Handling Pull Request mergers with Drone CD and the GitHub API

This is a very small example of a workaround to allow Drone.io to handle PR merge events. This repo is rather a proof of concept, but contributions are welcome to make it more effective, while waiting for the Drone team to implement some kind of a 'pull_request_merge' event.

Rationale

Pipelines that deploy a temporary version of an application for each pull request are quite common. But such on-the-fly environments then need to be destroyed once the pull request is merged. This script coudl allow to reach the branch of origin once the PR is merged, to destroy the environment that would have been created with the PR.

Requirements

The script requires jq and curl (not installed in Alpine by default).

How it works

The get_merged_branch.sh script reaches the GitHub API to get the right information from the GitHub API, and prints the orginial branch of the PR.

This script can be then called in a Drone pipeline (or any similar CI/CD platform). In the .drone.yml of this repo, the output is stored in an environment variable: export MERGED_FROM_BRANCH=$(./get_merged_branch.sh). If the output is empty, then the ongoing step stops with '[[ -z $MERGED_FROM_BRANCH ]] && exit'.