[RFC] colocated branches spec

[Jelmer Vernooij]

Hi,

As part of the native Bazaar colocated branches that I have been working
on and as required by bzr-git, I've been working on a spec for
supporting colocated branches in the Bazaar API.

== Summary ==

At the moment, each Bazaar branch has a separate directory in the file
system. While this works well, and makes it very easy to discover
branches there are several situations where it might be useful to also
support multiple branches under the same file system directory.

There is an experimental implementation for Bazaar available as a plugin
at http://people.samba.org/bzr/jelmer/bzr-local-branches/trunk. This was
the original proof-of-concept and doesn't yet use the API documented
here.

== Rationale ==

Allowing multiple branches to live under the same directory in the file
system means that it is possible to very easily share the same working
tree and repository between those branches, without having a lot of fs
infrastructure. 

Git and Mercurial (can) store multiple branches under a single directory
in the file system - per repository, so to speak. In order for this to
be accessible in Bazaar, Bazaar needs to have the right APIs and UI for
accessing these branches.

== Further Details ==

=== Assumptions ===

== Use Cases ==

## It is important that the description section covers the
functionality-related aspects (the "what") of the change. Providing
rationale (the "why") is always a plus.

Carla has a large C-based project with a large tree and a lot of .o
files that get generated as part of his build process. She doesn't want
to create a new working tree for each new branch but simply uses "bzr
switch" to switch between the different 
colocated branches that all use the same working tree.

Brad has a single project with a lot of related branches. He works on
them and occasionally pushes all of those branches to a remote host
using a single push command.

Joe follows one of his co-workers local branches in Mercurial by pulling
into Bazaar.

== Implementation ==

=== UI Changes  ===

Bazaar URLs need to have some way to specify a colocated branch other
than the current HEAD. Several options have been, each with its own
advantages and disadvantages: This was discussed on the mailing list,
most notably the use of a ";branch=NAME" suffix as well as a special
separation character (+, =, etc), but no final conclusion was reached. 

https://lists.ubuntu.com/archives/bazaar/2008q4/050105.html

=== Code Changes ===

BzrDir should support a BzrDir.get_colocated_branches() call returning a
colocated branch container, that can be used to add / remove colocated
branches as well as change the currently active colocated branch.

{{{
class ColocatedBranchContainer(object):

   def get_active_branch_name(self):
      """Returns the name of the currently active branch.

      This can be None if no branch is currently active.
      """

   def get_active_branch(self):
      """Returns the currently active branches' Branch object."""

   def get_branch(self, name):
      """Returns the Branch object for the specifie branch."""

   def available_branches(self):
      """Returns a set with the names of the available branches."""

   def set_active_branch(self, name):
      """Set the currently active branch."""

   def destroy_branch(self, name):
     """Destroy the specified branch.""" 
}}}

If the particular BzrDir implementation doesn't support colocated
branches, it can just return a dummy container that just contains a HEAD
branch. 

Looms can of course return a container with all their threads.

BzrDir.find_branches() should take into account the colocated branches
when iterating over its branches.

=== Schema Changes ===

No format changes are necessary at first; at least, even if Bazaar
provides the right infrastructure it doesn't have to support this
feature in its own file formats.

Eventually, Bazaar could easily support colocated branches by just
creating a new branch transport for each colocated branch and have a 
"regular" branch live there. This would require something like
BzrDirMeta2 though.

=== Unresolved Issues ===

 * What about colocated looms ?
 * What character to use to name colocated branches in URLs?

Cheers,

jelmer
-- 
Jelmer Vernooij <jelmer at samba.org> - http://samba.org/~jelmer/
Jabber: jelmer at jabber.fsfe.org