twobit auto builder part 2

In my last post on automating my project builds with buildbot I covered the relevant buildbot configs. The issue that I left unresolved was the triggering of the builds. Buildbot has tons of infrastructure to do this and their simple would be more than sufficient for handling my OE projects that typically have 3-5 git repos. But with OpenXT there are enough git repos to make polling with buildbot impractical. This post covers this last bit necessary to get builds triggering with a client side hook model.


Now that we’ve worked out the steps necessary to build the software we need to work out how to triggers builds when the source code is updated. Buildbot has extensive infrastructure for triggering builds ranging from classes to poll various SCM systems in a “pull” approach (pulled by buildbot) to scripts that plug into existing SCM repos to run as hook scripts in a “push” approach (pushed by the SCM).

Both approaches have their limitations. The easiest way to trigger events is to use the built in buildbot polling classes. I did use this method for a while with my OE projects but with OpenXT it’s a lost cause. This is because the xenclient-oe.git repo clones the other OpenXT repos at the head of their respective master branches. This means that unlike other OE layers the software being built may change without a subsequent change being made to the meta layer. To use the built in buildbot pollers you’d have to configure one for each OXT git repo and there’s about 60 of them. That’s a lot of git pollers and would require a really long and redundant master config. The alternative here is to set up a push mechanism like the one provided by the buildbot in the form of the script.

This sounds easy enough but given the distributed nature of OpenXT it’s a bit harder than I expected but not in a technical sense. Invoking a hook script from a git server is easy enough: just drop the script into the git repo generally as a ‘post-commit’ hook and every time someone pushes code to it the script will fire. If you control the git server where everyone pushes code this is easy enough. Github provides a similar mechanism but that would still makes for a tight coupling of the build master to the OpenXT Github org and here in lies the problem.

Tightly coupling the build master to the OpenXT Github infrastructure is the last thing I want to do. If there’s just one builder for a project this approach would work but we’d have to come up with a way to accommodate others wanting to trigger their build master as well. This could produce a system where there’s an “official” builder and then everyone else would be left hanging. Building something that leaves us in this situation knowingly would be a very bad thing as it would solve a technical problem but produce a “people problem” and those are exponentially harder. The ideal solution here is one that provides equal access / tooling such that anyone can stand up a build master with equal access.

Client-side hooks

The only alternative I could come up with here is to mirror every OpenXT repo (all 60 of them) on my build system and then invent a client side hook / build triggering mechanism. I had to “invent” the client side hooks because Git has almost no triggers for events that happen on the client side (including mirrors). My solution for this is in the twobit-git-poller.

It’s really nothing special. Just a simple daemon that takes a config specifying a pile of git repos to mirror. I took a cue from Dickon and his work here by using a small bit of the Github API to walk through all of the OpenXT repos so that my config file doesn’t become huge.

The thing that makes this unique is some magic I do to fake a hook mechanism as much like post-receive as possible. This allows us to use a script like the unmodified. So I expanded the config file to specify an arbitrary script so others can expand on this idea but this script is expected to take input identical to that produced by the post-receive and expected by the script.

This is implemented with a naive algorithm: every time the git poller daemon is run we just iterate over the available branches, grab the HEAD of each before and after the fetch and then dump this data into the hook script. I don’t do anything to detect whether or not there was a change even, I just dump data into the hook script. This allows us to use the script unmodified and buildbot is smart enough to ignore a hook even where the start and end hashes are the same. There are some details around passing authentication parameters and connection strings but those are just details and they’re in the code for the interested reader.

With this client side hooking mechanism we no longer need the poller objects in buildbot. We can just hook up a PBChangeSource that listens for data from the twobit-git-poller or any other change source. AFAIK this mechanism can live side by side with standard git hooks so if you’ve got an existing buildbot that’s triggered by some git repos that your team is pushing to using this poller shouldn’t interfere. If it does let me know about it so we can sort it out … or you could send me a pull request 🙂

Wrap Up

When I started this work I expected the git poller to be something that others may pick up and use and that maybe it would be adopted as part of the OpenXT build stuff. Now that I think about it though I expect the whole twobit-git-poller to be mostly disposable in the long run. We’ve already made huge progress in aligning the project with upstream Open Embedded and I expect that we’ll have a proper OE meta layer sooner than later. If this actually comes to pass there won’t be enough git repos to warrant such a heavy weight approach. The simple polling mechanisms in in buildbot should be sufficient eventually.

Still I plan to maintain this work for however long it’s useful. I’ll also be maintaining the renevant buildbot config which I hope to generalize a bit. It may even become as general as the Yocto autobuilder but time will tell.

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