Bound branches revisited

[John Arbash Meinel]

I want to re-open the discussion about bound branches, since I would
like to implement them soon.

I know it was discussed that doing a commit to a bound branch would do
the remote commit, and then just do a pull to the local system. I agree
that there are some nice properties. If the commit fails after
committing, but before pulling, you just 'bzr pull' and you are up to date.

It means that the commit will be slower. Rob mentioned that you could
pull information out of the local filesystem, but I don't think you can.
Because the weaves aren't necessarily perfectly in sync. (Think failed
merges, failed commits, shared weaves, etc).
It would be nice if you didn't have to download the remote weave, update
it, push it back up, and then download it again to update your local files.
I believe with knits, because we don't mesh things together, we can
compute the commit with just the ancestry, so we do know that the local
and remote knits at least contain that much information.
I would like to see bound branches before we see knits, though. Since I
don't know how long it will be before knits are perfected.

So as a first implementation, which can be optimized later. I would just
say that WorkingTree.commit() ends up checking for a bound branch, and
if it exists, it uses that as it's commit branch, and then updates the
local one afterwards.

As an interface issue, how do we handle 'bzr pull'. I think Robert
mentioned that it probably should use the bound location's parent as the
default pull target. And then you can use 'bzr update' to update versus
your bound branch. The problem is that you can update a checkout of a
bound branch.
There are a couple of things that can happen, but imagine this:

mainline-branch
	Publicly accessible, no working tree, (inside a repository?)
jam-dev-branch
	Publicly accessible, no working tree, has mainline as default
        parent
jam-laptop-branch
	Private, bound against jam-dev-branch, no working tree (inside
	repository on the laptop)
jam-working-tree
	Points to jam-laptop-branch
jam-working-tree2
	I forgot about my other working tree, also points to
        jam-laptop-branch
jam-desktop-branch
	On my desktop, I have a standalone branch, which is bound to
	jam-dev-branch (and has a working tree).
	

With this setup, I want to be able to edit inside jam-working-tree, and
when I commit, I want it to update the public jam-dev-branch, and the
local jam-laptop-branch. I need a way to update jam-desktop, so that it
is up-to-date against it's bound parent.

I go to jam-desktop and commit, which means my laptop-branch is now out
of date. I need a way to get it up-to-date. I also need a way that when
I recognize that my second working tree is also out of date, I can
update it.

Here is one possibility:

cd $jam-working-tree
bzr pull
	Reads the working tree in $jam-working-tree
	Sees that the branch is $jam-laptop-branch, which is bound to
		$jam-dev-branch
	Connects to $jam-dev-branch and sees that $mainline-branch is
		the parent location.
	Connects to $mainline-branch, and pulls the changes into
		$jam-dev-branch
	Pulls the changes into $jam-laptop-branch
	Updates $jam-working-tree

cd $jam-working-tree
bzr update
	Reads the working tree in $jam-working-tree
	Sees that the branch is $jam-laptop-branch, which is bound to
		$jam-dev-branch
	Connects to $jam-dev-branch and pulls any new changes into
		$jam-laptop-branch
	Updates the working tree

With the above situation, there is no way to update jam-working-tree2,
to just what exists in jam-laptop-branch, since it will always try to
connect to jam-dev-branch and pull changes first.
The alternative is:

cd $jam-working-tree
bzr pull
	Reads the working tree in $jam-working-tree
	Sees that the branch is $jam-laptop-branch, which is bound to
		$jam-dev-branch
	Connects to $jam-dev-branch, and pulls the changes into
		$jam-laptop-branch
	Updates $jam-working-tree

cd $jam-working-tree
bzr update
	Reads the working tree in $jam-working-tree
	Sees that the branch is $jam-laptop-branch
	Updates the working tree

Now, what do people think about the default behavior. If I am in a
checkout, and I'm bound, should "bzr update" do the full update, or just
against the local branch.
I think the full update would be nice, as long as it can fall back to a
local update without having to unbind. (otherwise there is no way to
update my local checkout when I'm on an airplane)

John
=:->
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