Continuous Integration for Github Pull Requests with Teamcity

Most developers with an interest in open source software these days have seen the Github interface for handling pull requests, and relatedly, Travis CI’s support for pull requests. And so we thought it’d be useful to have something similar for our internal CI system.

So, Github has the rather nice (albeit poorly advertised) feature that whenever a pull request is raised, they add a reference for both the HEAD revision of the remote branch, and a commit which merges said HEAD revision into master. For example:

: ceri@misssplendid; git ls-remote origin | fgrep pull
9d1b88f3a67d2704e08b142b776136ddb1bdf3c1    refs/pull/1/head
454fd4ab3d439e5b10c68b98ea60fbf7a0bf81cb    refs/pull/1/merge
80b11080005b5176564c369e1b4053268d3d4efb    refs/pull/2/head
2222b016401e0b3dc9e7455aeeeeef9a2c4a3b26    refs/pull/2/merge
358197ab037e5f50417ed7f5ea3d977e53a08006    refs/pull/3/head
371e7f88898217a72d5bd8322304bc1fc6b8e8ee    refs/pull/3/merge
: ceri@misssplendid;

Now, it’s possible to Configure git to fetch pull requests so you can check them out locally, but we can use this same trick to configure Teamcity (or another build CI system) to build and test our pull request applied to master.

Assuming that we already have a configuration to build the master branch of a project, then under “Version Control Settings”, and within the appropriate “VCS root”, add the following to the “Branch Specification” field:

+:refs/pull/*/merge

Once that’s saved, Teamcity will start polling all references for merged pull requests from github. Then the next time someone pushes to a branch referenced by a pull request, then it should show up as a branch under the build in Teamcity and (if configured to do so) be automatically built.

We haven’t gotten around to shaving the Github commit status API integration Yak just yet, but we’d love to hear if anyone has.