Add file_handle_cache FD reservations

deps/rabbit/src/rabbit_classic_queue_index_v2.erl

@@ -393,6 +393,7 @@ terminate(VHost, Terms, State0 = #mqistate { dir = Dir,
         ok = file:sync(Fd),
         ok = file:close(Fd)
     end, OpenFds),
+    file_handle_cache:release_reservation(),
     %% Write recovery terms for faster recovery.
     rabbit_recovery_terms:store(VHost, filename:basename(Dir),
                                 [{mqi_state, {?VERSION, Segments}} | Terms]),
@@ -408,6 +409,7 @@ delete_and_terminate(State = #mqistate { dir = Dir,
     _ = maps:map(fun(_, Fd) ->
         ok = file:close(Fd)
     end, OpenFds),
+    file_handle_cache:release_reservation(),
     %% Erase the data on disk.
     ok = erase_index_dir(Dir),
     State#mqistate{ segments = #{},
@@ -484,9 +486,18 @@ new_segment_file(Segment, State = #mqistate{ segments = Segments }) ->
 %% using too many FDs when the consumer lags a lot. We
 %% limit at 4 because we try to keep up to 2 for reading
 %% and 2 for writing.
-reduce_fd_usage(_, State = #mqistate{ fds = OpenFds })
+reduce_fd_usage(SegmentToOpen, State = #mqistate{ fds = OpenFds })
         when map_size(OpenFds) < 4 ->
+    %% The only case where we need to update reservations is
+    %% when we are opening a segment that wasn't already open,
+    %% and we are not closing another segment at the same time.
+    case OpenFds of
+        #{SegmentToOpen := _} ->
             State;
+        _ ->
+            file_handle_cache:set_reservation(map_size(OpenFds) + 1),
+            State
+    end;
 reduce_fd_usage(SegmentToOpen, State = #mqistate{ fds = OpenFds0 }) ->
     case OpenFds0 of
         #{SegmentToOpen := _} ->
@@ -719,6 +730,10 @@ delete_segment(Segment, State0 = #mqistate{ fds = OpenFds0 }) ->
     State = case maps:take(Segment, OpenFds0) of
         {Fd, OpenFds} ->
             ok = file:close(Fd),
+            case map_size(OpenFds) of
+                0 -> file_handle_cache:release_reservation();
+                N -> file_handle_cache:set_reservation(N)
+            end,
             State0#mqistate{ fds = OpenFds };
         error ->
             State0

deps/rabbit/test/backing_queue_SUITE.erl

@@ -124,8 +124,8 @@ setup_file_handle_cache(Config) ->
 
 setup_file_handle_cache1() ->
     %% FIXME: Why are we doing this?
-    application:set_env(rabbit, file_handles_high_watermark, 10),
+    application:set_env(rabbit, file_handles_high_watermark, 100),
-    ok = file_handle_cache:set_limit(10),
+    ok = file_handle_cache:set_limit(100),
     ok.
 
 end_per_group(Group, Config) ->