poolboy的bug

poolboy在checkout工作者进程的时候，每次都取第一个进程，导致就算是用cast异步处理耗时操作，也会被阻塞，换成随机取进程

老代码

handle_call({checkout, Block}, {FromPid, _} = From, State) ->
    #state{supervisor = Sup,
           workers = Workers,
           monitors = Monitors,
           overflow = Overflow,
           max_overflow = MaxOverflow} = State,
    case Workers of
        [Pid | Left] ->
            Ref = erlang:monitor(process, FromPid),
            true = ets:insert(Monitors, {Pid, Ref}),
            {reply, Pid, State#state{workers = Left}};
        [] when MaxOverflow > 0, Overflow < MaxOverflow ->
            {Pid, Ref} = new_worker(Sup, FromPid),
            true = ets:insert(Monitors, {Pid, Ref}),
            {reply, Pid, State#state{overflow = Overflow + 1}};
        [] when Block =:= false ->
            {reply, full, State};
        [] ->
            Ref = erlang:monitor(process, FromPid),
            Waiting = queue:in({From, Ref}, State#state.waiting),
            {noreply, State#state{waiting = Waiting}}
    end;

换成

handle_call({checkout, Block}, {FromPid, _} = From, State) ->
    #state{supervisor = Sup,
           workers = Workers,
           monitors = Monitors,
           overflow = Overflow,
           max_overflow = MaxOverflow} = State,

    case length(Workers) of
        0 when MaxOverflow > 0, Overflow < MaxOverflow ->
            {Pid, Ref} = new_worker(Sup, FromPid),
            true = ets:insert(Monitors, {Pid, Ref}),
            {reply, Pid, State#state{overflow = Overflow + 1}};

        0 when Block =:= false ->
            {reply, full, State};

        0 ->
            Ref = erlang:monitor(process, FromPid),
            Waiting = queue:in({From, Ref}, State#state.waiting),
            {noreply, State#state{waiting = Waiting}};

        _ when length(Workers) > 0 ->
            RadomIndex = random:uniform(999999999999) rem length(Workers),
            Pid = lists:nth(RadomIndex,Workers),
            Left = lists:delete(Pid,Workers),
            Ref = erlang:monitor(process, FromPid),
            true = ets:insert(Monitors, {Pid, Ref}),
            {reply, Pid, State#state{workers = Left}}
    end;