file_handle_cache: Move RabbitMQ-specific functions to rabbit_fhc_helpers

Those are the functions to clear the read buffer cache in various
internal RabbitMQ processes. file_handle_cache still reads the
`fhc_read_buffering` and `fhc_write_buffering` rabbit application
variables, though.

Thus, this change almost removes a dependency of file_handle_cache on
the broker. Reading the configuration variable is acceptable for now so
this will allow us to move file_handle_cache to rabbitmq-common.

[#118490793]

Author: dumbbell

src/file_handle_cache.erl

@@ -149,7 +149,7 @@
          open_with_absolute_path/3]).
 -export([obtain/0, obtain/1, release/0, release/1, transfer/1, transfer/2,
          set_limit/1, get_limit/0, info_keys/0, with_handle/1, with_handle/2,
-         info/0, info/1, clear_read_cache/0]).
+         info/0, info/1, clear_read_cache/0, clear_process_read_cache/0]).
 -export([ulimit/0]).
 
 -export([start_link/0, start_link/2, init/1, handle_call/3, handle_cast/2,
@@ -165,8 +165,6 @@
 -define(CLIENT_ETS_TABLE, file_handle_cache_client).
 -define(ELDERS_ETS_TABLE, file_handle_cache_elders).
 
--include("rabbit.hrl"). % For #amqqueue record definition.
-
 %%----------------------------------------------------------------------------
 
 -record(file,
@@ -601,35 +599,7 @@ info() -> info(?INFO_KEYS).
 info(Items) -> gen_server2:call(?SERVER, {info, Items}, infinity).
 
 clear_read_cache() ->
-    case application:get_env(rabbit, fhc_read_buffering) of
-        {ok, true} ->
-            gen_server2:cast(?SERVER, clear_read_cache),
-            clear_vhost_read_cache(rabbit_vhost:list());
-        _ -> %% undefined or {ok, false}
-            ok
-    end.
-
-clear_vhost_read_cache([]) ->
-    ok;
-clear_vhost_read_cache([VHost | Rest]) ->
-    clear_queue_read_cache(rabbit_amqqueue:list(VHost)),
-    clear_vhost_read_cache(Rest).
-
-clear_queue_read_cache([]) ->
-    ok;
-clear_queue_read_cache([#amqqueue{pid = MPid, slave_pids = SPids} | Rest]) ->
-    %% Limit the action to the current node.
-    Pids = [P || P <- [MPid | SPids], node(P) =:= node()],
-    %% This function is executed in the context of the backing queue
-    %% process because the read buffer is stored in the process
-    %% dictionary.
-    Fun = fun(_, State) ->
-                  _ = clear_process_read_cache(),
-                  State
-          end,
-    [rabbit_amqqueue:run_backing_queue(Pid, rabbit_variable_queue, Fun)
-     || Pid <- Pids],
-    clear_queue_read_cache(Rest).
+    gen_server2:cast(?SERVER, clear_read_cache).
 
 clear_process_read_cache() ->
     [

src/rabbit_fhc_helpers.erl

@@ -0,0 +1,52 @@
+%% The contents of this file are subject to the Mozilla Public License
+%% Version 1.1 (the "License"); you may not use this file except in
+%% compliance with the License. You may obtain a copy of the License
+%% at http://www.mozilla.org/MPL/
+%%
+%% Software distributed under the License is distributed on an "AS IS"
+%% basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
+%% the License for the specific language governing rights and
+%% limitations under the License.
+%%
+%% The Original Code is RabbitMQ.
+%%
+%% The Initial Developer of the Original Code is GoPivotal, Inc.
+%% Copyright (c) 2007-2017 Pivotal Software, Inc.  All rights reserved.
+%%
+
+-module(rabbit_fhc_helpers).
+
+-export([clear_read_cache/0]).
+
+-include("rabbit.hrl"). % For #amqqueue record definition.
+
+clear_read_cache() ->
+    case application:get_env(rabbit, fhc_read_buffering) of
+        {ok, true} ->
+            file_handle_cache:clear_read_cache(),
+            clear_vhost_read_cache(rabbit_vhost:list());
+        _ -> %% undefined or {ok, false}
+            ok
+    end.
+
+clear_vhost_read_cache([]) ->
+    ok;
+clear_vhost_read_cache([VHost | Rest]) ->
+    clear_queue_read_cache(rabbit_amqqueue:list(VHost)),
+    clear_vhost_read_cache(Rest).
+
+clear_queue_read_cache([]) ->
+    ok;
+clear_queue_read_cache([#amqqueue{pid = MPid, slave_pids = SPids} | Rest]) ->
+    %% Limit the action to the current node.
+    Pids = [P || P <- [MPid | SPids], node(P) =:= node()],
+    %% This function is executed in the context of the backing queue
+    %% process because the read buffer is stored in the process
+    %% dictionary.
+    Fun = fun(_, State) ->
+                  _ = file_handle_cache:clear_process_read_cache(),
+                  State
+          end,
+    [rabbit_amqqueue:run_backing_queue(Pid, rabbit_variable_queue, Fun)
+     || Pid <- Pids],
+    clear_queue_read_cache(Rest).