Fix after_omniauth_failure_path_for for engine-mounted Devise

app/controllers/active_admin/oidc/devise/omniauth_callbacks_controller.rb

@@ -74,14 +74,18 @@ def after_sign_in_path_for(resource)
           stored_location_for(resource) || '/admin'
         end
 
-        # Devise's default `after_omniauth_failure_path_for` calls
-        # `new_session_path(scope)`, a URL helper Devise only generates
-        # when :database_authenticatable mounts session routes. The
-        # engine mounts `new_<scope>_session_path` itself (see
-        # `mount_oidc_sessions_routes` initializer) regardless of which
-        # modules are loaded, so we always route through that helper.
+        # Devise's `new_session_path(scope)` is only generated when
+        # `:database_authenticatable` is in the mapping's `used_helpers`,
+        # so an OIDC-only model never gets it. The engine mounts
+        # `new_<scope>_session_path` itself, but the helper lives on
+        # whichever route set the mapping's `router_name` points at —
+        # main_app by default, or `<engine_name>` for hosts that mount
+        # Devise inside a Rails engine. Replicate Devise's own dispatch
+        # so the right context is asked.
         def after_omniauth_failure_path_for(scope)
-          public_send(:"new_#{scope}_session_path")
+          router_name = ::Devise.mappings[scope].router_name
+          context     = router_name ? send(router_name) : self
+          context.public_send(:"new_#{scope}_session_path")
         end
       end
     end

spec/engine/features/engine_mounted_login_spec.rb

@@ -46,4 +46,23 @@
     visit "/admin"
     expect(page).to have_current_path("/admin/login")
   end
+
+  context "OmniAuth failure path" do
+    around do |ex|
+      saved = OmniAuth.config.mock_auth[:oidc]
+      OmniAuth.config.mock_auth[:oidc] = :invalid_credentials
+      ex.run
+      OmniAuth.config.mock_auth[:oidc] = saved
+    end
+
+    scenario "failed OmniAuth callback lands back on the SSO landing page" do
+      # OmniauthCallbacksController#after_omniauth_failure_path_for has
+      # to resolve `new_<scope>_session_path` against the engine's
+      # route set (Devise.router_name pins helpers there) — calling the
+      # helper directly on the controller would raise NoMethodError.
+      visit "/admin/login"
+      click_button ActiveAdmin::Oidc.config.login_button_label
+      expect(page).to have_current_path("/admin/login")
+    end
+  end
 end

spec/isolated/features/isolated_engine_login_spec.rb

@@ -37,4 +37,23 @@
     expect(page.status_code).to eq(200)
     expect(page.body).to include(ActiveAdmin::Oidc.config.login_button_label)
   end
+
+  context "OmniAuth failure path" do
+    around do |ex|
+      saved = OmniAuth.config.mock_auth[:oidc]
+      OmniAuth.config.mock_auth[:oidc] = :invalid_credentials
+      ex.run
+      OmniAuth.config.mock_auth[:oidc] = saved
+    end
+
+    scenario "failed OmniAuth callback lands back on the SSO landing page" do
+      # As in the non-isolated case, the failure handler must resolve
+      # the session helper through the engine — and via the isolated
+      # engine's mount prefix the effective path stays `/admin/login`
+      # because `config.login_path = '/login'` is engine-relative.
+      visit "/admin/login"
+      click_button ActiveAdmin::Oidc.config.login_button_label
+      expect(page).to have_current_path("/admin/login")
+    end
+  end
 end