A site cannot override an installed app's route for a role with an external route #348

How a role resolves today

• getUrlByRouteRole walks siteConfig.apps in list order and each app's route tree depth-first, and returns the first route whose handle.roles includes the role. Only when no app declares the role does it read siteConfig.externalRoutes, again first match. There is no check for a second match and no warning, so when two apps declare the same role the one listed first in apps wins silently, and an external route for a role any app declares is never consulted.
• resolveRouteByRole and getLinkProps build on it, so every link resolved through a role follows the same order.
• ExternalRoute is a role and a URL; nothing on it or on the site config says it should take precedence.

What the docs say

• The migration guide's URL Config changes: URLs "should now be expressed as part of the apps config as route roles, or as externalRoutes, and resolved in code via resolveRouteByRole()". It names both sources and does not say what happens when both declare a role.

Where it bites

• Learning declares org.openedx.frontend.role.courseDates, courseOutline and courseProgress on its routes, which it must for the shell's course bar. Its progress tab is to link to the dates and outline tabs through those roles (Port the progress tab to frontend-base frontend-app-learning#2190). A site that then wants one of those links to go elsewhere cannot express that in site config: an external route for the role is never reached while Learning is installed.
• The same holds for any app that declares a role for a page it serves and any site that wants to point that role elsewhere.

Questions

• Should a site be able to override an installed app's route for a role with a destination of its own, without uninstalling the app?
• If so, how does the site say so?
• Should the resolver report when two sources declare the same role?