Erlang中使用Action ID调用wait函数无响应问题求助

重现步骤 #

• 在终端执行：

  A1 = async:new(fun(Args) -> lists:seq(1, Args) end, 10).
  

• 得到输出："#Ref<0.2297416181.3164864513.214889>"
• 调用Result = async:wait(A1).后无任何响应，预期结果应为[1,2,3,4,5,6,7,8,9,10]

完整代码 #

-module(async).
-export([new/2, wait/1, poll/1]).

new(Fun, Arg) ->
    Aid = make_ref(),  % Generate a unique action ID
    ComputationPid = spawn_link(fun() -> execute_computation(self(), Fun, Arg) end),
    State = {ComputationPid, none},
    erlang:put(Aid, State),
    Aid.

execute_computation(ParentPid, Fun, Arg) ->
    try
        Res = Fun(Arg),
        ParentPid ! {self(), {ok, Res}}
    catch
        throw:Ex ->
            ParentPid ! {self(), {exception, Ex}}
    end.

wait(Aid) ->
    receive
        {Aid, {ok, Res}} -> Res;
        {Aid, {exception, Ex}} -> throw(Ex)
    end.

poll(Aid) ->
    State = erlang:get(Aid),
    case State of
        undefined -> nothing;
        {_, none} -> nothing;
        {_, Result} -> Result
    end.

问题原因 #

1. 消息格式不匹配：子进程execute_computation发送的消息是{self(), {ok, Res}}（以自身PID为标识），但wait函数等待的是{Aid, ...}（以生成的引用为标识），导致消息无法被正确接收，进程一直阻塞在receive语句。
2. 进程字典状态未更新：子进程计算完成后，没有将结果写入进程字典，即使消息格式正确，poll函数也无法获取已完成的计算结果。

修复方案 #

修改代码，使子进程发送包含Aid的消息，并在wait函数中同时处理进程字典的状态检查与消息接收，确保结果能被正确获取：

-module(async).
-export([new/2, wait/1, poll/1]).

new(Fun, Arg) ->
    Aid = make_ref(),
    ComputationPid = spawn_link(fun() -> execute_computation(self(), Aid, Fun, Arg) end),
    State = {ComputationPid, none},
    erlang:put(Aid, State),
    Aid.

execute_computation(ParentPid, Aid, Fun, Arg) ->
    try
        Res = Fun(Arg),
        ParentPid ! {Aid, {ok, Res}}
    catch
        throw:Ex ->
            ParentPid ! {Aid, {exception, Ex}}
    end.

wait(Aid) ->
    case erlang:get(Aid) of
        {_, {ok, Res}} -> Res;
        {_, {exception, Ex}} -> throw(Ex);
        {Pid, none} ->
            receive
                {Aid, {ok, Res}} ->
                    erlang:put(Aid, {Pid, {ok, Res}}),
                    Res;
                {Aid, {exception, Ex}} ->
                    erlang:put(Aid, {Pid, {exception, Ex}}),
                    throw(Ex)
            end;
        undefined ->
            error(invalid_aid)
    end.

poll(Aid) ->
    State = erlang:get(Aid),
    case State of
        undefined -> nothing;
        {_, none} -> nothing;
        {_, Result} -> Result
    end.

修复说明 #

• 调整execute_computation的参数，传入Aid，让子进程发送包含Aid的消息，匹配wait的接收格式。
• 优化wait函数：先检查进程字典中是否已有计算结果，有则直接返回；无则等待消息，收到消息后更新进程字典状态，同时支持poll函数获取结果。
• 增加无效Aid的错误处理，避免无意义的阻塞。

内容的提问来源于stack exchange，提问作者Arraytics New