（一）redis源码阅读

• 1. srand()和rand()函数以及随机数种子
• 2. 守护进程及pid文件
• 3. 初始化服务器时的信号处理
• 4. 在程序coredump时记录堆栈信息StackTrace

1. srand()和rand()函数以及随机数种子

srand(time(NULL)^getpid());

struct timeval tv;
gettimeofday(&tv,NULL);
dictSetHashFunctionSeed(tv.tv_sec^tv.tv_usec^getpid());

2. 守护进程及pid文件

在main()函数中，通过配置文件判断是否以守护进程模式运行。

if (server.daemonize) daemonize();     // 守护进程
if (server.daemonize) createPidFile();  // 记录pid的文件

// 将程序以守护进程方式运行
void daemonize(void) {
    int fd;

    if (fork() != 0) exit(0); /* parent exits */
    setsid(); /* create a new session */

    /* Every output goes to /dev/null. If Redis is daemonized but
     * the 'logfile' is set to 'stdout' in the configuration file
     * it will not log at all. */
    if ((fd = open("/dev/null", O_RDWR, 0)) != -1) {
        dup2(fd, STDIN_FILENO);
        dup2(fd, STDOUT_FILENO);
        dup2(fd, STDERR_FILENO);
        if (fd > STDERR_FILENO) close(fd);
    }
}

// 保存进程ID到文件
void createPidFile(void) {
    /* Try to write the pid file in a best-effort way. */
    FILE *fp = fopen(server.pidfile,"w");
    if (fp) {
        fprintf(fp,"%d\n",(int)getpid());
        fclose(fp);
    }
}

3. 初始化服务器时的信号处理

// 初始化服务器函数
void initServer(void) {

    // 忽视信号: SIGHUP, SIGPIPE 
    signal(SIGHUP, SIG_IGN);
    signal(SIGPIPE, SIG_IGN);

    // 设置SIGTERM, SIGINT 信号处理函数
    setupSignalHandlers();

// 设置信号处理函数，当SIGTERN, SIGINT时，退出程序
void setupSignalHandlers(void) {
    struct sigaction act;

    /* When the SA_SIGINFO flag is set in sa_flags then sa_sigaction is used.
     * Otherwise, sa_handler is used. */
    sigemptyset(&act.sa_mask);
    act.sa_flags = 0;
    act.sa_handler = sigShutdownHandler;
    sigaction(SIGTERM, &act, NULL);
    sigaction(SIGINT, &act, NULL);

#ifdef HAVE_BACKTRACE
    sigemptyset(&act.sa_mask);
    act.sa_flags = SA_NODEFER | SA_RESETHAND | SA_SIGINFO;
    act.sa_sigaction = sigsegvHandler;
    sigaction(SIGSEGV, &act, NULL);
    sigaction(SIGBUS, &act, NULL);
    sigaction(SIGFPE, &act, NULL);
    sigaction(SIGILL, &act, NULL);
#endif
    return;
}

// 信号处理函数，ctrl+c 会产生 SIGINT信号，连续两次，会退出程序。
static void sigShutdownHandler(int sig) {
    char *msg;

    switch (sig) {
    case SIGINT:
        msg = "Received SIGINT scheduling shutdown...";
        break;
    case SIGTERM:
        msg = "Received SIGTERM scheduling shutdown...";
        break;
    default:
        msg = "Received shutdown signal, scheduling shutdown...";
    };

    /* SIGINT is often delivered via Ctrl+C in an interactive session.
     * If we receive the signal the second time, we interpret this as
     * the user really wanting to quit ASAP without waiting to persist
     * on disk. */
    if (server.shutdown_asap && sig == SIGINT) {
        redisLogFromHandler(REDIS_WARNING, "You insist... exiting now.");
        rdbRemoveTempFile(getpid());
        exit(1); /* Exit with an error since this was not a clean shutdown. */
    } else if (server.loading) {
        exit(0);
    }

    redisLogFromHandler(REDIS_WARNING, msg);

    // 在接收到SIGTERM/SIGINT信号后，在信号处理函数中将server.shutdown_asap置为1；
    // 当下次运行serverCron函数时，会检查该值，然后安全退出程序。
    server.shutdown_asap = 1;
}

4. 在程序coredump时记录堆栈信息StackTrace

/* Logs the stack trace using the backtrace() call. This function is designed
 * to be called from signal handlers safely. */
void logStackTrace(ucontext_t *uc) {
    void *trace[100];
    int trace_size = 0, fd;
    int log_to_stdout = server.logfile[0] == '\0';

    /* Open the log file in append mode. */
    fd = log_to_stdout ?
        STDOUT_FILENO :
        open(server.logfile, O_APPEND|O_CREAT|O_WRONLY, 0644);
    if (fd == -1) return;

    /* Generate the stack trace */
    trace_size = backtrace(trace, 100);

    /* overwrite sigaction with caller's address */
    if (getMcontextEip(uc) != NULL)
        trace[1] = getMcontextEip(uc);

    /* Write symbols to log file */
    backtrace_symbols_fd(trace, trace_size, fd);

    /* Cleanup */
    if (!log_to_stdout) close(fd);
}