
to extend it to allow interfacing with foreign (non-)SCM
systems.