erlang - gen_server 中的错误也会终止调用进程？

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.

它提供了一个start(不是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，我调用错误调用，由于被零除错误而终止服务器进程，但它也会终止 shell(调用进程)。我不明白为什么？它们没有链接，但 shell 仍然以新的 pid 重新启动。这是 gen_server 的预期行为，还是我做错了什么？

更新:它仍然无法工作，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 :(

首先我用了这个:-

c(some_test).
some_test:test_function().

并得到输出:-

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>

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

Answer 1

gen_server:call/2,3是一个同步消息传递函数。这意味着在 gen_server 向发送者发送回复之前它不会返回。在这种情况下，gen_server 崩溃了，因此永远不会发送响应。这样做的结果是调用函数超时，而超时的结果就是崩溃。

你会注意到被调用者立即崩溃，但调用者在 5 秒后崩溃。这是因为默认超时为 5000 毫秒(查看上面链接的文档)。尝试将其设置为无穷大——您的调用进程将挂起，永远阻塞，等待永远不会到来的响应。

[更新:直接来自 shell 的调用立即崩溃，因为在执行过程中引发了异常。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。 。尝试定义这个:

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

这只会导致被调用者崩溃；调用者将在消息消失后​​立即继续。这就像扔一个棒球然后走开，而不是扔一个回旋镖然后等待。

随着您获得 Erlang 的经验，您将倾向于编写具有尽可能多的 cast 的代码，并且仅在情况确实时才使用 call/em> 需要同步消息(某些状态值实际上取决于事件的顺序)。这使得系统在很多方面都更加松散耦合，并使您的调用函数能够抵抗它们向其发送数据的进程中的故障。

编辑

shell 崩溃是因为它所进行的调用在执行过程中崩溃了。不过，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您的模块来说明这一点。现在有一个名为 some_tester 的新模块，它会中继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的相关部分:

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}.

这是一个运行它的例子。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。