FIX: Do not preview chat channels to read-only users (#21700)

We want to simplify this case as it contains a lot of rabbit holes.

plugins/chat/app/controllers/chat/api/channels_controller.rb

@@ -121,7 +121,7 @@ class Chat::Api::ChannelsController < Chat::ApiController
     @channel ||=
       begin
         channel = Chat::Channel.find(params.require(:channel_id))
-        guardian.ensure_can_preview_chat_channel!(channel)
+        guardian.ensure_can_join_chat_channel!(channel)
         channel
       end
   end

plugins/chat/lib/chat/channel_fetcher.rb

@@ -230,7 +230,7 @@ module Chat
       end
 
       raise Discourse::NotFound if chat_channel.blank?
-      raise Discourse::InvalidAccess if !guardian.can_preview_chat_channel?(chat_channel)
+      raise Discourse::InvalidAccess if !guardian.can_join_chat_channel?(chat_channel)
       chat_channel
     end
   end

plugins/chat/spec/requests/chat_controller_spec.rb

@@ -1125,7 +1125,7 @@ RSpec.describe Chat::ChatController do
       channel = Fabricate(:category_channel, chatable: Fabricate(:category))
       message = Fabricate(:chat_message, chat_channel: channel)
 
-      Guardian.any_instance.expects(:can_preview_chat_channel?).with(channel)
+      Guardian.any_instance.expects(:can_join_chat_channel?).with(channel)
 
       sign_in(Fabricate(:user))
       get "/chat/message/#{message.id}.json"
@@ -1141,7 +1141,7 @@ RSpec.describe Chat::ChatController do
     before { sign_in(user) }
 
     it "ensures message's channel can be seen" do
-      Guardian.any_instance.expects(:can_preview_chat_channel?).with(channel)
+      Guardian.any_instance.expects(:can_join_chat_channel?).with(channel)
       get "/chat/lookup/#{message.id}.json", params: { chat_channel_id: channel.id }
     end
 

plugins/chat/spec/system/jit_messages_spec.rb

@@ -50,35 +50,6 @@ RSpec.describe "JIT messages", type: :system, js: true do
         )
       end
     end
-
-    context "when user can’t access a non read_restrictd channel" do
-      let!(:everyone) { Group.find(Group::AUTO_GROUPS[:everyone]) }
-      fab!(:category) { Fabricate(:category) }
-      fab!(:readonly_channel) { Fabricate(:category_channel, chatable: category) }
-
-      before do
-        Fabricate(
-          :category_group,
-          category: category,
-          group: everyone,
-          permission_type: CategoryGroup.permission_types[:readonly],
-        )
-        everyone.add(other_user)
-        readonly_channel.add(current_user)
-      end
-
-      it "displays a mention warning" do
-        Jobs.run_immediately!
-
-        chat.visit_channel(readonly_channel)
-        channel.send_message("hi @#{other_user.username}")
-
-        expect(page).to have_content(
-          I18n.t("js.chat.mention_warning.cannot_see", username: other_user.username),
-          wait: 5,
-        )
-      end
-    end
   end
 
   context "when category channel permission is readonly for everyone" do

plugins/chat/spec/system/visit_channel_spec.rb

@@ -110,17 +110,10 @@ RSpec.describe "Visit channel", type: :system, js: true do
           )
         end
 
-        it "doesn't allow user to join it" do
-          chat.visit_channel(readonly_category_channel_1)
+        it "shows an error" do
+          chat.visit_channel(inaccessible_dm_channel_1)
 
-          expect(page).not_to have_content(I18n.t("js.chat.channel_settings.join_channel"))
-        end
-
-        it "shows a preview of the channel" do
-          chat.visit_channel(readonly_category_channel_1)
-
-          expect(page).to have_content(readonly_category_channel_1.name)
-          expect(chat).to have_message(message_1)
+          expect(page).to have_content(I18n.t("invalid_access"))
         end
       end