Working copy as a pseudo-head

Working copy as a pseudo-head

[Steven Grimm]

I was trying to explain to someone today why "git pull" fails if you 
have working-copy edits of one of the files that changed in the other 
branch, and it occurred to me that there may be a way to model the world 
so it doesn't fail. I am still relatively new to git so I hope this is 
not a totally idiotic idea.

Right now there is a hard distinction between what's in the working copy 
and the revision history of the tree. Merge is an operation on the 
revision history, and the working copy doesn't enter into it aside from 
the sanity check in git-merge to prevent you from inadvertently 
overwriting local edits.

What if instead, we had a notion of a temporary commit, call it WORK, 
that is ahead of what we currently think of as HEAD? That is, where now 
we have

A----B----C----D(HEAD)

we instead allowed

A----B----C----D(HEAD)----E(WORK)

When you do some operation that currently would complain about 
working-copy edits, it instead commits a WORK revision but does not 
adjust HEAD. Then it performs the operation against HEAD (advancing HEAD 
as needed) and rebases WORK under the newly merged HEAD, updating the 
working copy as a normal rebase would. Then, with one exception I'll get 
to below, it removes the WORK revision from the version history but 
leaves the contents of the working copy alone.

For example, say you have a pristine working copy of a branch with some 
history:

A----B----C(HEAD)

and you make some edits to the working copy. Now you want to pull down 
the latest changes from upstream, revision "X". So the system silently 
commits your work in progress to a temporary revision, which (while the 
merge is running) changes your history to:

A----B----C(HEAD)----D(WORK)

Then it merges the new changes with HEAD to form a new "Y" revision:

            /--D(WORK)
           /
A----B----C----Y(HEAD)
 \            /
  \-------X--/

Then it rebases WORK:

A----B----C----Y(HEAD)----D(WORK)
 \            /
  \-------X--/

And finally it removes WORK from the version history.

A----B----C----Y(HEAD) + working-copy edits
 \            /
  \-------X--/

I believe something like the above is currently how Cogito does its 
"update from the origin without losing working copy edits" thing, though 
it has to stash the working-copy edits in a temporary branch. Having the 
underlying functionality supported in the core git tools would be far 
preferable to having to hack around the lack of it in a frontend.

The one exception to the above flow is when you want to switch branches. 
Here, we create a WORK temporary commit on the current branch and move 
over to the new branch as normal, blowing away the WORK commit on the 
*new* branch, if any, and leaving its edits in the working copy. That 
allows you to freely switch back and forth between branches without 
worrying about whether you need to save your changes in progress, and 
without cluttering your repository with intermediate copies of files 
you're editing.

This is a behavior change! But I think the proposed behavior makes more 
sense. It treats the working copy as an extension of whatever branch 
you're on, rather than a separate entity that can flit back and forth 
between branches if there aren't conflicts or prevent you from changing 
branches at all if there are. It has always seemed confusing and 
inconsistent to me that, in some sense, doing a commit makes my changes 
LESS sticky -- that is,

echo foo >> testfile
git checkout other-branch
tail -1 testfile

says "foo", but

echo foo >> testfile
git commit -a
git checkout other-branch
tail -1 testfile

doesn't. I know (or I think I do) why it's doing that, but from a user 
experience point of view it's just weird, because it makes it look like 
"commit" reduces the importance of my change. And it also makes it a lot 
less convenient to pop over to another branch for a moment to check out 
the old version of some code I'm in the middle of rewriting.

Also, I realize I can use StGIT to get something like the above. But 
again, I think it'd be cleaner to support this in the underlying tool; 
"pull down some updates while I'm still working" is a very common usage 
model that's handled fine by other SCMs (Subversion, etc.) and right now 
with Git it requires a bunch of awkward extra commands. And preserving 
working-copy edits when switching branches seems like a huge convenience 
win to me; I have a hard time imagining how the current behavior is a 
time-saver for anyone (though I'm sure I'll hear all about it now!)

Comments? Is this just nuts?

Re: Working copy as a pseudo-head

[Junio C Hamano]

Steven Grimm <koreth@midwinter.com> writes:

> Comments? Is this just nuts?

It is not "nuts", but we do not do it currently because it is a
bit too combersome to make it recoverable when things go wrong
while keeping the cost of making the necessary back-up for
recovery low.  When switching branches with local changes,
"git-checkout -m" needs to deal with a similar issue, but that
case only deal with two trees and a working tree.  To do this
for three-way merge, the conflicts you need to deal with become
more complex.

What needs to be done is very simple and straightforward.  You
first stash away the working tree state in a "virtual" tree,
then perform the usual 3-way merge using the common ancestor,
your HEAD and the other head, and come up with the merge result
without any of your changes.  Then you run another three-way
merge between the merge result and your previous working tree
state using the HEAD before the merge as the common ancestor.

       o---o---X (other head)
      /
  ---o---o---H (your head)
              \  
               W (your working tree)

==>

       o---o---X (other head)
      /         \
  ---o---o---H---M (merge between the base trees)
              \  
               W (your working tree)

==>

       o---o---X (other head)
      /         \
  ---o---o---H---M (merge between the base trees)
              \   \   
               W---W'(your working tree, updated for the merge result)

The design goal here is that you do not want to get any of your
local changes (i.e. "diff H W") to be included when you record
the result of the base merge 'M', and you would want the diff
between H and W to be forward-ported to the diff between M and
W'.

The first goal is already quite cumbersome when the merge
between X and H involve conflicing merges.  We use the working
tree to perform the file level merge (i.e. you edit them with
your favorite editor, and tell git when you are done), so while
that is happening we temporarily need to remove what was between
H and W.  If the merge is too complex and the user decides to
revert the whole thing, we would need to rewind the index and
working tree to the state at W (that means we would at least
need to stash the diff between H and index, and index and
working tree before starting the merge).

After the base merge is done, we would need to update the HEAD
to point at M.  Then we would perform the second three-way merge
between M and W using the original HEAD as their common
ancestor to come up with W'.  When this conflicts, there is no
easy way for the user to recover, other than going back to the
original state (that is, HEAD points at H and working tree is
W), even after having spent effort to merge between X and H to
produce M.

In common usage (call that "best current practice" if you may),
you do not have local changes in the working tree that would
conflict with the merge, so the current behaviour feels
cumbersome only when the safety valve kicks in.  After seeing it
fail due to the safety valve, you can do the same as what the
above pictures depict using existing tools.

 * preserve the local changes (i.e. come up with the virtual
   "W"):

	git commit -a -m WIP
	git tag -f WIP
        git reset --hard HEAD^

 * perform the merge in the now-clean tree:

	git pull ;# again from the same place
	edit edit
        test test
        git commit -a ;# resolve conflicts, test and commit

 * recover what you have preserved, and discard the virtual W:

	git pull --no-commit . WIP ;# merge in
	git reset ;# revert index to HEAD aka base merge result.
	git tag -d WIP