Skip testing for PR builds and non-lockfile push builds on greenkeeper branches

[rsimha]

greenkeeper branches are currently tested on Travis four times.

1. PR build for package.json update
2. Push build for package.json update
3. PR build for yarn.lock update
4. Push build for yarn.lock update

The only case in which we need to run all the AMP tests is case 4.

This PR skips testing in cases 1, 2, and 3. The pre and post script steps that actually update the lockfile will be run normally.

This will reduce test time from ~20 min to ~1 min in cases 1, 2, and 3, , thereby reducing the overall Travis CPU usage.

[rsimha]

/to @erwinmombay @cramforce

[cramforce]

Shouldn't this be possible to configure in .travis.yml / Travis settings? I thought the branches->only only option did that.

[rsimha]

@cramforce We'd like to run push builds on greenkeeper/* branches, but not PR builds. Removing this entry from branches->only will stop running push builds on greenkeeper branches. See https://docs.travis-ci.com/user/customizing-the-build/#Building-Specific-Branches

The Travis docs do not mention a way to specify that push builds be run for a particular branch, but not PR builds for PRs from that branch to master.

[erwinmombay]

says in https://docs.travis-ci.com/user/conditional-builds-stages-jobs/ that you can do if: type IN (push, pull_request) (we can remove pull_request from the IN check)

[rsimha]

That's promising, @erwinmombay :)

[rsimha]

@erwinmombay @cramforce PTAL.

[rsimha]

Bumping this.

[rsimha]

@cramforce @erwinmombay From testing, it turns out that if a build is conditionally skipped, continuous-integration/travis-ci/pr, which is a required check, fails with the message The Travis CI build could not complete due to an error, and blocks the PR from being merged. Therefore, we can't conditionally cancel greenkeeper PR builds. I'm going to have to go back to my pr-check.js logic to skip all testing.

[rsimha]

Tested this on an actual greenkeeper branch with push and PR builds, using lockfile and non-lockfile commits.

Non-lockfile commits:
PR: https://travis-ci.org/ampproject/amphtml/jobs/334940226#L1656 (skipped)
Push: https://travis-ci.org/ampproject/amphtml/jobs/334940209#L1672 (skipped)

Lockfile commits:
PR: https://travis-ci.org/ampproject/amphtml/jobs/334943871#L1665 (skipped)
Push: https://travis-ci.org/ampproject/amphtml/jobs/334943858#L1660 (run)