a8n: Implement `createCampaign` for Campaigns with a CampaignPlan

[mrnugget]

This is part of RFC 42. Read this section details.

When the createCampaign mutation is called with a CampaignPlan ID the following needs to happen:

• The Diffs of the CampaignPlan's CampaignJobs are turned into commits in their respective repositories
• Commits are made by a Sourcegraph bot
• These newly created commits are pushed up to the codehost
• The newly created commits are turned into pull requests on the code hosts
• On the Sourcegraph side, these new pull requests are turned into Changesets
• While this is happening asynchronously, the user can query campaignResolver.ChangesetCreationStatus

---

Technical Notes

(These do not cover everything in this issue, but are more of a braindump to help with the implementation)

The starting point is the CreateCampaign resolver here.

This ticket is about the case where a CampaignPlanID is given in the arguments. If none is given, the existing code is executed.

If a CampaignPlan is given, we need to:

• load the CampaignPlan
• check that it's finished executing its CampaignJob
• load all CampaignJobs
• create commits from CampaignJobs

Diff to commit

The Diff, RepoID, Rev fields on CampaignJob should be enough create a commit by talking to gitserver. Maybe we can use CreateCommitFromPatch on the gitserver.Client for this. Maybe it needs to be extended.

Commits should probably all be created under sourcegraph/* branches or any other naming scheme that's explicit.

After the commit is created, we need functionality in gitserver to push the newly-created branch.

We also need to make sure that the newly-created branch/commit is not cleaned up by gitserver's GC processes before we push it.

Creating Pull Requests

Once the commit/branch have been pushed, we need to use a CreatePullRequest method on the code host clients to create a pull request from branch/commit.

At the time of writing, these have not been implemented yet.

Pull requests to Changesets

Each created pull request needs to be turned into a Changeset and added to the Campaign.

Retrying

Keep in mind that Creating Pull Requests and PUll requests to Changesets might fail and that the whole process of CampaignJobs -> Pull Requests & Changesets need to be idempotently retryable. This issue here does not cover the retry-functionality, but it is something to keep in mind.

Clean up?

After the CampaignJobs have been successfully turned into Changesets, we probably want to "freeze" (or even clean up?) the CampaignJobs.

[tsenart]

check that it's finished executing its CampaignJobs

If this is not true at the time the client calls createCampaign with a CampaignPlan id, we can't proceed in creating branches in git-server and everything that follows, which makes me think this needs to be processed, again, as some sort of background job pulled from a durable queue.

I'm thinking that repo-updater can periodically load all pending campaign jobs that have an associated campaign (via a campaign plan) and process them then.

[tsenart]

Well, actually, the documentation in the schema says the mutation should error if the plan hasn't completed yet, so I guess that makes things simpler.

https://github.com/sourcegraph/sourcegraph/blob/b2a844dc7762146c9cf0ccdac2172bbfdbbb9259/cmd/frontend/graphqlbackend/schema.graphql#L436