Integrate nicely with systemd

Support systemd's watchdog feature and enable it by default in the unit
file, so that ejabberd is auto-restarted if the VM becomes unresponsive.
Also, set the systemd startup type to 'notify', so that startup of
followup units is delayed until ejabberd signals readiness.  While at
it, also notify systemd of configuration reload and shutdown states.

Note: "NotifyAccess=all" is required as long as "ejabberdctl foreground"
runs the VM as a new child process, rather than "exec"ing it.  This way,
systemd views the ejabberdctl process itself as the main service
process, and would discard notifications from other processes by
default.

ejabberd.service.template

@@ -3,14 +3,17 @@ Description=XMPP Server
 After=network.target
 
 [Service]
+Type=notify
 User=ejabberd
 Group=ejabberd
 LimitNOFILE=65536
 Restart=on-failure
 RestartSec=5
+WatchdogSec=30
 ExecStart=@ctlscriptpath@/ejabberdctl foreground
 ExecStop=/bin/sh -c '@ctlscriptpath@/ejabberdctl stop && @ctlscriptpath@/ejabberdctl stopped'
 ExecReload=@ctlscriptpath@/ejabberdctl reload_config
+NotifyAccess=all
 PrivateDevices=true
 TimeoutSec=300
 

src/ejabberd_app.erl

@@ -58,6 +58,7 @@ start(normal, _Args) ->
 			ejabberd_cluster:wait_for_sync(infinity),
 			ejabberd_hooks:run(ejabberd_started, []),
 			ejabberd:check_apps(),
+			ejabberd_systemd:ready(),
 			{T2, _} = statistics(wall_clock),
 			?INFO_MSG("ejabberd ~ts is started in the node ~p in ~.2fs",
 				  [ejabberd_option:version(),
@@ -96,6 +97,7 @@ start_included_apps() ->
 %% This function is called when an application is about to be stopped,
 %% before shutting down the processes of the application.
 prep_stop(State) ->
+    ejabberd_systemd:stopping(),
     ejabberd_hooks:run(ejabberd_stopping, []),
     ejabberd_listener:stop(),
     ejabberd_sm:stop(),

src/ejabberd_config.erl

@@ -93,10 +93,11 @@ load(Path) ->
 
 -spec reload() -> ok | error_return().
 reload() ->
+    ejabberd_systemd:reloading(),
     ConfigFile = path(),
     ?INFO_MSG("Reloading configuration from ~ts", [ConfigFile]),
     OldHosts = get_myhosts(),
-    case load_file(ConfigFile) of
+    Res = case load_file(ConfigFile) of
 	      ok ->
 		  NewHosts = get_myhosts(),
 		  AddHosts = NewHosts -- OldHosts,
@@ -116,7 +117,9 @@ reload() ->
 		  ?ERROR_MSG("Configuration reload aborted: ~ts",
 			     [format_error(Err)]),
 		  Err
-    end.
+	  end,
+    ejabberd_systemd:ready(),
+    Res.
 
 -spec dump() -> ok | error_return().
 dump() ->

src/ejabberd_sup.erl

@@ -37,7 +37,8 @@ start_link() ->
 
 init([]) ->
     {ok, {{one_for_one, 10, 1},
-	  [worker(ejabberd_hooks),
+	  [worker(ejabberd_systemd),
+	   worker(ejabberd_hooks),
 	   worker(ejabberd_cluster),
 	   worker(translate),
 	   worker(ejabberd_access_permissions),

src/ejabberd_systemd.erl

@@ -0,0 +1,196 @@
+%%%----------------------------------------------------------------------
+%%% File    : ejabberd_systemd.erl
+%%% Author  : Holger Weiss <holger@zedat.fu-berlin.de>
+%%% Purpose : Integrate with systemd
+%%% Created :  5 Jan 2021 by Holger Weiss <holger@zedat.fu-berlin.de>
+%%%
+%%%
+%%% ejabberd, Copyright (C) 2021   ProcessOne
+%%%
+%%% This program is free software; you can redistribute it and/or
+%%% modify it under the terms of the GNU General Public License as
+%%% published by the Free Software Foundation; either version 2 of the
+%%% License, or (at your option) any later version.
+%%%
+%%% This program is distributed in the hope that it will be useful,
+%%% but WITHOUT ANY WARRANTY; without even the implied warranty of
+%%% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+%%% General Public License for more details.
+%%%
+%%% You should have received a copy of the GNU General Public License along
+%%% with this program; if not, write to the Free Software Foundation, Inc.,
+%%% 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+%%%
+%%%----------------------------------------------------------------------
+
+-module(ejabberd_systemd).
+-author('holger@zedat.fu-berlin.de').
+-behaviour(gen_server).
+
+-export([start_link/0,
+	 ready/0,
+	 reloading/0,
+	 stopping/0]).
+-export([init/1,
+	 handle_call/3,
+	 handle_cast/2,
+	 handle_info/2,
+	 terminate/2,
+	 code_change/3]).
+
+-include("logger.hrl").
+
+-record(state,
+	{socket :: gen_udp:socket() | undefined,
+	 destination :: inet:local_address() | undefined,
+	 interval :: pos_integer() | undefined,
+	 last_ping :: integer() | undefined}).
+
+-type watchdog_timeout() :: pos_integer() | hibernate.
+-type state() :: #state{}.
+
+%%--------------------------------------------------------------------
+%% API.
+%%--------------------------------------------------------------------
+-spec start_link() -> {ok, pid()} | ignore | {error, term()}.
+start_link() ->
+    gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
+
+-spec ready() -> ok.
+ready() ->
+    cast_notification(<<"READY=1">>).
+
+-spec reloading() -> ok.
+reloading() ->
+    cast_notification(<<"RELOADING=1">>).
+
+-spec stopping() -> ok.
+stopping() ->
+    cast_notification(<<"STOPPING=1">>).
+
+%%--------------------------------------------------------------------
+%% gen_mod callbacks.
+%%--------------------------------------------------------------------
+-spec init(any())
+      -> {ok, state()} | {ok, state(), watchdog_timeout()} | {stop, term()}.
+init(_Opts) ->
+    process_flag(trap_exit, true),
+    case os:getenv("NOTIFY_SOCKET") of
+	[$@ | _Abstract] ->
+	    ?CRITICAL_MSG("Abstract NOTIFY_SOCKET not supported", []),
+	    {stop, esocktnosupport};
+	Path when is_list(Path), length(Path) > 0 ->
+	    ?DEBUG("Got NOTIFY_SOCKET: ~s", [Path]),
+	    Destination = {local, Path},
+	    case gen_udp:open(0, [local]) of
+		{ok, Socket} ->
+		    Interval = get_watchdog_interval(),
+		    State = #state{socket = Socket,
+				   destination = Destination,
+				   interval = Interval},
+		    if is_integer(Interval), Interval > 0 ->
+			    ?INFO_MSG("Watchdog notifications enabled", []),
+			    {ok, set_last_ping(State), Interval};
+		       true ->
+			    ?INFO_MSG("Watchdog notifications disabled", []),
+			    {ok, State}
+		    end;
+		{error, Reason} ->
+		    ?CRITICAL_MSG("Cannot open IPC socket: ~p", [Reason]),
+		    {stop, Reason}
+	    end;
+	_ ->
+	    ?INFO_MSG("Got no NOTIFY_SOCKET, notifications disabled", []),
+	    {ok, #state{}}
+    end.
+
+-spec handle_call(term(), {pid(), term()}, state())
+      -> {reply, {error, badarg}, state(), watchdog_timeout()}.
+handle_call(Request, From, State) ->
+    ?ERROR_MSG("Got unexpected request from ~p: ~p", [From, Request]),
+    {reply, {error, badarg}, State, get_timeout(State)}.
+
+-spec handle_cast({notify, binary()} | term(), state())
+      -> {noreply, state(), watchdog_timeout()}.
+handle_cast({notify, Notification},
+	    #state{destination = undefined} = State) ->
+    ?DEBUG("No NOTIFY_SOCKET, dropping ~s notification", [Notification]),
+    {noreply, State, get_timeout(State)};
+handle_cast({notify, Notification}, State) ->
+    try notify(State, Notification)
+    catch _:Err ->
+	    ?ERROR_MSG("Cannot send ~s notification: ~p", [Notification, Err])
+    end,
+    {noreply, State, get_timeout(State)};
+handle_cast(Msg, State) ->
+    ?ERROR_MSG("Got unexpected message: ~p", [Msg]),
+    {noreply, State, get_timeout(State)}.
+
+-spec handle_info(timeout | term(), state())
+      -> {noreply, state(), watchdog_timeout()}.
+handle_info(timeout, #state{interval = Interval} = State)
+  when is_integer(Interval), Interval > 0 ->
+    try notify(State, <<"WATCHDOG=1">>)
+    catch _:Err ->
+	    ?ERROR_MSG("Cannot ping watchdog: ~p", [Err])
+    end,
+    {noreply, set_last_ping(State), Interval};
+handle_info(Info, State) ->
+    ?ERROR_MSG("Got unexpected info: ~p", [Info]),
+    {noreply, State, get_timeout(State)}.
+
+-spec terminate(normal | shutdown | {shutdown, term()} | term(), state()) -> ok.
+terminate(Reason, #state{socket = undefined}) ->
+    ?DEBUG("Terminating ~s (~p)", [?MODULE, Reason]),
+    ok;
+terminate(Reason, #state{socket = Socket}) ->
+    ?DEBUG("Closing socket and terminating ~s (~p)", [?MODULE, Reason]),
+    ok = gen_udp:close(Socket).
+
+-spec code_change({down, term()} | term(), state(), term()) -> {ok, state()}.
+code_change(_OldVsn, State, _Extra) ->
+    ?INFO_MSG("Got code change request", []),
+    {ok, State}.
+
+%%--------------------------------------------------------------------
+%% Internal functions.
+%%--------------------------------------------------------------------
+-spec get_watchdog_interval() -> integer() | undefined.
+get_watchdog_interval() ->
+    case os:getenv("WATCHDOG_USEC") of
+	WatchdogUSec when is_list(WatchdogUSec), length(WatchdogUSec) > 0 ->
+	    Interval = round(0.5 * list_to_integer(WatchdogUSec)),
+	    ?DEBUG("Watchdog interval: ~B microseconds", [Interval]),
+	    erlang:convert_time_unit(Interval, microsecond, millisecond);
+	_ ->
+	    undefined
+    end.
+
+-spec get_timeout(state()) -> watchdog_timeout().
+get_timeout(#state{interval = undefined}) ->
+    ?DEBUG("Watchdog interval is undefined, hibernating", []),
+    hibernate;
+get_timeout(#state{interval = Interval, last_ping = LastPing}) ->
+    case Interval - (erlang:monotonic_time(millisecond) - LastPing) of
+	Timeout when Timeout > 0 ->
+	    ?DEBUG("Calculated new timeout value: ~B", [Timeout]),
+	    Timeout;
+	_ ->
+	    ?DEBUG("Calculated new timeout value: 1", []),
+	    1
+    end.
+
+-spec set_last_ping(state()) -> state().
+set_last_ping(State) ->
+    LastPing = erlang:monotonic_time(millisecond),
+    State#state{last_ping = LastPing}.
+
+-spec notify(state(), binary()) -> ok.
+notify(#state{socket = Socket, destination = Destination},
+       Notification) ->
+    ?DEBUG("Notifying systemd: ~s", [Notification]),
+    ok = gen_udp:send(Socket, Destination, 0, Notification).
+
+-spec cast_notification(binary()) -> ok.
+cast_notification(Notification) ->
+    gen_server:cast(?MODULE, {notify, Notification}).