Label PRs with merge conflicts

[bastelfreak]

Proper management for our issues and pull requests is important. Automation can help us a lot. Our conecpt: There are pull requests where we need to act (review, merge) and where we wait for the contributor. github provides the mergeable attribute in the API. It allows us to check if there is a merge conflict or not. If there is a conflict, we need to add the label. If it gets rebased, we need to remove the label

curl -s https://api.github.com/repos/voxpupuli/puppet-keepalived/pulls/150 | jq .'mergeable' 
false

If desired, we could configure the application within the repositories, so github will deliver events to us (so we don't need to poll for this, which would be stupid).

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The GitHub API allows us to update labels,

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GitHub does not send out emails to collaborators if their pull request isn't mergeable nor when a label gets added. So in addition to the labels we should probably add an automatic comment to the PR?

[Flipez]

Implemented with #35