Make it possible to disable the queue manager

src/wpool.erl

@@ -151,6 +151,12 @@
 %%
 %% This option can take values `lifo' or `fifo'. Defaults to `fifo'.
 
+-type enable_queues() :: boolean().
+%% A boolean value determining if `queue_manager' should be started for queueing requests.
+%%
+%% Defaults to `true'.
+%% Note that disabling this will disable `available_worker' and `next_available_worker' strategies.
+
 -type enable_callbacks() :: boolean().
 %% A boolean value determining if `event_manager' should be started for callback modules.
 %%
@@ -202,6 +208,7 @@
     {pool_sup_period, pool_sup_period()} |
     {queue_type, queue_type()} |
     {enable_callbacks, enable_callbacks()} |
+    {enable_queues, enable_queues()} |
     {callbacks, callbacks()}.
 %% Options that can be provided to a new pool.
 %%
@@ -221,6 +228,7 @@
                      pool_sup_period => pool_sup_period(),
                      queue_type => queue_type(),
                      enable_callbacks => enable_callbacks(),
+                     enable_queues => enable_queues(),
                      callbacks => callbacks(),
                      _ => _}.
 %% Options that can be provided to a new pool.

src/wpool_pool.erl

@@ -366,8 +366,9 @@ init({Name, Options}) ->
     _Wpool = store_wpool(Name, Size, Options),
 
     WorkerOpts0 =
-        [{queue_manager, QueueManagerName}, {time_checker, TimeCheckerName}
-         | maybe_event_manager(Options, {event_manager, EventManagerName})],
+        [{time_checker, TimeCheckerName}]
+        ++ maybe_queue_manager(Options, {queue_manager, QueueManagerName})
+        ++ maybe_event_manager(Options, {event_manager, EventManagerName}),
     WorkerOpts =
         maps:merge(
             maps:from_list(WorkerOpts0), Options),
@@ -406,7 +407,8 @@ init({Name, Options}) ->
          [wpool_process_sup]},
 
     Children =
-        [TimeCheckerSpec, QueueManagerSpec]
+        [TimeCheckerSpec]
+        ++ maybe_queue_manager(Options, QueueManagerSpec)
         ++ maybe_event_manager(Options, EventManagerSpec)
         ++ [ProcessSupSpec],
 
@@ -566,6 +568,11 @@ build_wpool(Name) ->
             undefined
     end.
 
+maybe_queue_manager(#{enable_queues := false}, _) ->
+    [];
+maybe_queue_manager(_, Item) ->
+    [Item].
+
 maybe_event_manager(#{enable_callbacks := true}, Item) ->
     [Item];
 maybe_event_manager(_, _) ->

src/wpool_queue_manager.erl

@@ -141,7 +141,12 @@ worker_dead(QueueManager, Worker) ->
 %% @see wpool_pool:stats/1
 -spec pending_task_count(queue_mgr()) -> non_neg_integer().
 pending_task_count(QueueManager) ->
-    gen_server:call(QueueManager, pending_task_count).
+    try
+        gen_server:call(QueueManager, pending_task_count)
+    catch
+        _:{noproc, _} ->
+            0
+    end.
 
 %%%===================================================================
 %%% gen_server callbacks

test/wpool_pool_SUITE.erl

@@ -27,7 +27,7 @@
 -export([init_per_suite/1, end_per_suite/1, init_per_testcase/2, end_per_testcase/2]).
 -export([stop_worker/1, best_worker/1, next_worker/1, random_worker/1, available_worker/1,
          hash_worker/1, custom_worker/1, next_available_worker/1, wpool_record/1,
-         queue_type_fifo/1, queue_type_lifo/1, get_workers/1]).
+         queue_type_fifo/1, queue_type_lifo/1, get_workers/1, no_queue_manager/1]).
 -export([manager_crash/1, super_fast/1, mess_up_with_store/1]).
 
 -elvis([{elvis_style, no_block_expressions, disable}]).
@@ -50,6 +50,9 @@ end_per_suite(Config) ->
     Config.
 
 -spec init_per_testcase(atom(), config()) -> config().
+init_per_testcase(no_queue_manager = TestCase, Config) ->
+    {ok, _} = wpool:start_pool(TestCase, [{workers, 1}, {enable_queues, false}]),
+    Config;
 init_per_testcase(queue_type_lifo = TestCase, Config) ->
     {ok, _} = wpool:start_pool(TestCase, [{workers, 1}, {queue_type, lifo}]),
     Config;
@@ -454,6 +457,37 @@ manager_crash(_Config) ->
 
     {comment, []}.
 
+-spec no_queue_manager(config()) -> {comment, []}.
+no_queue_manager(_Config) ->
+    Pool = no_queue_manager,
+
+    ct:log("Check that the pool is working"),
+    {ok, ok} = wpool:call(Pool, {io, format, ["ok!~n"]}, random_worker),
+    {ok, ok} = wpool:call(Pool, {io, format, ["ok!~n"]}, next_worker),
+    {ok, ok} = wpool:call(Pool, {io, format, ["ok!~n"]}, {hash_worker, self()}),
+
+    ct:log("Impossible task"),
+    Self = self(),
+    try wpool:call(Pool, {erlang, send, [Self, something]}, available_worker, 0) of
+        R ->
+            ct:fail("Unexpected ~p", [R])
+    catch
+        _:no_workers ->
+            ok
+    end,
+
+    0 = proplists:get_value(total_message_queue_len, wpool:stats(Pool)),
+
+    ct:log("Wait a second and nothing gets here"),
+    receive
+        X ->
+            ct:fail("Unexpected ~p", [X])
+    after 1000 ->
+        ok
+    end,
+
+    {comment, []}.
+
 -spec super_fast(config()) -> {comment, []}.
 super_fast(_Config) ->
     Pool = super_fast,