Erlang generic standard behaviours -- gen_server system msg

这是Erlang generic standard behaviors gen_server 分析的系列的最后一篇,主要分析gen_server module 辅助性的功能函数.

在gen_server 的MAIN loop 流程中,除了处理Parent 的'EXIT' 消息, user module 常规消息之外, 还处理了一类 system 消息. 这一类system 消息的来源是一个sys 的module,在Erlang OTP体系中,sys module 主要有两大类的作用,一个是热更,另一个是trace log statistics 相关的辅助功能.

这次主要分析trace log statistics 相关的内容,至于热更会放在之后的热更新系列中进行说明.

 decode_msg(Msg, Parent, Name, State, Mod, Time, Debug, Hib) ->
     case Msg of
     　　{system, From, Req} ->
        　　 sys:handle_system_msg(Req, From, Parent, ?MODULE, Debug,
           　　        [Name, State, Mod, Time], Hib);
     　　{'EXIT', Parent, Reason} ->
        　　 terminate(Reason, Name, Msg, Mod, State, Debug);
     　　_Msg when Debug =:= [] ->
         　　handle_msg(Msg, Parent, Name, State, Mod);
     　　_Msg ->
        　　 Debug1 = sys:handle_debug(Debug, fun print_event/3,
           　　            Name, {in, Msg}),
        　　 handle_msg(Msg, Parent, Name, State, Mod, Debug1)
     end.

由上面的代码片段中,可以看出gen_server 的 MAIN loop 会接收{system, From, Req} 消息(L3), 并且调用sys:handle_system_msg/7 函数(L4)进行处理. 此外还会根据是否有Debug option参数(L8)而调用sys:handle_debug/4 函数进行处理(L11).

system 消息来源

gen_server MAIL loop 中处理的{system, From, Req} 消息是来自sys module, 代码片段见下:

 %%-----------------------------------------------------------------
 %% All system messages sent are on the form {system, From, Msg}
 %% The receiving side should send Msg to handle_system_msg/5.
 %%-----------------------------------------------------------------
 send_system_msg(Name, Request) ->
     case catch gen:call(Name, system, Request) of
         {ok,Res} -> Res;
         {'EXIT', Reason} -> exit({Reason, mfa(Name, Request)})
     end.

 send_system_msg(Name, Request, Timeout) ->
     case catch gen:call(Name, system, Request, Timeout) of
         {ok,Res} -> Res;
         {'EXIT', Reason} -> exit({Reason, mfa(Name, Request, Timeout)})
     end.

这个代码片段的注释中的信息已经说的很明白.

而log trace statistics 相关的辅助功能的实现,都是通过调用send_system_msg 函数,向gen_server 进程发送system 消息, 如:

 trace(Name, Flag) ->
     send_system_msg(Name, {debug, {trace, Flag}}).
 statistics(Name, Flag) ->
     send_system_msg(Name, {debug, {statistics, Flag}}).

L1 是trace 函数, L3 是statistics 函数.

system 消息处理

当gen_server MAIN loop 接收到system 消息时, 调用sys:handle_system_msg/7 函数进行处理, sys:handle_system_msg/7 函数的实现:

 handle_system_msg(SysState, Msg, From, Parent, Mod, Debug, Misc, Hib) ->
     case do_cmd(SysState, Msg, Parent, Mod, Debug, Misc) of
         {suspended, Reply, NDebug, NMisc} ->
             _ = gen:reply(From, Reply),
             suspend_loop(suspended, Parent, Mod, NDebug, NMisc, Hib);
         {running, Reply, NDebug, NMisc} ->
             _ = gen:reply(From, Reply),
             Mod:system_continue(Parent, NDebug, NMisc)
     end.

也就是, handle_system_msg 函数调用了(L2)do_cmd 函数, 处理各种类型的system 消息

 do_cmd(_, suspend, _Parent, _Mod, Debug, Misc) ->
     {suspended, ok, Debug, Misc};
 do_cmd(_, resume, _Parent, _Mod, Debug, Misc) ->
     {running, ok, Debug, Misc};
 do_cmd(SysState, get_state, _Parent, Mod, Debug, Misc) ->
     {SysState, do_get_state(Mod, Misc), Debug, Misc};
 do_cmd(SysState, {replace_state, StateFun},  _Parent, Mod, Debug, Misc) ->
     {Res, NMisc} = do_replace_state(StateFun, Mod, Misc),
     {SysState, Res, Debug, NMisc};
 do_cmd(SysState, get_status, Parent, Mod, Debug, Misc) ->
     Res = get_status(SysState, Parent, Mod, Debug, Misc),
     {SysState, Res, Debug, Misc};
 do_cmd(SysState, {debug, What}, _Parent, _Mod, Debug, Misc) ->
     {Res, NDebug} = debug_cmd(What, Debug),
     {SysState, Res, NDebug, Misc};
 do_cmd(suspended, {change_code, Module, Vsn, Extra}, _Parent,
        Mod, Debug, Misc) ->
     {Res, NMisc} = do_change_code(Mod, Module, Vsn, Extra, Misc),
     {suspended, Res, Debug, NMisc};
 do_cmd(SysState, Other, _Parent, _Mod, Debug, Misc) ->
     {SysState, {error, {unknown_system_msg, Other}}, Debug, Misc}.

L1,L3,L16 的system 消息和热更相关.

L5,L7 的system 消息和gen_server 进程的state 相关.

L10 是获取gen_server 进程的所有状态信息, 包括但不仅限trace statistics state 信息.

L13 是trace log statistics 相关辅助函数的开关. 这些辅助函数功能对于gen_server 进程是可拔插的.

 debug_cmd({trace, true}, Debug) ->
     {ok, install_debug(trace, true, Debug)};
 debug_cmd({trace, false}, Debug) ->
     {ok, remove_debug(trace, Debug)};

仅以trace 功能举例,true false 表示该函数功能的生效与否.而install_debug 的实现就是lists [|] 的操作, remove_debug 调用的是lists:keydelete/3 函数实现.

触发debug

在gen_server 的MAIN loop 中,等Debug option 参数时, 在decode_msg, handle_msg, handle_common_reply, reply 函数处理时,即会调用sys:handle_debug 函数, 继而触发debug.

 %%-----------------------------------------------------------------
 %% Func: handle_debug/4
 %% Purpose: Called by a process that wishes to debug an event.
 %%          Func is a formatting function, called as Func(Device, Event).
 %% Returns: [debug_opts()]
 %%-----------------------------------------------------------------
 -spec handle_debug(Debug, FormFunc, Extra, Event) -> [dbg_opt()] when
       Debug :: [dbg_opt()],
       FormFunc :: format_fun(),
       Extra :: term(),
       Event :: system_event().
 handle_debug([{trace, true} | T], FormFunc, State, Event) ->
     print_event({Event, State, FormFunc}),
     [{trace, true} | handle_debug(T, FormFunc, State, Event)];
 handle_debug([{log, {N, LogData}} | T], FormFunc, State, Event) ->
     NLogData = [{Event, State, FormFunc} | trim(N, LogData)],
     [{log, {N, NLogData}} | handle_debug(T, FormFunc, State, Event)];
 handle_debug([{log_to_file, Fd} | T], FormFunc, State, Event) ->
     print_event(Fd, {Event, State, FormFunc}),
     [{log_to_file, Fd} | handle_debug(T, FormFunc, State, Event)];
 handle_debug([{statistics, StatData} | T], FormFunc, State, Event) ->
     NStatData = stat(Event, StatData),
     [{statistics, NStatData} | handle_debug(T, FormFunc, State, Event)];
 handle_debug([{Func, FuncState} | T], FormFunc, State, Event) ->
     case catch Func(FuncState, Event, State) of
     done -> handle_debug(T, FormFunc, State, Event);
     {'EXIT', _} -> handle_debug(T, FormFunc, State, Event);
     NFuncState ->
         [{Func, NFuncState} | handle_debug(T, FormFunc, State, Event)]
     end;
 handle_debug([], _FormFunc, _State, _Event) ->
     [].

1, trace 会调用print_event 函数,将trace 信息打印在console

2, log_to_file 同样调用print_event 函数, 将trace 信息写入到指定文件

3, 而statistics 是将统计信息更新至gen_server 进程的status

总结

在Erlang 的设计与源码实现中, 这种功能完备利于debug且可拔插的周边tools, 总能让人兴奋.