Bazaar

bzr commit –author

One of the features I recently discovered in Bazaar is the --author option for “bzr commit“.  This lets you make commits to a Bazaar branch on behalf of another person.  When used, the new revision credits two people: you as the committer and the other person as the author.

While Bazaar does make it easy for non-core contributors to send changes in a form that correctly attributes them (e.g. by publishing a branch or sending a bundle), I doubt we’ll ever see the end of pure patches.  Some cases include:

  • Patches based on a tarball release.   In these cases the contributor likely hasn’t even used the VCS.
  • People send simple diffs from e.g. “bzr diff” since that is sometimes the easiest solution (or what they do by default due to having transferred their knowledge from another VCS).
  • Some people use a VCS bridge so they can work with their favourite VCS.  They might not be able to provide their changes as Bazaar commits due to this.

The --author option lets you commit these changes in a way that credits the contributor for their work.  The author of the change will then be displayed in “bzr annotate” output and credited along with the you in the “bzr log” output.

The feature is also used by a number of plugins such as bzr-rebase: if you replay or rebase someone else’s changes, the new revisions will creit you as the committer and the original committer as the author.

Psycopg migrated to Bazaar

Last week we moved psycopg from Subversion to Bazaar.  I did the migration using Gustavo Niemeyer‘s svn2bzr tool with a few tweaks to map the old Subversion committer IDs to the email address form conventionally used by Bazaar.

The tool does a good job of following tree copies and create related Bazaar branches.  It doesn’t have any special handling for stuff in the tags/ directory (it produces new branches, as it does for other tree copies).  To get real Bazaar tags, I wrote a simple post-processing script to calculate the heads of all the branches in a tags/ directory and set them as tags in another branch (provided those revisions occur in its ancestry).  This worked pretty well except for a few revisions synthesised by a previous cvs2svn migration.  As these tags were from pretty old psycopg 1 releases I don’t know how much it matters.

As there is no code browsing set up on initd.org yet, I set up mirrors of the 2.0.x and 1.x branches on Launchpad to do this:

It is pretty cool having access to the entire revision history locally, and should make it easier to maintain full credit for contributions from non-core developers.

Looms Rock

While doing a bit of work on Storm, I decided to try out the loom plugin for Bazaar. The loom plugin is designed to help maintain a stack of changes to a base branch (similar to quilt). Some use cases where this sort of tool are useful include:

  1. Maintaining a long-running diff to a base branch. Distribution packaging is one such example.
  2. While developing a new feature, the underlying code may require some refactoring. A loom could be used to keep the refactoring separate from the feature work so that it can be merged ahead of the feature.
  3. For complex features, code reviewers often prefer to changes to be broken down into a sequence of simpler changes. A loom can help maintain the stack of changes in a coherent fashion.

A loom branch helps to manage these different threads in a coherent manner. Each thread in the loom contains all the changes from the threads below it, so the revision graph ends up looking something like this:

Sample Loom Timeline

Once the plugin has been installed, a normal branch can be converted to a loom with the “bzr loomify” command. The “bzr create-thread” command can be used to create a new thread above the current one.

The “bzr down-thread” and “bzr up-thread” commands can be used to switch between threads. When going up a thread, a merge will be performed if there are new changes from the lower thread. The “bzr show-loom” command shows the current state of the loom, and which thread is currently selected.

The “bzr export-loom” command can be used to explode the loom, creating a standard branch for each thread. The included HOWTO document gives a more detailed tutorial.

There are a few warts in the UI that I’ve encountered though:

  1. The “bzr combine-thread” command sounds like it should actually merge two threads. Instead it is an advisory command that can be used to remove a thread once its contents have been merged.
  2. After pulling new changes in from upstream on the bottom thread, it gets a bit tedious bubbling the changes up with “bzr up-thread” and “bzr commit“.
  3. As well as committing revisions to individual threads, the “bzr record” command can be used to commit the state of the loom as a whole. I haven’t really worked out when I should be using the command.
  4. No indication is given if there are changes in the loom that haven’t been recorded with “bzr record“. I’d expect some indication from “bzr status” to this effect.
  5. When using looms to break a larger feature down into smaller chunks, it’d be nice to have a command that generated a sequence of merge requests that built on top of each other. This would be the form needed to submit them for review on a mailing list.

Despite the quirks in the interface, it does make the relevant work flows easier.  It will be interesting to see how the plugin develops.

bzr-dbus hacking

When working on my bzr-avahi plugin, Robert asked me about how it should fit in with his bzr-dbus plugin. The two plugins offer complementary features, and could share a fair bit of infrastructure code. Furthermore, by not cooperating, there is a risk that the two plugins could break when both installed together.

Given the dependencies of the two packages, it made more sense to put common infrastructure in bzr-dbus and have bzr-avahi depend on it. That said, bzr-dbus is a bit more difficult to install than bzr-avahi, since it requires installation of a D-Bus service activation file. After looking at the code, it seemed that there was room to simplify how bzr-dbus worked and improve its reliability at the same time.

The primary purpose of bzr-dbus is to send signals over the session bus whenever the head revision of a branch changes. This was implemented using a daemon that is started using D-Bus activation, and sends out the signals in response to method calls made by short lived bzr processes.

While this seems to be the design the dbus-python tutorial guides you to use, I don’t think it is the best fit for bzr-dbus. The approach I took was to do away with the daemon altogether: the D-Bus session bus does a pretty good job of broadcasting the signals on its own.

The code that previously asked the broadcast daemon to send the revision signal was changed to simply send the signal. The following helper made this pretty easy to do without having to write any extra classes to emit the signals:

def send_signal(bus, dbus_interface, signal_name, signature, *args):
    """Send a signal on the bus."""
    message = dbus.lowlevel.SignalMessage('/', dbus_interface, signal_name)
    message.append(signature=signature, *args)
    bus.send_message(message)

With these changes, the commit hook now only needs to connect to the session bus and fire off the signal and return. Previously it was connecting to the bus, getting an the broadcast service (which might involve activating it), sending a method call message and waiting for a method return message. The new code is faster and if no one is listening for the signals, it only wakes the bus.

For code that was consuming the signals, they had to switch to the bus.add_signal_receiver() method to register the callbacks, which allows you to subscribe to a signal irrespective of its origin.

The only missing feature with these changes was annotating the signals with additional URLs when the branch was being shared over the network. As these additional URLs are only really interesting when accessing the branch remotely, I moved the functionality to the “bzr lan-notify” command so that it annotates the revision announcements just before broadcasting them to the local network.

With all the changes applied, the D-Bus API consists entirely of signal emissions, which gives a looser coupling between the various components: each component will happily function in the absence of the others, which is great for reliability.

Once the patches are merged, I’ll have to look at porting bzr-avahi to this infrastructure. Together, these two plugins offer compelling features for local network collaboration.

Zeroconf Branch Sharing with Bazaar

Bazaar logoAt Canonical, one of the approaches taken to accelerate development is to hold coding sprints (otherwise known as hackathons, hackfests or similar). Certain things get done a lot quicker face to face compared to mailing lists, IRC or VoIP.

When collaborating with someone at one of these sprints the usual way to let others look at my work would be to commit the changes so that they could be pulled or merged by others. With legacy version control systems like CVS or Subversion, this would generally result in me uploading all my changes to a server in another country only for them to be downloaded back to the sprint location by others.

In contrast, with a modern VCS like Bazaar we should be able to avoid this since the full history of the branch is available locally – enough information to let others pull or merge the changes. That said, we’ve often ended up using a server on the internet to exchange changes despite this. This is the same work flow we use when working from home, so I guess the pain of switching to a new work flow outweighs the potential productivity gains.

The Solution

Bazaar makes it easy to run a read only server locally:

bzr serve [--directory=DIR]

However, there is still the issue of others finding the branch. They’d need to know the IP address assigned to my computer at the sprint, and the path to the branch on the server. Ideally they’d just need to know the name of the my branch. As it happens, we’ve got the technology to fix this.

Avahi logoAvahi makes it trivial to advertise and browse for services on the local network without having to worry about what IP addresses have been assigned or what people name their computer. So the solution is to hook Avahi and Bazaar together. This was fairly easy due to Avahi’s DBus interface and the dbus-python bindings.

The result is my bzr-avahi plugin. You can either download tarballs or install the latest version directly with from Bazaar:

bzr branch lp:bzr-avahi ~/.bazaar/plugins/avahi

To use the plugin, you must have at least version 1.1 of Bazaar, the Python bindings for DBus and Avahi, and a working Avahi setup. Once the plugin is installed, it hooks into the standard “bzr serve” command to do the following:

  • scan the directory being served for branches that the user has asked to advertise.
  • ask Avahi to advertise said branches

You can ask to advertise a branch using the new “bzr advertise” command:

bzr advertise [BRANCH-NAME]

If no name is specified, the branch’s nickname is used. The advertise command sends a signal over the session bus to tell any running servers about the change, so there is no need to restart “bzr serve” to see the change.

At this point, the advertised branches should be visible with a service browser like avahi-discover, so that’s half the problem solved. From the client side two things are provided: a special redirecting transport and a command to list all advertised branches on the local network.

The transport allows you to access the branch by its advertised name with most Bazaar commands. For example, merging a branch is as simple as:

$ bzr merge local:BRANCH-NAME
local:BRANCH-NAME is redirected to bzr://hostname.local:4155/path/to/branch
...
All changes applied successfully.
$

If you want to get a list of all advertised branches on the network, the “bzr browse” command will print out a list of branch names and the URLs they translate to.

I believe using these tools together should offer a low enough overhead for direct sharing of branches at sprints that people would actually bother using it. It should be quite useful at the next sprint I go to.