Use more specific check for :format in route path

The current check for whether to add an optional format to the path
is very lax and will match things like `:format_id` where there are
nested resources, e.g:

    resources :formats do
      resources :items
    end

Fix this by using a more restrictive regex pattern that looks for
the patterns `(.:format)`, `.:format` or `/` at the end of the path.
Note that we need to allow for multiple closing parenthesis since
the route may be of this form:

    get "/books(/:action(.:format))", controller: "books"

This probably isn't what's intended since it means that the default
index action route doesn't support a format but we have a test for
it so we need to allow it.

Fixes #28517.

2 files changed, 20 insertions, 1 deletions

diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 8ad17504ae..74904e3d45 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -54,6 +54,7 @@ module ActionDispatch
 
       class Mapping #:nodoc:
         ANCHOR_CHARACTERS_REGEX = %r{\A(\\A|\^)|(\\Z|\\z|\$)\Z}
+        OPTIONAL_FORMAT_REGEX = %r{(?:\(\.:format\)+|\.:format|/)\Z}
 
         attr_reader :requirements, :defaults
         attr_reader :to, :default_controller, :default_action
@@ -93,7 +94,7 @@ module ActionDispatch
         end
 
         def self.optional_format?(path, format)
-          format != false && !path.include?(":format") && !path.end_with?("/")
+          format != false && path !~ OPTIONAL_FORMAT_REGEX
         end
 
         def initialize(set, ast, defaults, controller, default_action, modyoule, to, formatted, scope_constraints, blocks, via, options_constraints, anchor, options)
diff --git a/actionpack/test/dispatch/routing_test.rb b/actionpack/test/dispatch/routing_test.rb
index 64818e6ca1..fdc47743fa 100644
--- a/actionpack/test/dispatch/routing_test.rb
+++ b/actionpack/test/dispatch/routing_test.rb
@@ -3706,6 +3706,24 @@ class TestRoutingMapper < ActionDispatch::IntegrationTest
     assert_equal "/bar", bar_root_path
   end
 
+  def test_nested_routes_under_format_resource
+    draw do
+      resources :formats do
+        resources :items
+      end
+    end
+
+    get "/formats/1/items.json"
+    assert_equal 200, @response.status
+    assert_equal "items#index", @response.body
+    assert_equal "/formats/1/items.json", format_items_path(1, :json)
+
+    get "/formats/1/items/2.json"
+    assert_equal 200, @response.status
+    assert_equal "items#show", @response.body
+    assert_equal "/formats/1/items/2.json", format_item_path(1, 2, :json)
+  end
+
 private
 
   def draw(&block)