How to use hook_url_outbound_alter for emails only?

Question

I have a Rule set up that sends a mail to a certain user with a link to a certain node. This link is altered using hook_url_outbound_alter to include additional details.

The problem is that hook_url_outbound_alter also alters the URL everywhere else. Closing an overlay on the node view page rewrites the URL, clearing the cache (admin toolbar + devel) on the node pages rewrites the URL and so on.

I already attempted to check for an absolute URL in $options, but that does not solve the problems above.

function mymodule_url_outbound_alter(&$path, &$options, $original_path) {
  if (preg_match('|^node/([0-9]*)|', $path, $matches) && $options['absolute'] {
      $options['query']['parameter'] = $foo;
    }
  }
}

Can I use hook_url_outbound_alter() to only alter URLs when it was invoked from a specific module or something?

Answer 1

Can I use hook_url_outbound_alter() to only alter URLs when it was invoked from a specific module or something?

You cannot, if that module doesn't put in $options (the same argument passed to url()) something that makes you understand that. To make an example, entity_uri() uses $options['entity_type'] and $options['entity']; if you were trying to intercept the calls to url() done from entity_uri(), you could check the presence of those options.
Otherwise, the context given to hook_url_outbound_alter() doesn't allow to understand which module called url(). Maybe you could get that using debugging functions, but I am not sure I would put that kind of code in a module.

Since you seem to trying to alter links in an email sent from Drupal, you could implement hook_mail_alter(), assuming the emails are sent using the correct way of sending them with Drupal.