path: root/src/mod_mqtt.erl

%%%-------------------------------------------------------------------
%%% @author Evgeny Khramtsov <ekhramtsov@process-one.net>
%%% @copyright (C) 2002-2022 ProcessOne, SARL. All Rights Reserved.
%%%
%%% Licensed under the Apache License, Version 2.0 (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.apache.org/licenses/LICENSE-2.0
%%%
%%% Unless required by applicable law or agreed to in writing, software
%%% distributed under the License is distributed on an "AS IS" BASIS,
%%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
%%% See the License for the specific language governing permissions and
%%% limitations under the License.
%%%
%%%-------------------------------------------------------------------
-module(mod_mqtt).
-behaviour(p1_server).
-behaviour(gen_mod).
-behaviour(ejabberd_listener).
-dialyzer({no_improper_lists, join_filter/1}).

%% gen_mod API
-export([start/2, stop/1, reload/3, depends/2, mod_options/1, mod_opt_type/1]).
-export([mod_doc/0]).
%% gen_server callbacks
-export([init/1, handle_call/3, handle_cast/2, handle_info/2,
	 terminate/2, code_change/3]).
%% ejabberd_listener API
-export([start/3, start_link/3, listen_opt_type/1, listen_options/0, accept/1]).
%% ejabberd_http API
-export([socket_handoff/3]).
%% Legacy ejabberd_listener API
-export([become_controller/2, socket_type/0]).
%% API
-export([open_session/1, close_session/1, lookup_session/1,
	 publish/3, subscribe/4, unsubscribe/2, select_retained/4,
         check_publish_access/2, check_subscribe_access/2]).
%% ejabberd_hooks
-export([remove_user/2]).

-include("logger.hrl").
-include("mqtt.hrl").
-include("translate.hrl").

-define(MQTT_TOPIC_CACHE, mqtt_topic_cache).
-define(MQTT_PAYLOAD_CACHE, mqtt_payload_cache).

-type continuation() :: term().
-type seconds() :: non_neg_integer().

%% RAM backend callbacks
-callback init() -> ok | {error, any()}.
-callback open_session(jid:ljid()) -> ok | {error, db_failure}.
-callback close_session(jid:ljid()) -> ok | {error, db_failure}.
-callback lookup_session(jid:ljid()) -> {ok, pid()} | {error, notfound | db_failure}.
-callback get_sessions(binary(), binary()) -> [jid:ljid()].
-callback subscribe(jid:ljid(), binary(), sub_opts(), non_neg_integer()) -> ok | {error, db_failure}.
-callback unsubscribe(jid:ljid(), binary()) -> ok | {error, notfound | db_failure}.
-callback find_subscriber(binary(), binary() | continuation()) ->
          {ok, {pid(), qos()}, continuation()} | {error, notfound | db_failure}.
%% Disc backend callbacks
-callback init(binary(), gen_mod:opts()) -> ok | {error, any()}.
-callback publish(jid:ljid(), binary(), binary(), qos(), properties(), seconds()) ->
          ok | {error, db_failure}.
-callback delete_published(jid:ljid(), binary()) -> ok | {error, db_failure}.
-callback lookup_published(jid:ljid(), binary()) ->
          {ok, {binary(), qos(), properties(), seconds()}} |
          {error, notfound | db_failure}.
-callback list_topics(binary()) -> {ok, [binary()]} | {error, db_failure}.
-callback use_cache(binary()) -> boolean().
-callback cache_nodes(binary()) -> [node()].

-optional_callbacks([use_cache/1, cache_nodes/1]).

-record(state, {host :: binary()}).

%%%===================================================================
%%% API
%%%===================================================================
start(SockMod, Sock, ListenOpts) ->
    mod_mqtt_session:start(SockMod, Sock, ListenOpts).

start(Host, Opts) ->
    gen_mod:start_child(?MODULE, Host, Opts).

start_link(SockMod, Sock, ListenOpts) ->
    mod_mqtt_session:start_link(SockMod, Sock, ListenOpts).

stop(Host) ->
    gen_mod:stop_child(?MODULE, Host).

reload(_Host, _NewOpts, _OldOpts) ->
    ok.

depends(_Host, _Opts) ->
    [].

socket_type() ->
    raw.

become_controller(Pid, _) ->
    accept(Pid).

accept(Pid) ->
    mod_mqtt_session:accept(Pid).

socket_handoff(LocalPath, Request, Opts) ->
    mod_mqtt_ws:socket_handoff(LocalPath, Request, Opts).

open_session({U, S, R}) ->
    Mod = gen_mod:ram_db_mod(S, ?MODULE),
    Mod:open_session({U, S, R}).

close_session({U, S, R}) ->
    Mod = gen_mod:ram_db_mod(S, ?MODULE),
    Mod:close_session({U, S, R}).

lookup_session({U, S, R}) ->
    Mod = gen_mod:ram_db_mod(S, ?MODULE),
    Mod:lookup_session({U, S, R}).

-spec publish(jid:ljid(), publish(), seconds()) ->
              {ok, non_neg_integer()} | {error, db_failure | publish_forbidden}.
publish({_, S, _} = USR, Pkt, ExpiryTime) ->
    case check_publish_access(Pkt#publish.topic, USR) of
        allow ->
            case retain(USR, Pkt, ExpiryTime) of
                ok ->
		    ejabberd_hooks:run(mqtt_publish, S, [USR, Pkt, ExpiryTime]),
                    Mod = gen_mod:ram_db_mod(S, ?MODULE),
                    route(Mod, S, Pkt, ExpiryTime);
                {error, _} = Err ->
                    Err
            end;
        deny ->
            {error, publish_forbidden}
    end.

-spec subscribe(jid:ljid(), binary(), sub_opts(), non_neg_integer()) ->
                       ok | {error, db_failure | subscribe_forbidden}.
subscribe({_, S, _} = USR, TopicFilter, SubOpts, ID) ->
    Mod = gen_mod:ram_db_mod(S, ?MODULE),
    Limit = mod_mqtt_opt:max_topic_depth(S),
    case check_topic_depth(TopicFilter, Limit) of
	allow ->
            case check_subscribe_access(TopicFilter, USR) of
                allow ->
                    Mod:subscribe(USR, TopicFilter, SubOpts, ID);
                deny ->
                    {error, subscribe_forbidden}
            end;
	deny ->
	    {error, subscribe_forbidden}
    end.

-spec unsubscribe(jid:ljid(), binary()) -> ok | {error, notfound | db_failure}.
unsubscribe({U, S, R}, Topic) ->
    Mod = gen_mod:ram_db_mod(S, ?MODULE),
    Mod:unsubscribe({U, S, R}, Topic).

-spec select_retained(jid:ljid(), binary(), qos(), non_neg_integer()) ->
                             [{publish(), seconds()}].
select_retained({_, S, _} = USR, TopicFilter, QoS, SubID) ->
    Mod = gen_mod:db_mod(S, ?MODULE),
    Limit = mod_mqtt_opt:match_retained_limit(S),
    select_retained(Mod, USR, TopicFilter, QoS, SubID, Limit).

remove_user(User, Server) ->
    LUser = jid:nodeprep(User),
    LServer = jid:nameprep(Server),
    Mod = gen_mod:ram_db_mod(LServer, ?MODULE),
    Sessions = Mod:get_sessions(LUser, LServer),
    [close_session(Session) || Session <- Sessions].

%%%===================================================================
%%% gen_server callbacks
%%%===================================================================
init([Host|_]) ->
    Opts = gen_mod:get_module_opts(Host, ?MODULE),
    Mod = gen_mod:db_mod(Opts, ?MODULE),
    RMod = gen_mod:ram_db_mod(Opts, ?MODULE),
    ejabberd_hooks:add(remove_user, Host, ?MODULE, remove_user, 50),
    try
	ok = Mod:init(Host, Opts),
	ok = RMod:init(),
	ok = init_cache(Mod, Host, Opts),
	{ok, #state{host = Host}}
    catch _:{badmatch, {error, Why}} ->
	    {stop, Why}
    end.

handle_call(Request, From, State) ->
    ?WARNING_MSG("Unexpected call from ~p: ~p", [From, Request]),
    {noreply, State}.

handle_cast(Msg, State) ->
    ?WARNING_MSG("Unexpected cast: ~p", [Msg]),
    {noreply, State}.

handle_info(Info, State) ->
    ?WARNING_MSG("Unexpected info: ~p", [Info]),
    {noreply, State}.

terminate(_Reason, #state{host = Host}) ->
    ejabberd_hooks:delete(remove_user, Host, ?MODULE, remove_user, 50),
    ok.

code_change(_OldVsn, State, _Extra) ->
    {ok, State}.

%%%===================================================================
%%% Options
%%%===================================================================
-spec mod_options(binary()) -> [{access_publish, [{[binary()], acl:acl()}]} |
				{access_subscribe, [{[binary()], acl:acl()}]} |
				{atom(), any()}].
mod_options(Host) ->
    [{match_retained_limit, 1000},
     {max_topic_depth, 8},
     {max_topic_aliases, 100},
     {session_expiry, timer:minutes(5)},
     {max_queue, 5000},
     {access_subscribe, []},
     {access_publish, []},
     {db_type, ejabberd_config:default_db(Host, ?MODULE)},
     {ram_db_type, ejabberd_config:default_ram_db(Host, ?MODULE)},
     {queue_type, ejabberd_option:queue_type(Host)},
     {use_cache, ejabberd_option:use_cache(Host)},
     {cache_size, ejabberd_option:cache_size(Host)},
     {cache_missed, ejabberd_option:cache_missed(Host)},
     {cache_life_time, ejabberd_option:cache_life_time(Host)}].

mod_opt_type(max_queue) ->
    econf:pos_int(unlimited);
mod_opt_type(session_expiry) ->
    econf:either(
      econf:int(0, 0),
      econf:timeout(second));
mod_opt_type(match_retained_limit) ->
    econf:pos_int(infinity);
mod_opt_type(max_topic_depth) ->
    econf:pos_int(infinity);
mod_opt_type(max_topic_aliases) ->
    econf:int(0, 65535);
mod_opt_type(access_subscribe) ->
    topic_access_validator();
mod_opt_type(access_publish) ->
    topic_access_validator();
mod_opt_type(queue_type) ->
    econf:queue_type();
mod_opt_type(db_type) ->
    econf:db_type(?MODULE);
mod_opt_type(ram_db_type) ->
    econf:db_type(?MODULE);
mod_opt_type(use_cache) ->
    econf:bool();
mod_opt_type(cache_size) ->
    econf:pos_int(infinity);
mod_opt_type(cache_missed) ->
    econf:bool();
mod_opt_type(cache_life_time) ->
    econf:timeout(second, infinity).

listen_opt_type(tls_verify) ->
    econf:bool();
listen_opt_type(max_payload_size) ->
    econf:pos_int(infinity).

listen_options() ->
    [{max_fsm_queue, 10000},
     {max_payload_size, infinity},
     {tls, false},
     {tls_verify, false}].

%%%===================================================================
%%% Doc
%%%===================================================================
mod_doc() ->
    #{desc =>
          ?T("This module adds "
             "https://docs.ejabberd.im/admin/guide/mqtt/[support for the MQTT] "
             "protocol version '3.1.1' and '5.0'. Remember to configure "
	     "'mod_mqtt' in 'modules' and  'listen' sections."),
      opts =>
          [{access_subscribe,
            #{value => "{TopicFilter: AccessName}",
              desc =>
                  ?T("Access rules to restrict access to topics "
                     "for subscribers. By default there are no restrictions.")}},
           {access_publish,
            #{value => "{TopicFilter: AccessName}",
              desc =>
                  ?T("Access rules to restrict access to topics "
                     "for publishers. By default there are no restrictions.")}},
           {max_queue,
            #{value => ?T("Size"),
              desc =>
                  ?T("Maximum queue size for outgoing packets. "
                     "The default value is '5000'.")}},
           {session_expiry,
            #{value => "timeout()",
              desc =>
                  ?T("The option specifies how long to wait for "
                     "an MQTT session resumption. When '0' is set, "
                     "the session gets destroyed when the underlying "
                     "client connection is closed. The default value is "
                     "'5' minutes.")}},
           {max_topic_depth,
            #{value => ?T("Depth"),
              desc =>
                  ?T("The maximum topic depth, i.e. the number of "
                     "slashes ('/') in the topic. The default "
                     "value is '8'.")}},
           {max_topic_aliases,
            #{value => "0..65535",
              desc =>
                  ?T("The maximum number of aliases a client "
                     "is able to associate with the topics. "
                     "The default value is '100'.")}},
           {match_retained_limit,
            #{value => "pos_integer() | infinity",
              desc =>
                  ?T("The option limits the number of retained messages "
                     "returned to a client when it subscribes to some "
                     "topic filter. The default value is '1000'.")}},
           {queue_type,
            #{value => "ram | file",
              desc =>
                  ?T("Same as top-level _`queue_type`_ option, "
                     "but applied to this module only.")}},
           {ram_db_type,
            #{value => "mnesia",
              desc =>
                  ?T("Same as top-level _`default_ram_db`_ option, "
                     "but applied to this module only.")}},
           {db_type,
            #{value => "mnesia | sql",
              desc =>
                  ?T("Same as top-level _`default_db`_ option, "
                     "but applied to this module only.")}},
           {use_cache,
            #{value => "true | false",
              desc =>
                  ?T("Same as top-level _`use_cache`_ option, "
                     "but applied to this module only.")}},
           {cache_size,
            #{value => "pos_integer() | infinity",
              desc =>
                  ?T("Same as top-level _`cache_size`_ option, "
                     "but applied to this module only.")}},
           {cache_missed,
            #{value => "true | false",
              desc =>
                  ?T("Same as top-level _`cache_missed`_ option, "
                     "but applied to this module only.")}},
           {cache_life_time,
            #{value => "timeout()",
              desc =>
                  ?T("Same as top-level _`cache_life_time`_ option, "
                     "but applied to this module only.")}}]}.

%%%===================================================================
%%% Internal functions
%%%===================================================================
route(Mod, LServer, Pkt, ExpiryTime) ->
    route(Mod, LServer, Pkt, ExpiryTime, Pkt#publish.topic, 0).

route(Mod, LServer, Pkt, ExpiryTime, Continuation, Num) ->
    case Mod:find_subscriber(LServer, Continuation) of
        {ok, {Pid, #sub_opts{no_local = true}, _}, Continuation1}
          when Pid == self() ->
            route(Mod, LServer, Pkt, ExpiryTime, Continuation1, Num);
	{ok, {Pid, SubOpts, ID}, Continuation1} ->
	    ?DEBUG("Route to ~p: ~ts", [Pid, Pkt#publish.topic]),
            MinQoS = min(SubOpts#sub_opts.qos, Pkt#publish.qos),
            Retain = case SubOpts#sub_opts.retain_as_published of
                         false -> false;
                         true -> Pkt#publish.retain
                     end,
            Props = set_sub_id(ID, Pkt#publish.properties),
	    mod_mqtt_session:route(
              Pid, {Pkt#publish{qos = MinQoS,
                                dup = false,
                                retain = Retain,
                                properties = Props},
                    ExpiryTime}),
	    route(Mod, LServer, Pkt, ExpiryTime, Continuation1, Num+1);
	{error, _} ->
	    {ok, Num}
    end.

select_retained(Mod, {_, LServer, _} = USR, TopicFilter, QoS, SubID, Limit) ->
    Topics = match_topics(TopicFilter, LServer, Limit),
    lists:filtermap(
      fun({{Filter, _}, Topic}) ->
	      case lookup_published(Mod, USR, Topic) of
		  {ok, {Payload, QoS1, Props, ExpiryTime}} ->
                      Props1 = set_sub_id(SubID, Props),
		      {true, {#publish{topic = Topic,
                                       payload = Payload,
                                       retain = true,
                                       properties = Props1,
                                       qos = min(QoS, QoS1)},
                              ExpiryTime}};
		  error ->
		      ets:delete(?MQTT_TOPIC_CACHE, {Filter, LServer}),
		      false;
		  _ ->
		      false
	      end
      end, Topics).

match_topics(Topic, LServer, Limit) ->
    Filter = topic_filter(Topic),
    case Limit of
	infinity ->
	    ets:match_object(?MQTT_TOPIC_CACHE, {{Filter, LServer}, '_'});
	_ ->
	    case ets:select(?MQTT_TOPIC_CACHE,
			    [{{{Filter, LServer}, '_'}, [], ['$_']}], Limit) of
		{Topics, _} -> Topics;
		'$end_of_table' -> []
	    end
    end.

retain({_, S, _} = USR, #publish{retain = true,
                                 topic = Topic, payload = Data,
                                 qos = QoS, properties = Props},
       ExpiryTime) ->
    Mod = gen_mod:db_mod(S, ?MODULE),
    TopicKey = topic_key(Topic),
    case Data of
	<<>> ->
	    ets:delete(?MQTT_TOPIC_CACHE, {TopicKey, S}),
	    case use_cache(Mod, S) of
		true ->
		    ets_cache:delete(?MQTT_PAYLOAD_CACHE, {S, Topic},
				     cache_nodes(Mod, S));
		false ->
		    ok
	    end,
	    Mod:delete_published(USR, Topic);
	_ ->
	    ets:insert(?MQTT_TOPIC_CACHE, {{TopicKey, S}, Topic}),
	    case use_cache(Mod, S) of
		true ->
		    case ets_cache:update(
			   ?MQTT_PAYLOAD_CACHE, {S, Topic},
			   {ok, {Data, QoS, Props, ExpiryTime}},
			   fun() ->
				   Mod:publish(USR, Topic, Data,
                                               QoS, Props, ExpiryTime)
			   end, cache_nodes(Mod, S)) of
			{ok, _} -> ok;
			{error, _} = Err -> Err
		    end;
		false ->
		    Mod:publish(USR, Topic, Data, QoS, Props, ExpiryTime)
	    end
    end;
retain(_, _, _) ->
    ok.

lookup_published(Mod, {_, LServer, _} = USR, Topic) ->
    case use_cache(Mod, LServer) of
	true ->
	    ets_cache:lookup(
	      ?MQTT_PAYLOAD_CACHE, {LServer, Topic},
	      fun() ->
		      Mod:lookup_published(USR, Topic)
	      end);
	false ->
	    Mod:lookup_published(USR, Topic)
    end.

set_sub_id(0, Props) ->
    Props;
set_sub_id(ID, Props) ->
    Props#{subscription_identifier => [ID]}.

%%%===================================================================
%%% Matching functions
%%%===================================================================
topic_key(S) ->
    Parts = split_path(S),
    case join_key(Parts) of
	[<<>>|T] -> T;
	T -> T
    end.

topic_filter(S) ->
    Parts = split_path(S),
    case join_filter(Parts) of
	[<<>>|T] -> T;
	T -> T
    end.

join_key([X,Y|T]) ->
    [X, $/|join_key([Y|T])];
join_key([X]) ->
    [X];
join_key([]) ->
    [].

join_filter([X, <<$#>>]) ->
    [wildcard(X)|'_'];
join_filter([X,Y|T]) ->
    [wildcard(X), $/|join_filter([Y|T])];
join_filter([<<>>]) ->
    [];
join_filter([<<$#>>]) ->
    '_';
join_filter([X]) ->
    [wildcard(X)];
join_filter([]) ->
    [].

wildcard(<<$+>>) -> '_';
wildcard(Bin) -> Bin.

check_topic_depth(_Topic, infinity) ->
    allow;
check_topic_depth(_, N) when N=<0 ->
    deny;
check_topic_depth(<<$/, T/binary>>, N) ->
    check_topic_depth(T, N-1);
check_topic_depth(<<_, T/binary>>, N) ->
    check_topic_depth(T, N);
check_topic_depth(<<>>, _) ->
    allow.

split_path(Path) ->
    binary:split(Path, <<$/>>, [global]).

%%%===================================================================
%%% Validators
%%%===================================================================
-spec topic_access_validator() -> econf:validator().
topic_access_validator() ->
    econf:and_then(
      econf:map(
	fun(TF) ->
		try split_path(mqtt_codec:topic_filter(TF))
		catch _:{mqtt_codec, _} = Reason ->
			econf:fail(Reason)
		end
	end,
	econf:acl(),
	[{return, orddict}]),
      fun lists:reverse/1).

%%%===================================================================
%%% ACL checks
%%%===================================================================
check_subscribe_access(Topic, {_, S, _} = USR) ->
    Rules = mod_mqtt_opt:access_subscribe(S),
    check_access(Topic, USR, Rules).

check_publish_access(<<$$, _/binary>>, _) ->
    deny;
check_publish_access(Topic, {_, S, _} = USR) ->
    Rules = mod_mqtt_opt:access_publish(S),
    check_access(Topic, USR, Rules).

check_access(_, _, []) ->
    allow;
check_access(Topic, {U, S, R} = USR, FilterRules) ->
    TopicParts = binary:split(Topic, <<$/>>, [global]),
    case lists:any(
           fun({FilterParts, Rule}) ->
                   case match(TopicParts, FilterParts, U, S, R) of
                       true ->
                           allow == acl:match_rule(S, Rule, USR);
                       false ->
                           false
                   end
           end, FilterRules) of
        true -> allow;
        false -> deny
    end.

match(_, [<<"#">>|_], _, _, _) ->
    true;
match([], [<<>>, <<"#">>|_], _, _, _) ->
    true;
match([_|T1], [<<"+">>|T2], U, S, R) ->
    match(T1, T2, U, S, R);
match([H|T1], [<<"%u">>|T2], U, S, R) ->
    case jid:nodeprep(H) of
        U -> match(T1, T2, U, S, R);
        _ -> false
    end;
match([H|T1], [<<"%d">>|T2], U, S, R) ->
    case jid:nameprep(H) of
        S -> match(T1, T2, U, S, R);
        _ -> false
    end;
match([H|T1], [<<"%c">>|T2], U, S, R) ->
    case jid:resourceprep(H) of
        R -> match(T1, T2, U, S, R);
        _ -> false
    end;
match([H|T1], [<<"%g">>|T2], U, S, R) ->
    case jid:resourceprep(H) of
        H ->
            case acl:loaded_shared_roster_module(S) of
                undefined -> false;
                Mod ->
                    case Mod:get_group_opts(S, H) of
                        error -> false;
                        _ ->
                            case Mod:is_user_in_group({U, S}, H, S) of
                                true -> match(T1, T2, U, S, R);
                                _ -> false
                            end
                    end
            end;
        _ -> false
    end;
match([H|T1], [H|T2], U, S, R) ->
    match(T1, T2, U, S, R);
match([], [], _, _, _) ->
    true;
match(_, _, _, _, _) ->
    false.

%%%===================================================================
%%% Cache stuff
%%%===================================================================
-spec init_cache(module(), binary(), gen_mod:opts()) -> ok | {error, db_failure}.
init_cache(Mod, Host, Opts) ->
    init_payload_cache(Mod, Host, Opts),
    init_topic_cache(Mod, Host).

-spec init_topic_cache(module(), binary()) -> ok | {error, db_failure}.
init_topic_cache(Mod, Host) ->
    catch ets:new(?MQTT_TOPIC_CACHE,
                  [named_table, ordered_set, public,
                   {heir, erlang:group_leader(), none}]),
    ?INFO_MSG("Building MQTT cache for ~ts, this may take a while", [Host]),
    case Mod:list_topics(Host) of
        {ok, Topics} ->
            lists:foreach(
              fun(Topic) ->
                      ets:insert(?MQTT_TOPIC_CACHE,
                                 {{topic_key(Topic), Host}, Topic})
              end, Topics);
        {error, _} = Err ->
            Err
    end.

-spec init_payload_cache(module(), binary(), gen_mod:opts()) -> ok.
init_payload_cache(Mod, Host, Opts) ->
    case use_cache(Mod, Host) of
        true ->
            CacheOpts = cache_opts(Opts),
            ets_cache:new(?MQTT_PAYLOAD_CACHE, CacheOpts);
        false ->
            ets_cache:delete(?MQTT_PAYLOAD_CACHE)
    end.

-spec cache_opts(gen_mod:opts()) -> [proplists:property()].
cache_opts(Opts) ->
    MaxSize = mod_mqtt_opt:cache_size(Opts),
    CacheMissed = mod_mqtt_opt:cache_missed(Opts),
    LifeTime = mod_mqtt_opt:cache_life_time(Opts),
    [{max_size, MaxSize}, {cache_missed, CacheMissed}, {life_time, LifeTime}].

-spec use_cache(module(), binary()) -> boolean().
use_cache(Mod, Host) ->
    case erlang:function_exported(Mod, use_cache, 1) of
        true -> Mod:use_cache(Host);
        false -> mod_mqtt_opt:use_cache(Host)
    end.

-spec cache_nodes(module(), binary()) -> [node()].
cache_nodes(Mod, Host) ->
    case erlang:function_exported(Mod, cache_nodes, 1) of
        true -> Mod:cache_nodes(Host);
        false -> ejabberd_cluster:get_nodes()
    end.