Use the count feature of i18n to fix plurals

config/locales/en.yml

@@ -100,7 +100,10 @@ en:
         unauthorized_client: 'The client is not authorized to perform this request using this method.'
         access_denied: 'The resource owner or authorization server denied the request.'
         invalid_scope: 'The requested scope is invalid, unknown, or malformed.'
-        invalid_code_challenge_method: 'The code challenge method must be one of %{challenge_methods}.'
+        invalid_code_challenge_method:
+          zero: 'There are no acceptable code_challenge methods'
+          one: 'The code challenge method must be %{challenge_methods}.'
+          other: 'The code challenge method must be one of %{challenge_methods}.'
         server_error: 'The authorization server encountered an unexpected condition which prevented it from fulfilling the request.'
         temporarily_unavailable: 'The authorization server is currently unable to handle the request due to a temporary overloading or maintenance of the server.'
 

lib/doorkeeper/errors.rb

@@ -51,8 +51,10 @@ def self.name_for_response
 
     class InvalidCodeChallengeMethod < BaseResponseError
       def self.translate_options
+        challenge_methods = Doorkeeper.config.pkce_code_challenge_methods_supported
         {
-          challenge_methods: Doorkeeper.config.pkce_code_challenge_methods_supported.join(", ")
+          challenge_methods: challenge_methods.join(", "),
+          count: challenge_methods.length
         }
       end
     end

spec/lib/oauth/pre_authorization_spec.rb

@@ -331,6 +331,22 @@
         expect(pre_auth).not_to be_authorizable
       end
 
+      context "when pkce_code_challenge_methods is set to none" do
+        before do
+          Doorkeeper.configure do
+            pkce_code_challenge_methods []
+          end
+        end
+
+        it "rejects plain as a code_challenge_method" do
+          attributes[:code_challenge] = "a45a9fea-0676-477e-95b1-a40f72ac3cfb"
+          attributes[:code_challenge_method] = "plain"
+
+          expect(pre_auth).to_not be_authorizable
+          expect(pre_auth.error_response.description).to eq("There are no acceptable code_challenge methods")
+        end
+      end
+
       context "when pkce_code_challenge_methods is set to only S256" do
         before do
           Doorkeeper.configure do
@@ -350,7 +366,7 @@
           attributes[:code_challenge_method] = "plain"
 
           expect(pre_auth).to_not be_authorizable
-          expect(pre_auth.error_response.description).to eq("The code challenge method must be one of S256.")
+          expect(pre_auth.error_response.description).to eq("The code challenge method must be S256.")
         end
       end