Question

我的gen_server包含这样的方法：

handle_call(error, From, State) ->
    io:format("Inside the handle_call error~n"),
    1/0.

它提供了start（非start_link）函数：

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

normal_call() ->
    gen_server:call(?MODULE, {normal}).

error_call() ->
    gen_server:call(?MODULE, error).

我从shell调用start函数：

c(some_module).
some_module:start().

然后我调用终止服务器进程的错误调用，因为除零错误，但它也终止了shell（调用进程）。我不明白为什么？它们没有链接但仍然使用新的pid重新启动shell。这是gen_server的预期行为，还是我做错了什么？

更新：它仍然不起作用，以帮助我发布完整的代码。

-module(some_test).
-behaviour(gen_server).
-compile(export_all).

%% api functions, can be directly used by the user

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

normal_call() ->
    gen_server:call(?MODULE, normal, infinity).

error_call() ->
    gen_server:call(?MODULE, error, infinity).

%% service specific function shoule not be call directly

init(_) ->
    io:format("Inside the init ( ~p )~n", [self()]),
    io:format("Inside the init...~n"),
    {ok, nil}.

handle_call(normal, _, _) ->
    io:format("Inside the handle_call normal~n"),
    {reply, ok, nil};

handle_call(error, _, nil) ->
    io:format("Inside the handle_call error~n"),
    1/0.

terminate(Reason, _) ->
    io:format("Inside the terminate~p~n", [Reason]).

%% just to complete

handle_info(_, _) ->
    {noreply, nil}.

handle_cast(_, _) ->
    {noreply, nil}.

code_change(_, _, _) ->
    {ok, nil}.

%% a single test function that prove that called process was removed immeditely, did not wait for 5 seconds

test_function() ->
    io:format("Id is : ~p~n", [self()]),
    ?MODULE:start(),
    ?MODULE:normal_call(),
    ?MODULE:error_call(),
    io:format("Id is : ~p~n", [self()]).   %% never reached :(

并开始使用它： -

c(some_test).
some_test:test_function().

得到了输出： -

20> some_test:test_function().
Id is : <0.84.0>
Inside the init ( <0.92.0> )
Inside the init...
Inside the handle_call normal
Inside the handle_call error
Inside the terminate{badarith,
                        [{some_test,handle_call,3,
                             [{file,"some_test.erl"},{line,29}]},
                         {gen_server,try_handle_call,4,
                             [{file,"gen_server.erl"},{line,629}]},
                         {gen_server,handle_msg,5,
                             [{file,"gen_server.erl"},{line,661}]},
                         {proc_lib,init_p_do_apply,3,
                             [{file,"proc_lib.erl"},{line,240}]}]}

=ERROR REPORT==== 3-Dec-2015::18:36:22 ===
** Generic server some_test terminating
** Last message in was error
** When Server state == nil
** Reason for termination ==
** {badarith,[{some_test,handle_call,3,[{file,"some_test.erl"},{line,29
              {gen_server,try_handle_call,4,
                          [{file,"gen_server.erl"},{line,629}]},
              {gen_server,handle_msg,5,[{file,"gen_server.erl"},{line,6
              {proc_lib,init_p_do_apply,3,
                        [{file,"proc_lib.erl"},{line,240}]}]}
** exception exit: {{badarith,
                        [{some_test,handle_call,3,
                             [{file,"some_test.erl"},{line,29}]},
                         {gen_server,try_handle_call,4,
                             [{file,"gen_server.erl"},{line,629}]},
                         {gen_server,handle_msg,5,
                             [{file,"gen_server.erl"},{line,661}]},
                         {proc_lib,init_p_do_apply,3,
                             [{file,"proc_lib.erl"},{line,240}]}]},
                    {gen_server,call,[some_test,error,infinity]}}
     in function  gen_server:call/3 (gen_server.erl, line 212)
     in call from some_test:test_function/0 (some_test.erl, line 51)
21>

因此我们可以看到some_test之后的最后一行：error_call（）永远不会被调用？因为调用进程也终止了？

Answer 1

gen_server:call/2,3是同步消息传递功能。这意味着在gen_server向发件人发送回复之前它不会返回。在这种情况下，gen_server崩溃，因此永远不会发送响应。结果是调用函数超时，并且该超时的结果是崩溃。

您会注意到被叫方立即崩溃，但是呼叫者在5秒后崩溃。那是因为默认超时是5000毫秒（检查上面链接的文档）。尝试将其设置为infinity - 您的呼叫过程将会挂起，永远阻止等待永不响应的响应。

[ UPDATE：直接来自shell的调用会立即崩溃，因为在执行过程中会引发异常。这与超时到期不同 - 两种情况都会导致崩溃，但异常是即时的。]

解决这个问题的方法是使用gen_server:cast/2发送异步消息。尝试定义：

handle_cast(error, State) ->
    io:format("Inside the handle_cast error~n"),
    1/0.

这只会导致被叫方崩溃;呼叫者将在消息离开的瞬间继续。它就像扔棒球而走开而不是扔飞去旋转等待。

当您获得使用Erlang的经验时，您将倾向于使用尽可能多cast的代码编写代码，并且当情境确实要求同步时，仅使用call s消息（某些状态值实际上取决于事件序列）。这使得系统在许多方面更松散地耦合，并使您的调用函数能够抵抗它们发送数据的过程中的失败。

修改

shell崩溃是因为它在执行期间进行了调用崩溃。但是，使用异步消息不会发生这种情况。我添加了另一个模块并扩展了你的模块以说明这一点。现在有一个名为some_tester的新模块可以传递我们发送的所有内容，因此它会崩溃，而不是shell。以下是相关内容：

-module(some_tester). -compile(export_all). start() -> gen_server:start({local, ?MODULE}, ?MODULE, [], []). relay(Nature, Message) -> gen_server:cast(?MODULE, {Nature, Message}). init(State) -> ok = z("Starting..."), {ok, State}. handle_call(Message, From, S) -> ok = z("Received ~tp from ~tp", [Message, From]), {reply, ok, S}. handle_cast({cast, Message}, S) -> ok = some_test:cast(Message), {noreply, S}; handle_cast({call, Message}, S) -> ok = z("Sending call with ~tp", [Message]), Reply = some_test:normal_call(Message), ok = z("Received ~tp as reply", [Reply]), {noreply, S}; handle_cast({infinite, Message}, S) -> ok = z("Sending infinite with ~tp", [Message]), Reply = some_test:infinite_call(Message), ok = z("Received ~tp as reply", [Reply]), {noreply, S}; handle_cast(Message, S) -> ok = z("Unexpected ~tp", [Message]), {noreply, S}.

以下是some_test的相关位：

start() -> gen_server:start({local, ?MODULE}, ?MODULE, [], []). normal_call(Message) -> gen_server:call(?MODULE, Message). infinite_call(Message) -> gen_server:call(?MODULE, Message, infinity). cast(Message) -> gen_server:cast(?MODULE, Message). % ... handle_call(normal, _, S) -> io:format("Inside the handle_call normal~n"), {reply, ok, S}; handle_call(error, _, S) -> io:format("Inside the handle_call error~n"), {reply, 1/0, S}; handle_call(bad_reply, _, S) -> io:format("Inside the handle_call error~n"), foo; handle_call(Message, From, S) -> io:format("Received ~tp from ~tp~n", [Message, From]), {reply, ok, S}. handle_cast(error, S) -> io:format("Bad arith: ~tp~n", [1/0]), {noreply, S}; handle_cast(Message, S) -> io:format("Received ~tp~n", [Message]), {noreply, S}.

这是一个玩弄它的游戏。请注意shell自己调用self()和flush()：
的输出
1> c(some_test). some_test.erl:31: Warning: this expression will fail with a 'badarith' exception some_test.erl:32: Warning: variable 'S' is unused some_test.erl:40: Warning: this expression will fail with a 'badarith' exception {ok,some_test} 2> c(some_tester). {ok,some_tester} 3> {ok, Test} = some_test:start(). Inside the init ( <0.45.0> ) Inside the init... {ok,<0.45.0>} 4> {ok, Tester} = some_tester:start(). <0.47.0> some_tester: Starting... {ok,<0.47.0>} 5> monitor(process, Test). #Ref<0.0.2.178> 6> monitor(process, Tester). #Ref<0.0.2.183> 7> self(). <0.33.0> 8> some_tester:relay(call, foo). <0.47.0> some_tester: Sending call with foo ok Received foo from {<0.47.0>,#Ref<0.0.2.196>} <0.47.0> some_tester: Received ok as reply 9> some_tester:relay(cast, bar). Received bar ok 10> some_tester:relay(call, error). <0.47.0> some_tester: Sending call with error ok Inside the handle_call error Inside the terminate{badarith, [{some_test,handle_call,3, [{file,"some_test.erl"},{line,31}]}, {gen_server,try_handle_call,4, [{file,"gen_server.erl"},{line,629}]}, {gen_server,handle_msg,5, [{file,"gen_server.erl"},{line,661}]}, {proc_lib,init_p_do_apply,3, [{file,"proc_lib.erl"},{line,240}]}]} Inside the terminate{{badarith, [{some_test,handle_call,3, [{file,"some_test.erl"},{line,31}]}, {gen_server,try_handle_call,4, [{file,"gen_server.erl"},{line,629}]}, {gen_server,handle_msg,5, [{file,"gen_server.erl"},{line,661}]}, {proc_lib,init_p_do_apply,3, [{file,"proc_lib.erl"},{line,240}]}]}, {gen_server,call,[some_test,error]}} 11> =ERROR REPORT==== 3-Dec-2015::22:52:17 === ** Generic server some_test terminating ** Last message in was error ** When Server state == nil ** Reason for termination == ** {badarith,[{some_test,handle_call,3,[{file,"some_test.erl"},{line,31}]}, {gen_server,try_handle_call,4, [{file,"gen_server.erl"},{line,629}]}, {gen_server,handle_msg,5,[{file,"gen_server.erl"},{line,661}]}, {proc_lib,init_p_do_apply,3, [{file,"proc_lib.erl"},{line,240}]}]} =ERROR REPORT==== 3-Dec-2015::22:52:17 === ** Generic server some_tester terminating ** Last message in was {'$gen_cast',{call,error}} ** When Server state == [] ** Reason for termination == ** {{{badarith,[{some_test,handle_call,3,[{file,"some_test.erl"},{line,31}]}, {gen_server,try_handle_call,4, [{file,"gen_server.erl"},{line,629}]}, {gen_server,handle_msg,5,[{file,"gen_server.erl"},{line,661}]}, {proc_lib,init_p_do_apply,3, [{file,"proc_lib.erl"},{line,240}]}]}, {gen_server,call,[some_test,error]}}, [{gen_server,call,2,[{file,"gen_server.erl"},{line,204}]}, {some_tester,handle_cast,2,[{file,"some_tester.erl"},{line,24}]}, {gen_server,try_dispatch,4,[{file,"gen_server.erl"},{line,615}]}, {gen_server,handle_msg,5,[{file,"gen_server.erl"},{line,681}]}, {proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,240}]}]} 11> self(). <0.33.0> 12> flush(). Shell got {'DOWN',#Ref<0.0.2.178>,process,<0.45.0>, {badarith, [{some_test,handle_call,3, [{file,"some_test.erl"},{line,31}]}, {gen_server,try_handle_call,4, [{file,"gen_server.erl"},{line,629}]}, {gen_server,handle_msg,5, [{file,"gen_server.erl"},{line,661}]}, {proc_lib,init_p_do_apply,3, [{file,"proc_lib.erl"},{line,240}]}]}} Shell got {'DOWN',#Ref<0.0.2.183>,process,<0.47.0>, {{badarith, [{some_test,handle_call,3, [{file,"some_test.erl"},{line,31}]}, {gen_server,try_handle_call,4, [{file,"gen_server.erl"},{line,629}]}, {gen_server,handle_msg,5, [{file,"gen_server.erl"},{line,661}]}, {proc_lib,init_p_do_apply,3, [{file,"proc_lib.erl"},{line,240}]}]}, {gen_server,call,[some_test,error]}}} ok

仔细阅读。

gen_server中的错误也会终止调用进程吗？

1 个答案: