Request Forgery takes relative paths into account

Passing relative paths into form_for and related helpers led to invalid
token generations, as the tokens did not match the request.path on the
POST endpoint. Variants, such as:

form_for url:
* ""
* "./"
* "./post_one"
* "post_one"

are now handled according to [RFC 3986 5.2 - 5.4](https://tools.ietf.org/html/rfc3986#section-5.2)

Limitations: double dots are not handled (../../path)

relevant issue: #31191

actionpack/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Request Forgery takes relative paths into account.
+
+    *Stefan Wienert*
+
 *   Add ".test" as a default allowed host in development to ensure smooth golden-path setup with puma.dev.
 
     *DHH*

actionpack/lib/action_controller/metal/request_forgery_protection.rb

@@ -635,6 +635,15 @@ def valid_request_origin? # :doc:
 
       def normalize_action_path(action_path) # :doc:
         uri = URI.parse(action_path)
+
+        if uri.relative? && (action_path.blank? || !action_path.start_with?("/"))
+          uri = URI.parse(request.path)
+          # add the action path to the request.path
+          uri.path += "/#{action_path}"
+          # relative path with "./path"
+          uri.path.gsub!("/./", "/")
+        end
+
         uri.path.chomp("/")
       end
 

actionpack/test/controller/request_forgery_protection_test.rb

@@ -1112,6 +1112,45 @@ def test_ignores_trailing_slash_during_generation
     assert_response :success
   end
 
+  def test_handles_empty_path_as_request_path
+    get :index, params: { form_path: "" }
+
+    form_token = assert_presence_and_fetch_form_csrf_token
+
+    # This is required because PATH_INFO isn't reset between requests.
+    @request.env["PATH_INFO"] = "/per_form_tokens"
+    assert_nothing_raised do
+      post :post_one, params: { custom_authenticity_token: form_token }
+    end
+    assert_response :success
+  end
+
+  def test_handles_relative_paths
+    get :index, params: { form_path: "post_one" }
+
+    form_token = assert_presence_and_fetch_form_csrf_token
+
+    # This is required because PATH_INFO isn't reset between requests.
+    @request.env["PATH_INFO"] = "/per_form_tokens/post_one"
+    assert_nothing_raised do
+      post :post_one, params: { custom_authenticity_token: form_token }
+    end
+    assert_response :success
+  end
+
+  def test_handles_relative_paths_with_dot
+    get :index, params: { form_path: "./post_one" }
+
+    form_token = assert_presence_and_fetch_form_csrf_token
+
+    # This is required because PATH_INFO isn't reset between requests.
+    @request.env["PATH_INFO"] = "/per_form_tokens/post_one"
+    assert_nothing_raised do
+      post :post_one, params: { custom_authenticity_token: form_token }
+    end
+    assert_response :success
+  end
+
   def test_ignores_origin_during_generation
     get :index, params: { form_path: "https://example.com/per_form_tokens/post_one/" }