Restore safe parts of 0278980 CQ shared store GC improvements

Restore three independent improvements from the reverted commit that
are unrelated to the broken current_file_removes mechanism:

- Relax index_update_fields assertion: true= -> _= so a missing key
  does not crash the process
- Add prioritise_cast/3 to rabbit_msg_store_gc so delete requests are
  processed before compaction requests, avoiding unnecessary compaction
  of files that are already pending deletion
- compact_file/2 early-exit guard was already present after the revert

Author: lukebakken

deps/rabbit/src/rabbit_msg_store.erl

@@ -1180,18 +1180,16 @@ remove_message(MsgId, CRef, State = #msstate{ index_ets = IndexEts }) ->
           when RefCount > 0 ->
             %% only update field, otherwise bad interaction with
             %% concurrent GC
-            Dec = fun () -> index_update_ref_counter(IndexEts, MsgId, -1) end,
+            ok = index_update_ref_counter(IndexEts, MsgId, -1),
             case RefCount of
                 %% Don't remove from cur_file_cache_ets here because
                 %% there may be further writes in the mailbox for the
                 %% same msg.
-                1 -> ok = Dec(),
-                     delete_file_if_empty(
+                1 -> delete_file_if_empty(
                        File, gc_candidate(File,
                          adjust_valid_total_size(
                                File, -TotalSize, State)));
-                _ -> ok = Dec(),
-                     gc_candidate(File, State)
+                _ -> gc_candidate(File, State)
             end
     end.
 
@@ -1656,7 +1654,7 @@ index_update(IndexEts, Obj) ->
     ok.
 
 index_update_fields(IndexEts, Key, Updates) ->
-    true = ets:update_element(IndexEts, Key, Updates),
+    _ = ets:update_element(IndexEts, Key, Updates),
     ok.
 
 index_update_ref_counter(IndexEts, Key, RefCount) ->

deps/rabbit/src/rabbit_msg_store_gc.erl

@@ -12,7 +12,7 @@
 -export([start_link/1, compact/2, truncate/4, delete/2, stop/1]).
 
 -export([init/1, handle_call/3, handle_cast/2, handle_info/2,
-         terminate/2, code_change/3]).
+         terminate/2, code_change/3, prioritise_cast/3]).
 
 -record(state,
         { pending,
@@ -51,6 +51,10 @@ delete(Server, File) ->
 stop(Server) ->
     gen_server2:call(Server, stop, infinity).
 
+%% TODO replace with priority messages for OTP28+
+prioritise_cast({delete, _}, _Len, _State) -> 5;
+prioritise_cast(_, _Len, _State) -> 0.
+
 %%----------------------------------------------------------------------------
 
 init([MsgStoreState]) ->