In Drush configuration files, it is possible to define additional paths where command files and alias file paths may be found. For example, the following config will make the Drush 9 configuration behavior similar to what Drush 8 does:

drush:
  paths:
    include:
      - '${env.home}/.drush/commands'
      - /usr/share/drush/commands
    alias-path:
      - '${env.home}/.drush/sites'
      - /etc/drush/sites

A problem arises, though, when there are multiple drush.yml files, each with their own include &/or alias-path configurations, then the current implementation will return the paths from only one of the configuration files. The paths in all of the other configuration files will be overwritten and ignored.

What is needed to fix this limitation is a merge strategy that unions together all of the paths from all of the drush.yml files. However, it would not necessarily be appropriate to union all of the arrays from all configuration files.

The preliminary idea in this PR is to provide an API to specify the exact path for config items where array values should be unioned rather than replaced.

Here is how it would be used in Drush \Drush\Config\ConfigLocator:

    public function addConfigPaths($contextName, $paths)
    {
        $loader = new YamlConfigLoader();
        // Make all of the config values parsed so far available in evaluations.
        $reference = $this->config()->export();
        $processor = new ConfigProcessor();
        $processor->selectMergeStrategy(['drush.paths.include', 'drush.paths.alias-path']);

I'm not sure what the right solution is here, though. If we go with a solution similar to this one, I think I'll switch from an array of strings to a class with an API.