Fix mounted engine named routes regression

When generating the url for a mounted engine through its proxy, the path should be the sum of three parts:

1. Any `SCRIPT_NAME` request header or the value of `ActionDispatch::Routing::RouteSet#relative_url_root`.
2. A prefix (the engine's mounted path).
3. The path of the named route inside the engine.

Since commit https://github.com/rails/rails/commit/44ff0313c121f528a68b3bd21d6c7a96f313e3d3, this has been broken. Step 2 has been changed to:

2. A prefix (the value of `ActionDispatch::Routing::RouteSet#relative_url_root` + the engine's mounted path).

The value of `ActionDispatch::Routing::RouteSet#relative_url_root` is taken into account in step 1 of the route generation and should be ignored when generating the mounted engine's prefix in step 2.

This commit fixes the regression by having `ActionDispatch::Routing::RouteSet#url_for` check `options[:relative_url_root]` before falling back to `ActionDispatch::Routing::RouteSet#relative_url_root`. The prefix generating code then sets `options[:relative_url_root]` to an empty string. This empty string is used instead of `ActionDispatch::Routing::RouteSet#relative_url_root` and avoids the duplicate `relative_url_root` value in the final result.

This resolves #20920 and resolves #21459

2 files changed, 7 insertions, 2 deletions

diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index cfd50f616d..2f1c2afb91 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -665,6 +665,7 @@ module ActionDispatch
                   super(options)
                 else
                   prefix_options = options.slice(*_route.segment_keys)
+                  prefix_options[:relative_url_root] = ''.freeze
                   # we must actually delete prefix segment keys to avoid passing them to next url_for
                   _route.segment_keys.each { |k| options.delete(k) }
                   _routes.url_helpers.send("#{name}_path", prefix_options)
diff --git a/actionpack/lib/action_dispatch/routing/route_set.rb b/actionpack/lib/action_dispatch/routing/route_set.rb
index e4b8d5993e..339e2b7c4a 100644
--- a/actionpack/lib/action_dispatch/routing/route_set.rb
+++ b/actionpack/lib/action_dispatch/routing/route_set.rb
@@ -656,14 +656,18 @@ module ActionDispatch
 
       RESERVED_OPTIONS = [:host, :protocol, :port, :subdomain, :domain, :tld_length,
                           :trailing_slash, :anchor, :params, :only_path, :script_name,
-                          :original_script_name]
+                          :original_script_name, :relative_url_root]
 
       def optimize_routes_generation?
         default_url_options.empty?
       end
 
       def find_script_name(options)
-        options.delete(:script_name) || relative_url_root || ''
+        options.delete(:script_name) || find_relative_url_root(options) || ''
+      end
+
+      def find_relative_url_root(options)
+        options.delete(:relative_url_root) || relative_url_root
       end
 
       def path_for(options, route_name = nil)