triggers: Polling a repository to detect changes and trigger a pipeline

To Poll a repository at a certain interval to look for any changes and trigger a pipelinerun/taskrun.

Use cases:

• Some users might not have permission to setup a webhook so this could be helpful to trigger a pipelinerun/taskrun after some changes.
• another use case https://github.com/tektoncd/triggers/issues/480#issuecomment-620605920

This can be done using a cronjob but could to a part of triggers. Similar Issue: https://github.com/tektoncd/triggers/issues/480

Proposal: To have this feature as a part of triggers. I did a poc around this where I have a new SyncRepo CRD

apiVersion: triggers.tekton.dev/v1alpha1
kind: SyncRepo
metadata:
  name: test
spec:
  repo: https://github.com/tektoncd/hub
  branch: main
  frequency: 3m
  binding: pipeline-binding
  template: pipeline-template

Approach:

• This looks for changes on repository using GitHub Rest API and triggers a pipelines.
• The API returns the response in json format for example or https://api.github.com/repos/sm43/hub/commits/tekton-ci
• It uses trigger binding to get values from the json response and use them in the trigger template and triggers the pipelinerun.
• Later it saves the last commit on which it was triggered and use it in next reconcilation to look for changes.

Things can be improved:

• Support for private repositories
• Gitlab also have similar API which we can use
• We can also directly allow users to pass pipelineRef which can be directly used if a user doesn’t want to use TriggerBinding/TriggerTemplate
• A user can provide multiple pipeline templates which can be triggered together

Available solution: There is an operator by Kevin https://github.com/bigkevmcd/tekton-polling-operator which polls a repo and create a pipelinerun/taskrun if something is changes. This is independent of TektonBinding and TektonTemplate, it take PipelineRef as input.

cc @vdemeester