prow/config/jobs.go
func (ps Presubmit) ShouldRun(baseRef string, changes ChangedFilesProvider, forced, defaults bool) (bool, error) {
	if !ps.CouldRun(baseRef) {
		return false, nil
	}
	if ps.AlwaysRun {
		return true, nil
	}
	if forced {
		return true, nil
	}
	if determined, shouldRun, err := ps.RegexpChangeMatcher.ShouldRun(changes); err != nil {
		return false, err
	} else if determined {
		return shouldRun, nil
	}
	return defaults, nil
}
func (ps Presubmit) ShouldRun(baseRef string, changes ChangedFilesProvider, forced, defaults bool) (bool, error) {
	if !ps.CouldRun(baseRef) {
		return false, nil
	}
	if ps.AlwaysRun {
		return true, nil
	}
	if forced {
		return true, nil
	}
	if determined, shouldRun, err := ps.RegexpChangeMatcher.ShouldRun(changes); err != nil {
		return false, err
	} else if determined {
		return shouldRun, nil
	}
	return defaults, nil
}
// ShouldRun determines if the presubmit should run against a specific
// base ref, or in response to a set of changes. The latter mechanism
// is evaluated lazily, if necessary.