[erlang 002]gen_server中何时会跑到terminate函数

一、从start方法产出的独立gen_server进程

实验代码：

%%%--------------------------------------
%%% @Module  :
%%% @Author  :
%%% @Email   :
%%% @Created :
%%% @Description:
%%%--------------------------------------
-module(ter_a).
-behaviour(gen_server).

%% gen_server callbacks exports
-export([init/1, handle_call/3, handle_cast/2, handle_info/2,
     terminate/2, code_change/3]).
%% gen_server api exports
-export([start/1, link/1, is_alive/0, mistake/0, stop/1]).

-record(state, {}).

%%====================================================================
%% API
%%====================================================================
start(Flag) ->
    gen_server:start({local, ?MODULE}, ?MODULE, [Flag], []).

link(Flag) ->
    gen_server:call(?MODULE, {link, Flag}).

is_alive() ->
    gen_server:call(?MODULE, is_alive).

mistake() ->
    gen_server:cast(?MODULE, mistake).

stop(Reason) ->
    gen_server:cast(?MODULE, {stop, Reason}).

%%====================================================================
%% gen_server callbacks
%%====================================================================
init([Flag]) ->
    case Flag of
        0 -> skip;
        _ -> process_flag(trap_exit, true)
    end,
    {ok, #state{}}.

handle_call({link, Flag}, _From, State) ->
    {ok, Pid} = ter_b:start_link(Flag),
    io:format("handle_call, link to ter_b, LinkPid:~p~n", [Pid]),
    {reply, Pid, State};

handle_call(is_alive, _From, State) ->
    io:format("yes, i'm alive~n"),
    {reply, alive, State};

handle_call(Request, _From, State) ->
    io:format("handle_call, Request:~p~n", [Request]),
    Reply = ok,
    {reply, Reply, State}.

handle_cast(mistake, State) ->
    A = 0,
    _B = 1 / A,
    {noreply, State};

handle_cast({stop, Reason}, State) ->
    io:format("handle_cast, stop, Reason:~p~n", [Reason]),
    {stop, Reason, State};

handle_cast(Msg, State) ->
    io:format("handle_cast, Msg:~p~n", [Msg]),
    {noreply, State}.

handle_info(Info, State) ->
    io:format("handle_info, Info:~p~n", [Info]),
    {noreply, State}.

terminate(Reason, _State) ->
    io:format("terminate, Reason:~p~n", [Reason]),
    ok.

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

1) 无论有没有在初始化的时候捕捉退出即process_flag(trap_exit, true)，只要回调函数以{stop, Reason, State}或者{stop, Reason, State}结束，则都会跑到terminate/2。特别的如果Reson为normal、shutdown或者{shutdown, ElseInfo}，进程会正常退出，否则进程会报错然后才退出。部分实验数据如下：

1> ter_a:start(0).
{ok,<0.33.0>}
2> ter_a:stop(normal).
handle_cast, stop, Reason:normal
terminate, Reason:normal
ok
3> ter_a:start(1).
{ok,<0.36.0>}
4> ter_a:stop(else).
handle_cast, stop, Reason:else
terminate, Reason:else
ok
5> 
=ERROR REPORT==== 28-Apr-2015::10:38:42 ===
** Generic server ter_a terminating 
** Last message in was {'$gen_cast',{stop,else}}
** When Server state == {state}
** Reason for termination == 
** else

实际上，可以通过gen_server的源码了解到这些东西：

dispatch({'$gen_cast', Msg}, Mod, State) ->
    Mod:handle_cast(Msg, State);
dispatch(Info, Mod, State) ->
    Mod:handle_info(Info, State).

handle_msg({'$gen_call', From, Msg}, Parent, Name, State, Mod) ->
    case catch Mod:handle_call(Msg, From, State) of
    ...
    {stop, Reason, Reply, NState} ->
        {'EXIT', R} = 
        (catch terminate(Reason, Name, Msg, Mod, NState, [])),
        reply(From, Reply),
        exit(R);
    Other -> handle_common_reply(Other, Parent, Name, Msg, Mod, State)
    end;
handle_msg(Msg, Parent, Name, State, Mod) ->
    Reply = (catch dispatch(Msg, Mod, State)),
    handle_common_reply(Reply, Parent, Name, Msg, Mod, State).

handle_msg({'$gen_call', From, Msg}, Parent, Name, State, Mod, Debug) ->
    case catch Mod:handle_call(Msg, From, State) of
    ...
    {stop, Reason, Reply, NState} ->
        {'EXIT', R} = 
        (catch terminate(Reason, Name, Msg, Mod, NState, Debug)),
        _ = reply(Name, From, Reply, NState, Debug),
        exit(R);
    Other ->
        handle_common_reply(Other, Parent, Name, Msg, Mod, State, Debug)
    end;
handle_msg(Msg, Parent, Name, State, Mod, Debug) ->
    Reply = (catch dispatch(Msg, Mod, State)),
    handle_common_reply(Reply, Parent, Name, Msg, Mod, State, Debug).

handle_common_reply(Reply, Parent, Name, Msg, Mod, State) ->
    case Reply of
    ...
    {stop, Reason, NState} ->
        terminate(Reason, Name, Msg, Mod, NState, []);
    {'EXIT', What} ->

        terminate(What, Name, Msg, Mod, State, []);
    _ ->
        terminate({bad_return_value, Reply}, Name, Msg, Mod, State, [])
    end.

handle_common_reply(Reply, Parent, Name, Msg, Mod, State, Debug) ->
    case Reply of
    ...
    {stop, Reason, NState} ->
        terminate(Reason, Name, Msg, Mod, NState, Debug);
    {'EXIT', What} ->

        terminate(What, Name, Msg, Mod, State, Debug);
    _ ->
        terminate({bad_return_value, Reply}, Name, Msg, Mod, State, Debug)
    end.

terminate(Reason, Name, Msg, Mod, State, Debug) ->
    case catch Mod:terminate(Reason, State) of
    {'EXIT', R} ->
        FmtState = format_status(terminate, Mod, get(), State),
        error_info(R, Name, Msg, FmtState, Debug),
        exit(R);
    _ ->
        case Reason of
        normal ->
            exit(normal);
        shutdown ->
            exit(shutdown);
        {shutdown,_}=Shutdown ->
            exit(Shutdown);
        _ ->
            FmtState = format_status(terminate, Mod, get(), State),
            error_info(Reason, Name, Msg, FmtState, Debug),
            exit(Reason)
        end
    end.

2) 从外部用exit(Pid, Reason)去杀死这个进程的情况

    a) 只要Reason不是normal或者kill，如果进程没有捕捉退出，则进程会以Reason的理由直接退出，不会跑到terminate/2;如果有捕捉退出，进程不会退出，而且这个操作会转换成一条法外消息{'EXIT', From, Reason}发到进程，由handle_info处理：

1> {ok, Pid} = ter_a:start(0).
{ok,<0.33.0>}
2> exit(Pid, else).
true
3> ter_a:is_alive().
** exception exit: {noproc,{gen_server,call,[ter_a,is_alive]}}
     in function  gen_server:call/2 (gen_server.erl, line 182)
4> {ok, Pid1} = ter_a:start(1).
{ok,<0.38.0>}
5> exit(Pid1, else).           
handle_info, Info:{'EXIT',<0.36.0>,else}
true
6> ter_a:is_alive().
yes, i'm alive
alive

    b) Reason是normal，如果进程没有捕捉退出，则进程不会退出；如果有捕捉退出，进程也不会退出，只会转为{'EXIT', From, normal}消息投递到Info的消息{'EXIT', From, normal}消息投递到进程信箱，由handle_info处理：

1> {ok, Pid0} = ter_a:start(0).
{ok,<0.33.0>}
2> exit(Pid0, normal).
true
3> ter_a:is_alive().
yes, i'm alive
alive
4> ter_a:stop(normal).
handle_cast, stop, Reason:normal
terminate, Reason:normal
ok
5> {ok, Pid1} = ter_a:start(1).
{ok,<0.38.0>}
6> exit(Pid1, normal).
handle_info, Info:{'EXIT',<0.31.0>,normal}
true
7> ter_a:is_alive().
yes, i'm alive
alive

    c) Reason是kill，无论进程有没有捕捉退出，进程都会无条件退出，而且不会跑到terminate/2：

1> {ok, Pid0} = ter_a:start(0).
{ok,<0.33.0>}
2> exit(Pid0, kill).
true
3> ter_a:is_alive().
** exception exit: {noproc,{gen_server,call,[ter_a,is_alive]}}
     in function  gen_server:call/2 (gen_server.erl, line 182)
4> {ok, Pid1} = ter_a:start(1).
{ok,<0.38.0>}
5> exit(Pid1, kill).           
true
6> ter_a:is_alive().           
** exception exit: {noproc,{gen_server,call,[ter_a,is_alive]}}
     in function  gen_server:call/2 (gen_server.erl, line 182)

二、gen_server进程相互链接的情况（假如有进程A、B）

进程B的代码如下：

%%%--------------------------------------
%%% @Module  : 
%%% @Author  : 
%%% @Email   : 
%%% @Created : 
%%% @Description:
%%%--------------------------------------
-module(ter_b).
-behaviour(gen_server).

%% gen_server callbacks exports
-export([init/1, handle_call/3, handle_cast/2, handle_info/2,
     terminate/2, code_change/3]).

%% gen_server api exports
-export([start_link/1, is_alive/0]).
-record(state, {}).

%%====================================================================
%% API
%%====================================================================
start_link(Flag) ->
    gen_server:start_link({local, ?MODULE}, ?MODULE, [Flag], []).

is_alive() ->
    gen_server:call(?MODULE, is_alive).

%%====================================================================
%% gen_server callbacks
%%====================================================================

init([Flag]) ->
    case Flag of 
        0 -> skip;
        _ ->
            process_flag(trap_exit, true)
    end,
    {ok, #state{}}.

handle_call(is_alive, _From, State) ->
    io:format("ter_b, i'm alive~n"),
    {reply, alive, State};

handle_call(_Request, _From, State) ->
    io:format("ter_b, handle_call,  _Request:~p~n", [_Request]),
    Reply = ok,
    {reply, Reply, State}.

handle_cast(_Msg, State) ->
    io:format("ter_b, handle_cast, Msg:~p~n", [_Msg]),
    {noreply, State}.

handle_info(_Info, State) ->
    io:format(ter_b, "handle_info, Msg:~p~n", [_Info]),
    {noreply, State}.

terminate(_Reason, _State) ->
    io:format("ter_b, terminate~n"),
    ok.

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

1) B不捕捉退出，如果A异常退出，则B也会随之退出，A进程会跑到terminate/2而B进程不会；如果A正常退出，则B不做任何处理：

异常退出：

1> ter_a:start(0).
{ok,<0.33.0>}
2> ter_a:link(0). 
handle_call, link to ter_b, LinkPid:<0.35.0>
<0.35.0>
3> ter_a:mistake().
terminate, Reason:{badarith,
                      [{ter_a,handle_cast,2,[{file,"ter_a.erl"},{line,63}]},
                       {gen_server,handle_msg,5,
                           [{file,"gen_server.erl"},{line,599}]},
                       {proc_lib,init_p_do_apply,3,
                           [{file,"proc_lib.erl"},{line,237}]}]}
ok
4> 
=ERROR REPORT==== 28-Apr-2015::11:35:25 ===
** Generic server ter_a terminating 
** Last message in was {'$gen_cast',mistake}
** When Server state == {state}
** Reason for termination == 
** {badarith,[{ter_a,handle_cast,2,[{file,"ter_a.erl"},{line,63}]},
              {gen_server,handle_msg,5,[{file,"gen_server.erl"},{line,599}]},
              {proc_lib,init_p_do_apply,3,
                        [{file,"proc_lib.erl"},{line,237}]}]}

4> ter_a:is_alive().
** exception exit: {noproc,{gen_server,call,[ter_a,is_alive]}}
     in function  gen_server:call/2 (gen_server.erl, line 182)
5> ter_b:is_alive().
** exception exit: {noproc,{gen_server,call,[ter_b,is_alive]}}
     in function  gen_server:call/2 (gen_server.erl, line 182)

正常退出：

1> ter_a:start(0).
{ok,<0.33.0>}
2> ter_a:link(0).
handle_call, link to ter_b, LinkPid:<0.35.0>
<0.35.0>
3> ter_a:stop(normal).
handle_cast, stop, Reason:normal
terminate, Reason:normal
ok
4> ter_a:is_alive().
** exception exit: {noproc,{gen_server,call,[ter_a,is_alive]}}
     in function  gen_server:call/2 (gen_server.erl, line 182)
5> ter_b:is_alive().
ter_b, i'm alive
alive

2) B捕捉退出，只要A退出，B都会随着退出，两个进程都会跑到terminate/2：

异常退出：

Eshell V6.2  (abort with ^G)
1> ter_a:start(0).
{ok,<0.33.0>}
2> ter_a:link(1).
handle_call, link to ter_b, LinkPid:<0.35.0>
<0.35.0>
3> ter_a:mistake().
terminate, Reason:{badarith,
                      [{ter_a,handle_cast,2,[{file,"ter_a.erl"},{line,63}]},
                       {gen_server,handle_msg,5,
                           [{file,"gen_server.erl"},{line,599}]},
                       {proc_lib,init_p_do_apply,3,
                           [{file,"proc_lib.erl"},{line,237}]}]}
ter_b, terminate
ok
4> 
=ERROR REPORT==== 28-Apr-2015::11:40:11 ===
** Generic server ter_a terminating 
** Last message in was {'$gen_cast',mistake}
** When Server state == {state}
** Reason for termination == 
** {badarith,[{ter_a,handle_cast,2,[{file,"ter_a.erl"},{line,63}]},
              {gen_server,handle_msg,5,[{file,"gen_server.erl"},{line,599}]},
              {proc_lib,init_p_do_apply,3,
                        [{file,"proc_lib.erl"},{line,237}]}]}

=ERROR REPORT==== 28-Apr-2015::11:40:11 ===
** Generic server ter_b terminating 
** Last message in was {'EXIT',<0.33.0>,
                           {badarith,
                               [{ter_a,handle_cast,2,
                                    [{file,"ter_a.erl"},{line,63}]},
                                {gen_server,handle_msg,5,
                                    [{file,"gen_server.erl"},{line,599}]},
                                {proc_lib,init_p_do_apply,3,
                                    [{file,"proc_lib.erl"},{line,237}]}]}}
** When Server state == {state}
** Reason for termination == 
** {badarith,[{ter_a,handle_cast,2,[{file,"ter_a.erl"},{line,63}]},
              {gen_server,handle_msg,5,[{file,"gen_server.erl"},{line,599}]},
              {proc_lib,init_p_do_apply,3,
                        [{file,"proc_lib.erl"},{line,237}]}]}

4> ter_a:is_alive().
** exception exit: {noproc,{gen_server,call,[ter_a,is_alive]}}
     in function  gen_server:call/2 (gen_server.erl, line 182)
5> ter_b:is_alive().
** exception exit: {noproc,{gen_server,call,[ter_b,is_alive]}}
     in function  gen_server:call/2 (gen_server.erl, line 182)

正常退出：

1> ter_a:start(0).
{ok,<0.33.0>}
2> ter_a:link(1).
handle_call, link to ter_b, LinkPid:<0.35.0>
<0.35.0>
3> ter_a:stop(normal).
handle_cast, stop, Reason:normal
terminate, Reason:normal
ok
ter_b, terminate

三、一些结论和补充

1) 在gen_server中，进程结束时不是什么情况都会跑到terminat/2函数；

2)  如果有两个gen_server进程相互链接，需要让两个进程同时存在同时消亡（无论原因），并且在消亡的时候都保证要跑到terminate/2去，则需要给每个进程捕获退出消息process_flag(trap_exit, true)；

3) 如果gen_server进程是supervision tree的一部分，并且由supervision去终止，只要符合以下条件terminate/2就会以shutdown作为Reason被调用

    a) gen_server进程捕获退出;

    b) 在supervision关于这个gen_server子策略中，Shutdown的值是一个整数，而非brutal_kill .

（完）