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.

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