Caffeinated Coder

What are Subversion hooks and why would you use them?

Hooks are extension points that allow you to add behavior at various stages of the commit process through an executable file.

The most useful one is the probably the pre-commit hook because it allows you to prevent a commit from occurring, thus providing a way to enforce whatever policies that can dream up to help your development process run more smoothly.

We use it to ensure that developers enter a valid Jira key into the commit notes, which is a crucial step in making our code review process as painless as possible.

How do pre-commit hooks work?

All you have to do is create a console application or script named pre-commit and copy it in the hooks directory of your repository. Subversion will automatically execute any application following this convention before each commit.

The repository path and transaction id associated with the commit will be passed in as the first two arguments, thus allowing you to look up relevant information about the revision before making the decision about whether or not to allow the commit to occur.

To prevent the commit from happening, all you have to do is exit with a ‘1′. You can even display an explanatory message to the user (at least in TortoiseSVN) by writing to Console.Error.

Using SharpSVN to Avoiding Command Line Parsing Hell

During my first attempt at implementing a hook, I programmatically spawned a process and executed svnlook.exe to extract the information I needed about the revision.

This worked fine for simple cases, but quickly began to require some serious Command Line Parsing-Fu in order to accomplish what I wanted. At that point my “There-has-to-be-an-easier-way” alarm went off in my head, so I began to search around for a pre-built API to simplify the task.

I soon found SharpSVN, a stable open source project that is used by AnkhSVN and SharpDevelop.

Aside from a few annoying testability issues that forced me to create wrapper classes in order to do unit testing, I had a good experience with the framework and would definitely recommend it over rolling your own.

Integrating with Jira

Jira exposes a large portion of their functionality through web services that you can access by simply turning the “Accept Remote API calls” option on in the General Configuration area of the administration page.

We use the key parsed from the comment to query the Jira web service and verify that the referenced issue is in an opened state and has a fix version that has not already been released.

Downloading the Code

I zipped up a sanitized version of our pre-commit hook that you can download and either use as a learning sample or modify for your own use.

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