Utilizing Terraform github_repository_pull_request for Efficient Workflow Operations

Automating PR Creation in GitHub with Terraform When File Content Drifts

As someone who frequently works with infrastructure as code, I often encounter scenarios where maintaining consistency across different branches in a GitHub repository becomes paramount. A common challenge is ensuring that files in a non-default branch remain in sync with those in the master branch. However, things can get tricky when these files diverge, leaving us needing a robust mechanism to handle discrepancies and re-align the branches.

Here, I’d like to discuss a practical approach using Terraform—specifically the GitHub provider—to automate the creation of pull requests whenever file contents between a master and a synced branch differ. This process not only saves manual effort but also keeps all changes transparent and reviewable by repository owners.

The Challenge

Imagine having two branches in a GitHub repo: master and sync. The file foo.txt in both branches initially contains different text:

• master branch’s foo.txt: contains “bar”

• sync branch’s foo.txt: contains “foo”

The goal is for the sync branch to mirror the master branch. Whenever these files differ, I want to automatically create a pull request using Terraform to propose changes from sync to master. This way, the repository owner can review and decide on merging the changes.

However, complications arise after a pull request is merged. Suppose the content in master‘s foo.txt changes again, diverging from the sync branch. Unfortunately, trying to recreate a pull request for the same file content change leads to an issue: GitHub sees it as a duplicate of the previously merged (and now closed) pull request, thus the new PR fails to create.

Solution Strategy

To tackle this, one strategy involves managing pull request creation dynamically based on file content changes. Given Terraform’s stateful nature and the capabilities of the GitHub provider, we must engineer a condition where a new pull request is triggered only when actual content differences exist between the two branches. This ensures we’re not repeatedly creating identical or unnecessary pull requests.

Steps to implement:

1. File Content Monitoring: Utilize a mechanism to monitor file content on both branches effectively. This can be a script or an automated job that periodically checks for differences.

1. Dynamic PR Identifier: Craft a dynamic solution that generates a unique identifier for each unique state of foo.txt. This identifier can be composed of a hash of the file contents from both branches.

1. Terraform Resource Configuration: Use Terraform’s github_repository_pull_request resource, ensuring that the pull request is tied to this dynamic identifier. The idea is to let Terraform recreate the resource when the identifier changes, which corresponds to a change in file content.

1. Automation Script:

resource "github_repository_pull_request" "sync_to_master" {
     base_repository = "example-repo"
     base_branch     = "master"
     head_branch     = "${filemd5("sync/foo.txt")}-${filemd5("master/foo.txt")}-sync-branch"
     title           = "Update foo.txt"
     body            = "Automated PR to sync foo.txt content"
   }

This resource configuration hypothetically uses the MD5 hashes of the file contents from both branches to construct a unique head_branch. Whenever the content of foo.txt changes on either branch, the resulting MD5 will change, and thus the head_branch will effectively be a new branch, prompting Terraform to manage a new pull request.

Conclusion

By leveraging Terraform in conjunction with a strategic handling of GitHub branches and pull requests, we can automate the synchronization process between branches efficiently. This approach minimizes manual oversight and enhances consistency across development workflows. Adjustable and scalable, this method shields the integrity of the codebase while facilitating a smoother review and integration process for repository owners.