prow/config/branch_protection.go
func BranchRequirements(org, repo, branch string, presubmits map[string][]Presubmit) ([]string, []string, []string) {
	jobs, ok := presubmits[org+"/"+repo]
	if !ok {
		return nil, nil, nil
	}
	var required, requiredIfPresent, optional []string
	for _, j := range jobs {
		if !j.CouldRun(branch) {
			continue
		}

		if j.ContextRequired() {
			if j.TriggersConditionally() {
				// jobs that trigger conditionally cannot be
				// required as their status may not exist on PRs
				requiredIfPresent = append(requiredIfPresent, j.Context)
			} else {
				// jobs that produce required contexts and will
				// always run should be required at all times
				required = append(required, j.Context)
			}
		} else {
			optional = append(optional, j.Context)
		}
	}
	return required, requiredIfPresent, optional
}
func BranchRequirements(org, repo, branch string, presubmits map[string][]Presubmit) ([]string, []string, []string) {
	jobs, ok := presubmits[org+"/"+repo]
	if !ok {
		return nil, nil, nil
	}
	var required, requiredIfPresent, optional []string
	for _, j := range jobs {
		if !j.CouldRun(branch) {
			continue
		}

		if j.ContextRequired() {
			if j.TriggersConditionally() {
				// jobs that trigger conditionally cannot be
				// required as their status may not exist on PRs
				requiredIfPresent = append(requiredIfPresent, j.Context)
			} else {
				// jobs that produce required contexts and will
				// always run should be required at all times
				required = append(required, j.Context)
			}
		} else {
			optional = append(optional, j.Context)
		}
	}
	return required, requiredIfPresent, optional
}
// BranchRequirements partitions status contexts for a given org, repo branch into three buckets:
//  - contexts that are always required to be present
//  - contexts that are required, _if_ present
//  - contexts that are always optional