co-located branches
===================

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.

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.

Use Cases
---------

Carla has a large C-based project with a large tree and a lot of .o
files that get generated as part of her 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 address colocated branches in 
directories that contain multiple branches. 

Per RFC3986 we have picked the comma (",") to allow the specification of 
colocated branch names. Comma's in path names would have to be 
urlencoded at first to avoid ambiguity, though perhaps it would be 
possible to support heuristics later when interpreting user-specified URLs.

An example URL would be:

 bzr://bazaar.launchpad.net/~jelmer/bzr/bzr.dev,colo-urls

The segment after the comma will initially be interpreted as a colocated 
branch name but we would like to keep the option to allow 
key=value style specifications in the future and DWIM for segments that 
do not contain an =. Following the RFC the comma would be interpreted within
the scope of a path segment. In other words, in the URL:

 git://git.debian.org/pkg-python-debian/python-debian.git,unstable/README

unstable is interpreted as the colocated branch living in the python-debian.git
control directory; README is a path inside of the branch.

Control directories will also have the notion of an "active" branch. This is 
the branch that is being used by a working tree, if present and the branch 
that will be used if no explicit colocated branch is specified. The 
active branch support makes it easier to deal with existing bzrdirs and 
is useful when accessing foreign control directories that have the concept 
as well.

A new command 'bzr rmbranch' needs to be added to make it possible to 
remove colocated branches, as this won't be possible by simple 
directory removal, at least not of a user-visible directory.

Code Changes
~~~~~~~~~~~~

BzrDirFormat will need a supports_colocated_branches property that 
indicates whether a format supports the creation, removal and accessing of 
colocated branches.

Several methods on BzrDir will need to be updated to take an option branch_name 
parameter. If this parameter is not specified or None, the active branch 
will be used.

The methods that will have to be extended are:

 * BzrDir.open_branch()
 * BzrDir.create_branch()
 * BzrDir.destroy_branch()
 * BzrDir.get_branch_transport()

 * BranchFormat.initialise()
 * BranchFormat.open()

A new BzrDir.list_branches() method will return all colocated branches 
present in a control directory. 

Any URL interpreting methods (e.g. Branch.open) will need to be updated 
to extract a colocated branch name and need to pass that into the 
relevant methods.

Existing callers of BzrDir.{create,open,destroy}_branch() need to 
be updated to pass in branch names and optionally be changed to use 
BzrDir.list_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. An example of this is implemented in the 
lp:bzr-colocated plugin

Further integration
-------------------

Loggerhead and Launchpad need to be updated to show colocated branches
(perhaps in a similar way as they would show tags?).

qbzr/bzr-gtk need to be updated to support colocated branches.