Prototype GitLab support for webhooks

[mnuttall]

GitLab is a popular source control system that many users will wish to integrate with Tekton via webhooks. We currently use the Knative GitHub Event Source which does not support GitLab. This issue is part of Epic #42 which is ranked lower than Epic #40, 'Stop using Knative for webhooks.' Ideally #40 should be progressed before this issue - in particular, we need a clear design for

• how we expect to register webhooks
• how we expect to receive them
• how we expect to trigger pipeline and taskruns as a result

This issue should focus on the GitLab-specific work that must be performed in all cases:

• Is there a functional Golang library that we can use to register webhooks against GitLab?
• What forms of credentials are best used?
• Are the GibLab APIs consistent between the hosted and locally installed versions?

User this issue, develop a working prototype that:

• Registers a webhook against a locally installed GitLab server
• Triggers a PipelineRun as a result of a git push

[vincent-pli]

Please check this knative/eventing-contrib#382
I guess Knative will support gitlab as eventing source soon.

[mnuttall]

Thanks for the pointer @vincent-pli - that's a huge pull request :-o

[dibbles]

Will start looking at this after I finish #362 which will give us a client we can use to talk to different git providers.

[dibbles]

I have a very hacked together proof-of-concept for some of this:

My Changes:

1. Create client (GitHub/Gitlab) based on repo URL (as first pass) as create/delete webhook
2. Modify git pipelineresource to take a tlsVerify flag to disable certificate validation on git commands as using a self signed cert.
3. Modified interceptor to look for different headers/values
4. Modified test pipeline triggertemplate/bindings
5. Modified monitor binding for gitlab

Not verified:

6. Monitor task not coded for gitlab - would need changing
7. PullRequest pipelineresource not coded for gitlab - would either need fixing or need monitor task to use gitlab api directly for interactions with the merge request
8. API endpoints not fully investigated

[dibbles]

Prototyped - implemented and delivered