hiredis aeStop仅在redis命令的回调函数中生效 分析

hiredis 是 redis 的client端C语言 lib,  hiredis拥有同步和异步的API， 异步API的实现有多种方法，分别依赖libev, libevent, libuv, ae等等，其中ae是redis内部实现的一个异步事件处理模块。

稍微修改了hiredis的example-ae.c代码：在一个线程里面循环10次执行命令ping, 检查redisserver, 如下所示, 线程发完10次ping后，调用disconnect, 发现aeMain函数并未退出，程序一直阻塞住.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <thread>
#include<functional>

#ifdef __cplusplus
    extern "C"{
#endif

#include <hiredis.h>
#include <async.h>
#include <adapters/ae.h>

#ifdef __cplusplus
           }
#endif

/* Put event loop in the global scope, so it can be explicitly stopped */
static aeEventLoop *loop;

void getCallback(redisAsyncContext *c, void *r, void *privdata) {
    redisReply *reply = r;
    if (reply == NULL) return;
    printf("argv[%s]: %s\n", (char*)privdata, reply->str);
}

void connectCallback(const redisAsyncContext *c, int status) {
    if (status != REDIS_OK) {
        printf("Error: %s\n", c->errstr);
        aeStop(loop);
        return;
    }

    printf("Connected...\n");
}

void disconnectCallback(const redisAsyncContext *c, int status) {
    if (status != REDIS_OK) {
        printf("Error: %s\n", c->errstr);
        aeStop(loop);
        return;
    }

    printf("Disconnected...\n");
    aeStop(loop);
}

void quitConnCallBack(redisAsyncContext *c, void *r, void *privdata) {
    printf("quit");
    redisAsyncDisconnect(c);
}

void testThreadLoop(void * p) {
    static int num = 10;
    char c11[64];
    strcpy(c11, "test");
    while(1) {
        std::this_thread::sleep_for(std::chrono::milliseconds(1500));
        num--;
        if (num < 0) {

            //在这里调用disconnect, 并不能使aeMain退出
            redisAsyncDisconnect((redisAsyncContext *)p);
            //正确做法，应该调用如下
　　　　　　　//redisAsyncCommand((redisAsyncContext *)p, quitConnCallBack, c11, "quit");
            printf("exit\n");
            return;
        }

        redisAsyncCommand((redisAsyncContext *)p, getCallback, c11, "ping");
    }
}

int main (int argc, char **argv) {
    signal(SIGPIPE, SIG_IGN);

    redisAsyncContext *c = redisAsyncConnect("127.0.0.1", 6379);
    if (c->err) {
        /* Let *c leak for now... */
        printf("Error: %s\n", c->errstr);
        return 1;
    }

    loop = aeCreateEventLoop(64);
    redisAeAttach(loop, c);
    redisAsyncSetConnectCallback(c,connectCallback);
    redisAsyncSetDisconnectCallback(c,disconnectCallback);

    std::thread t(testThreadLoop, c);
    t.detach();
    aeMain(loop);
    return 0;
} 

首先检查下两个主要函数aeStop, aeMain的逻辑：

aeStop, aeMain函数代码如下：

void aeMain(aeEventLoop *eventLoop) {
    eventLoop->stop = 0;
    while (!eventLoop->stop) {
        if (eventLoop->beforesleep != NULL)
            eventLoop->beforesleep(eventLoop);
        aeProcessEvents(eventLoop, AE_ALL_EVENTS);
    }
}
void aeStop(aeEventLoop *eventLoop) {
    eventLoop->stop = 1;
}

1. stop分析

aeStop仅设置stop标志为true, aeMain里面在一直循环处理事件，第一印象是，直接设了stop为true后，aeMain在处理完事件后，跳出aeProcessEvents函数后，检查stop为true就会跳出while循环。但是事实是aeMain并未跳出循环，难道因为是不同线程间操作，要将stop设置为volatile类型？尝试修改了stop为volatile int类型，测试结果：aeMain 仍然未推出，程序阻塞，无法推出。

2.aeProcessEvents分析

这时就只能推测由于aeProcessEvents没有退出，导致aeMain执行无法检测stop值，分析该函数，推测可能阻塞在aeApiPoll函数，同时发现tvp变量是个NULL, 查看aeApiPoll代码（ae_epoll.c），如下

int aeProcessEvents(aeEventLoop *eventLoop, int flags)
{
　　　　 //...
　　　　 //tvp 会是NULL, 推测阻塞在aeApiPoll, 查看aeApiPoll代码进行证实
        numevents = aeApiPoll(eventLoop, tvp);
        for (j = 0; j < numevents; j++) {
            aeFileEvent *fe = &eventLoop->events[eventLoop->fired[j].fd];
            int mask = eventLoop->fired[j].mask;
            int fd = eventLoop->fired[j].fd;
            int rfired = 0;

        /* note the fe->mask & mask & ... code: maybe an already processed
             * event removed an element that fired and we still didn‘t
             * processed, so we check if the event is still valid. */
            if (fe->mask & mask & AE_READABLE) {
                rfired = 1;
                fe->rfileProc(eventLoop,fd,fe->clientData,mask);
            }
            if (fe->mask & mask & AE_WRITABLE) {
                if (!rfired || fe->wfileProc != fe->rfileProc)
                    fe->wfileProc(eventLoop,fd,fe->clientData,mask);
            }
            processed++;
        }
    }
    /* Check time events */
    if (flags & AE_TIME_EVENTS)
        processed += processTimeEvents(eventLoop);

    return processed; /* return the number of processed file/time events */
}
static int aeApiPoll(aeEventLoop *eventLoop, struct timeval *tvp) {
    aeApiState *state = eventLoop->apidata;
    int retval, numevents = 0;

　　//真相在这边，epoll_wait, 第三个参数为-1, epoll_wait将一直等待下去！
    retval = epoll_wait(state->epfd,state->events,eventLoop->setsize,
            tvp ? (tvp->tv_sec*1000 + tvp->tv_usec/1000) : -1);
    if (retval > 0) {
        int j;

        numevents = retval;
        for (j = 0; j < numevents; j++) {
            int mask = 0;
            struct epoll_event *e = state->events+j;

            if (e->events & EPOLLIN) mask |= AE_READABLE;
            if (e->events & EPOLLOUT) mask |= AE_WRITABLE;
            if (e->events & EPOLLERR) mask |= AE_WRITABLE;
            if (e->events & EPOLLHUP) mask |= AE_WRITABLE;
            eventLoop->fired[j].fd = e->data.fd;
            eventLoop->fired[j].mask = mask;
        }
    }
    return numevents;
}

整理下aeMain的流程如下图所示，

我们的disconnect回调, 内部调用 aeStop函数，如果刚好发生在processEvents之后，aeMain检查stop值之前，那么就没问题，当然这种概率极其小，如果这都中了，那可以买彩票了~~，现在我们知道aestop调用是有立即生效的限制范围，我们最好在processEvents的时候，判断是否应该退出aeMain, 如果是就调用aeStop. processEvents内部会调用到我们外部定义的各种命令的回调函数， 刚好redis有个quit的命令（让redisserver关闭连接）， 我们就增加一个quit命令回调函数调用aeStop:

redisAsyncCommand((redisAsyncContext *)p, quitConnCallBack, c11, "quit");

void quitConnCallBack(redisAsyncContext *c, void *r, void *privdata) {
    printf("quit");
    redisAsyncDisconnect(c);
}