Feature: Install argument for a leaner installation to enable static analyses

[Dagefoerde]

Travis has a cool new feature called "stages" that facilitate splitting tasks. We want to leverage this since we have a large build matrix (for PHP version, Moodle version, database): Right now, all scripts are executed for every instance of that matrix. However, most scripts are static analyses that will have the same result in every instance, so it would be sufficient to run them once.

Example:

The single run in Static now calls all static analyses, e.g. CPD and basic plugin checks. In contrast, Test contains the full build matrix, running PHPUnit and Behat on each instance.

Regarding the issue: We have to execute moodle-plugin-ci install on every single job. For the static analyses we actually only do this to have the specified Moodle, but it installs the full database regardless: https://github.com/moodlerooms/moodle-plugin-ci/blob/173764f0235cc26b955d4864d8abf418366fdeda/src/Installer/InstallerFactory.php#L82

Would it be possible to add an argument to install to suppress installing the DB? I envision something like moodle-plugin-ci install --no-install-db that just makes sure that all files are in place for the Static stage, but that does not perform a full installation.

Cheers!

(Also, as an example, here is the .travis.yml that results in the above screenshot: https://github.com/Dagefoerde/moodle-repository_owncloud/blob/c6d19cea78a138b98a35ffadc809723bcb7b4210/.travis.yml)

[Dagefoerde]

Oh, also, sorry for even explaining stages... It was actually the pull request against your repo that made me realise that stages are a thing in Travis. Thanks a lot for that! :)