feature: Add discovery endpoint

app/controllers/doorkeeper/openid_connect/discovery_controller.rb

@@ -0,0 +1,53 @@
+module Doorkeeper
+  module OpenidConnect
+    class DiscoveryController < ::Doorkeeper::ApplicationController
+      include Doorkeeper::Helpers::Controller
+
+      def show
+        render json: provider_configuration
+      end
+
+      private
+
+      def provider_configuration
+        doorkeeper = ::Doorkeeper.configuration
+        openid_connect = ::Doorkeeper::OpenidConnect.configuration
+
+        {
+          issuer: openid_connect.issuer,
+          authorization_endpoint: oauth_authorization_url(protocol: :https),
+          token_endpoint: oauth_token_url(protocol: :https),
+          userinfo_endpoint: oauth_userinfo_url(protocol: :https),
+
+          # TODO: implement controller
+          #jwks_uri: oauth_keys_url(protocol: :https),
+
+          scopes_supported: doorkeeper.scopes,
+
+          # TODO: support id_token response type
+          response_types_supported: doorkeeper.authorization_response_types,
+          response_modes_supported: [ 'query', 'fragment' ],
+
+          token_endpoint_auth_methods_supported: [
+            'client_secret_basic',
+            'client_secret_post',
+
+            # TODO: look into doorkeeper-jwt_assertion for these
+            #'client_secret_jwt',
+            #'private_key_jwt'
+          ],
+
+          # TODO: make this configurable
+          subject_types_supported: [
+            'public',
+          ],
+
+          # TODO: make this configurable
+          id_token_signing_alg_values_supported: [
+            'RS256',
+          ],
+        }
+      end
+    end
+  end
+end

lib/doorkeeper/openid_connect/rails/routes.rb

@@ -26,6 +26,10 @@ def generate_routes!(options)
           routes.scope options[:scope] || 'oauth', as: 'oauth' do
             map_route(:userinfo, :userinfo_routes)
           end
+
+          routes.scope as: 'oauth' do
+            map_route(:discovery, :discovery_routes)
+          end
         end
 
         private
@@ -44,6 +48,15 @@ def userinfo_routes(mapping)
             controller: mapping[:controllers]
           )
         end
+
+        def discovery_routes(mapping)
+          routes.resource(
+            :discovery,
+            path: '.well-known/openid-configuration',
+            only: [:show], as: mapping[:as],
+            controller: mapping[:controllers]
+          )
+        end
       end
     end
   end

lib/doorkeeper/openid_connect/rails/routes/mapping.rb

@@ -7,11 +7,13 @@ class Mapping
 
           def initialize
             @controllers = {
-              userinfo: 'doorkeeper/openid_connect/userinfo'
+              userinfo: 'doorkeeper/openid_connect/userinfo',
+              discovery: 'doorkeeper/openid_connect/discovery'
             }
 
             @as = {
-              userinfo: :userinfo
+              userinfo: :userinfo,
+              discovery: :discovery
             }
 
             @skips = []

spec/controllers/doorkeeper/openid_connect/discovery_controller_spec.rb

@@ -0,0 +1,35 @@
+require 'rails_helper'
+
+describe Doorkeeper::OpenidConnect::DiscoveryController, type: :controller do
+  describe '#show' do
+    it 'returns the provider configuration' do
+      get :show
+      configuration = JSON.parse(response.body)
+
+      expect(configuration.sort).to eq({
+        'issuer' => 'dummy',
+        'authorization_endpoint' => 'https://test.host/oauth/authorize',
+        'token_endpoint' => 'https://test.host/oauth/token',
+        'userinfo_endpoint' => 'https://test.host/oauth/userinfo',
+
+        'scopes_supported' => ['openid'],
+
+        'response_types_supported' => ['code'],
+        'response_modes_supported' => ['query', 'fragment'],
+
+        'token_endpoint_auth_methods_supported' => [
+          'client_secret_basic',
+          'client_secret_post',
+        ],
+
+        'subject_types_supported' => [
+          'public',
+        ],
+
+        'id_token_signing_alg_values_supported' => [
+          'RS256',
+        ],
+      }.sort)
+    end
+  end
+end

spec/dummy/config/initializers/doorkeeper_openid_connect.rb

@@ -1,4 +1,6 @@
 Doorkeeper::OpenidConnect.configure do
+  issuer 'dummy'
+
   resource_owner_from_access_token do |access_token|
     User.find_by(id: access_token.resource_owner_id)
   end

spec/lib/doorkeeper/openid_connect/routes_spec.rb

@@ -0,0 +1,17 @@
+require 'rails_helper'
+
+describe Doorkeeper::OpenidConnect::Rails::Routes, type: :routing do
+  it 'maps userinfo#show' do
+    expect(get: 'oauth/userinfo').to route_to(
+      controller: 'doorkeeper/openid_connect/userinfo',
+      action: 'show'
+    )
+  end
+
+  it 'maps discovery#show' do
+    expect(get: '.well-known/openid-configuration').to route_to(
+      controller: 'doorkeeper/openid_connect/discovery',
+      action: 'show'
+    )
+  end
+end