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[数据库]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);}