FIX: nullifies target message id when not readable (#24540)

This bug was very reproducible when your last read was a message you didn't read and an admin would delete it. When coming back to the channel you would get a not found, in this case we will now reset last read and present you the last message of the channel.

We could be more fancy and  try to detect the next readable message but that would be more code and complexity for such a rare case.

plugins/chat/app/services/chat/list_channel_messages.rb

@@ -84,11 +84,18 @@ module Chat
 
     def target_message_exists(channel:, guardian:, **)
       return true if context.target_message_id.blank?
+
       target_message =
         Chat::Message.with_deleted.find_by(id: context.target_message_id, chat_channel: channel)
       return false if target_message.blank?
+
       return true if !target_message.trashed?
-      target_message.user_id == guardian.user.id || guardian.is_staff?
+      if target_message.trashed? && target_message.user_id == guardian.user.id || guardian.is_staff?
+        return true
+      end
+
+      context.target_message_id = nil
+      true
     end
 
     def fetch_messages(channel:, contract:, guardian:, enabled_threads:, **)

plugins/chat/spec/services/chat/list_channel_messages_spec.rb

@@ -109,7 +109,9 @@ RSpec.describe Chat::ListChannelMessages do
       before { target_message.trash! }
 
       context "when user is regular" do
-        it { is_expected.to fail_a_policy(:target_message_exists) }
+        it "nullifies target_message_id" do
+          expect(result.target_message_id).to be_blank
+        end
       end
 
       context "when user is the message creator" do