Refactor handling of :action default in routing

The longstanding convention in Rails is that if the :action parameter
is missing or nil then it defaults to 'index'. Up until Rails 5.0.0.beta1
this was handled slightly differently than other routing defaults by
deleting it from the route options and adding it to the recall parameters.

With the recent focus of removing unnecessary duplications this has
exposed a problem in this strategy - we are now mutating the request's
path parameters and causing problems for later url generation. This will
typically affect url_for rather a named url helper since the latter
explicitly pass :controller, :action, etc.

The fix is to add a default for :action in the route class if the path
contains an :action segment and no default is passed. This change also
revealed an issue with the parameterized part expiry in that it doesn't
follow a right to left order - as soon as a dynamic segment is required
then all other segments become required.

Fixes #23019.

4 files changed, 13 insertions, 18 deletions

diff --git a/actionpack/lib/action_dispatch/journey/formatter.rb b/actionpack/lib/action_dispatch/journey/formatter.rb
index 0323360faa..200477b002 100644
--- a/actionpack/lib/action_dispatch/journey/formatter.rb
+++ b/actionpack/lib/action_dispatch/journey/formatter.rb
@@ -32,8 +32,13 @@ module ActionDispatch
 
           defaults       = route.defaults
           required_parts = route.required_parts
-          parameterized_parts.keep_if do |key, value|
-            (defaults[key].nil? && value.present?) || value.to_s != defaults[key].to_s || required_parts.include?(key)
+
+          route.parts.reverse_each do |key|
+            break if defaults[key].nil? && parameterized_parts[key].present?
+            break if parameterized_parts[key].to_s != defaults[key].to_s
+            break if required_parts.include?(key)
+
+            parameterized_parts.delete(key)
           end
 
           return [route.format(parameterized_parts), params]
diff --git a/actionpack/lib/action_dispatch/routing/inspector.rb b/actionpack/lib/action_dispatch/routing/inspector.rb
index 983f1daeb3..4ee5717b3e 100644
--- a/actionpack/lib/action_dispatch/routing/inspector.rb
+++ b/actionpack/lib/action_dispatch/routing/inspector.rb
@@ -33,11 +33,11 @@ module ActionDispatch
       end
 
       def controller
-        requirements[:controller] || ':controller'
+        parts.include?(:controller) ? ':controller' : requirements[:controller]
       end
 
       def action
-        requirements[:action] || ':action'
+        parts.include?(:action) ? ':action' : requirements[:action]
       end
 
       def internal?
diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index afbaa45d20..92205a89f6 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -137,6 +137,10 @@ module ActionDispatch
           @conditions = Hash[conditions]
           @defaults = formats[:defaults].merge(@defaults).merge(normalize_defaults(options))
 
+          if path_params.include?(:action) && !@requirements.key?(:action)
+            @defaults[:action] ||= 'index'
+          end
+
           @required_defaults = (split_options[:required_defaults] || []).map(&:first)
         end
 
diff --git a/actionpack/lib/action_dispatch/routing/route_set.rb b/actionpack/lib/action_dispatch/routing/route_set.rb
index 310e98f584..de291d249b 100644
--- a/actionpack/lib/action_dispatch/routing/route_set.rb
+++ b/actionpack/lib/action_dispatch/routing/route_set.rb
@@ -533,12 +533,10 @@ module ActionDispatch
           @recall      = recall
           @set         = set
 
-          normalize_recall!
           normalize_options!
           normalize_controller_action_id!
           use_relative_controller!
           normalize_controller!
-          normalize_action!
         end
 
         def controller
@@ -557,11 +555,6 @@ module ActionDispatch
           end
         end
 
-        # Set 'index' as default action for recall
-        def normalize_recall!
-          @recall[:action] ||= 'index'
-        end
-
         def normalize_options!
           # If an explicit :controller was given, always make :action explicit
           # too, so that action expiry works as expected for things like
@@ -615,13 +608,6 @@ module ActionDispatch
           end
         end
 
-        # Move 'index' action from options to recall
-        def normalize_action!
-          if @options[:action] == 'index'.freeze
-            @recall[:action] = @options.delete(:action)
-          end
-        end
-
         # Generates a path from routes, returns [path, params].
         # If no route is generated the formatter will raise ActionController::UrlGenerationError
         def generate