Use Azure Functions to enforce Pull Request Reviewers in Azure DevOps

Use Azure Functions to enforce Pull Request Reviewers in Azure DevOps

I’m currently working on a project that requires several engineers creating and updating multiple Git repositories within the same project in Azure DevOps. To ensure that all the code uploaded to these repositories and merged to their master branch follows the team standards, I wanted to create a Branch Policy for all repositories (the existing ones, and the ones which don’t exist yet) that enforced an Azure DevOps Team as reviewer for every Pull Request. While doing some research I found a Microsoft document which proposed using Azure Functions to create custom branch policies, unfortunately this didn’t cover exactly what I needed (still, I do recommend reading this document as it will cover the basics on creating a Function App and how to register the Service Hook in Azure DevOps, and I’ll not be covering those details here). But even so, it certainly was a good starting point.

After giving some through to it, it was clear to me that, besides the Function App I also needed an Azure Key Vault to store some information (such as the PAT for interacting with Azure DevOps). And in order to be able to retrieve the Secrets that I was planning to store in the Azure Key Vault, I needed some form of authentication / authorization against the Azure Key Vault API. In summary, my process should look like this:

  1. A Pull Request is submitted, Azure DevOps invokes the Function App via the Service Hook
  2. The Function app requests a token to Azure AD by using the Managed Service Identity (for authenticating against Azure AD) assigned to the Function App. (Follow this link if you want to read more about  Managed Service Identities).
  3. The Function App connects to the Azure Key Vault and retrieves the Secret containing the PAT.
  4. The Function App connects back to Azure DevOps and performs the programmed functionality.

In order for all these resources to talk between each other, there are a few tasks you’ll need to accomplish (I won’t cover in detail what each of these imply as they are rather basic and you’ll find plenty of posts online explaining how to do it if you don’t know it already).

  • Enable Managed Service Identity for you Function App
  • Allow the SPN generated for the Function App access to the Azure Key Vault Secrets by adding a policy in the AKV
  • Create a PAT in Azure DevOps and store it as a Secret in the AKV

Once you have deployed and connected all the components, the customization left to be done is quite simple.

First of all, you’ll need, 2 Application Settings to be registered in your Function App:

  • secretURI: this is the URI to the secret stored in your Azure Key Vault.
  • reviewerId: here I’ve stored the ID of the object (a group in my case) that I want to enforce to every Pull Request. In order to understand how to obtain this information please refer to (TIP: as this is a simple GET, you can paste the URI on a tab of a browser that’s already validated against your Azure DevOps Organization).

After that is done, in your Azure Function, you’ll need a ‘function.proj’ file which contains the following references to the NuGet packages used to interact with Azure AD and Azure Key Vault:

And finally your ‘run.csx’ file should look like this:

Hernán J. Larrea

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