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%% 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) 2011-2015 Pivotal Software, Inc. All rights reserved.
%%
-module(pmon).
%% Process Monitor
%% ================
%%
%% This module monitors processes so that every process has at most
%% 1 monitor.
%% Processes monitored can be dynamically added and removed.
%%
%% Unlike erlang:[de]monitor* functions, this module
%% provides basic querying capability and avoids contacting down nodes.
%%
%% It is used to monitor nodes, queue mirrors, and by
%% the queue collector, among other things.
-export([new/0, new/1, monitor/2, monitor_all/2, demonitor/2,
is_monitored/2, erase/2, monitored/1, is_empty/1]).
-compile({no_auto_import, [monitor/2]}).
-record(state, {dict, module}).
-ifdef(use_specs).
%%----------------------------------------------------------------------------
-export_type([?MODULE/0]).
-opaque(?MODULE() :: #state{dict :: dict:dict(),
module :: atom()}).
-type(item() :: pid() | {atom(), node()}).
-spec(new/0 :: () -> ?MODULE()).
-spec(new/1 :: ('erlang' | 'delegate') -> ?MODULE()).
-spec(monitor/2 :: (item(), ?MODULE()) -> ?MODULE()).
-spec(monitor_all/2 :: ([item()], ?MODULE()) -> ?MODULE()).
-spec(demonitor/2 :: (item(), ?MODULE()) -> ?MODULE()).
-spec(is_monitored/2 :: (item(), ?MODULE()) -> boolean()).
-spec(erase/2 :: (item(), ?MODULE()) -> ?MODULE()).
-spec(monitored/1 :: (?MODULE()) -> [item()]).
-spec(is_empty/1 :: (?MODULE()) -> boolean()).
-endif.
new() -> new(erlang).
new(Module) -> #state{dict = dict:new(),
module = Module}.
monitor(Item, S = #state{dict = M, module = Module}) ->
case dict:is_key(Item, M) of
true -> S;
false -> case node_alive_shortcut(Item) of
true -> Ref = Module:monitor(process, Item),
S#state{dict = dict:store(Item, Ref, M)};
false -> self() ! {'DOWN', fake_ref, process, Item,
nodedown},
S
end
end.
monitor_all([], S) -> S; %% optimisation
monitor_all([Item], S) -> monitor(Item, S); %% optimisation
monitor_all(Items, S) -> lists:foldl(fun monitor/2, S, Items).
demonitor(Item, S = #state{dict = M, module = Module}) ->
case dict:find(Item, M) of
{ok, MRef} -> Module:demonitor(MRef),
S#state{dict = dict:erase(Item, M)};
error -> M
end.
is_monitored(Item, #state{dict = M}) -> dict:is_key(Item, M).
erase(Item, S = #state{dict = M}) -> S#state{dict = dict:erase(Item, M)}.
monitored(#state{dict = M}) -> dict:fetch_keys(M).
is_empty(#state{dict = M}) -> dict:size(M) == 0.
%%----------------------------------------------------------------------------
%% We check here to see if the node is alive in order to avoid trying
%% to connect to it if it isn't - this can cause substantial
%% slowdowns. We can't perform this shortcut if passed {Name, Node}
%% since we would need to convert that into a pid for the fake 'DOWN'
%% message, so we always return true here - but that's OK, it's just
%% an optimisation.
node_alive_shortcut(P) when is_pid(P) ->
lists:member(node(P), [node() | nodes()]);
node_alive_shortcut({_Name, _Node}) ->
true.
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