redis-软看门狗实现分析

redis在2.6?版本实现了软看门狗，利用setitimer产生SIGALRM信号量实现，并且需要定期喂狗，如果没有喂则回触发一次

样例代码如下，可利用代码实现慢操作跟踪/logStackTrace可用于实现coredump的函数栈打印

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
 #include 

//backtrace
#include 

#define HAVE_BACKTRACE 1

typedef struct server{
	int watchdog_period;
	int hz;                     /* serverCron() calls frequency in hertz */
	char logfile[10];
}server_t;

server_t server = {
		.watchdog_period = 0,
		.hz = 10,
		.logfile[0] = '\0',
	};


#ifdef HAVE_BACKTRACE
static void *getMcontextEip(ucontext_t *uc) {
	return (void*) uc->uc_mcontext.gregs[16]; /* Linux 64 */
}
#endif

/* Return a file descriptor to write directly to the Redis log with the
 * write(2) syscall, that can be used in critical sections of the code
 * where the rest of Redis can't be trusted (for example during the memory
 * test) or when an API call requires a raw fd.
 *
 * Close it with closeDirectLogFiledes(). */
int openDirectLogFiledes(void) {
    int log_to_stdout = server.logfile[0] == '\0';
    int fd = log_to_stdout ?
        STDOUT_FILENO :
        open(server.logfile, O_APPEND|O_CREAT|O_WRONLY, 0644);
    return fd;
}

/* Used to close what closeDirectLogFiledes() returns. */
void closeDirectLogFiledes(int fd) {
    int log_to_stdout = server.logfile[0] == '\0';
    if (!log_to_stdout) close(fd);
}


/* 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[101];
    int trace_size = 0, fd = openDirectLogFiledes();

    if (fd == -1) return; /* If we can't log there is anything to do. */

    /* Generate the stack trace */
    trace_size = backtrace(trace+1, 100);

    if (getMcontextEip(uc) != NULL) {
        char *msg1 = "EIP:\n";
        char *msg2 = "\nBacktrace:\n";
        if (write(fd,msg1,strlen(msg1)) == -1) {/* Avoid warning. */};
        trace[0] = getMcontextEip(uc);
        backtrace_symbols_fd(trace, 1, fd);
        if (write(fd,msg2,strlen(msg2)) == -1) {/* Avoid warning. */};
    }

    /* Write symbols to log file */
    backtrace_symbols_fd(trace+1, trace_size, fd);

    /* Cleanup */
    closeDirectLogFiledes(fd);
}


void watchdogSignalHandler(int sig, siginfo_t *info, void *secret) {
#ifdef HAVE_BACKTRACE
    ucontext_t *uc = (ucontext_t*) secret;
#else
    (void)secret;
#endif

    //serverLogFromHandler(LL_WARNING,"\n--- WATCHDOG TIMER EXPIRED ---");
#ifdef HAVE_BACKTRACE
    logStackTrace(uc);
#else
    
#endif
    printf("--------\n");
}


void watchdogScheduleSignal(int period) {
    struct itimerval it;

    /* Will stop the timer if period is 0. */
    it.it_value.tv_sec = period/1000;
    it.it_value.tv_usec = (period%1000)*1000;
    /* Don't automatically restart. */
    it.it_interval.tv_sec = 0;
    it.it_interval.tv_usec = 0;
    setitimer(ITIMER_REAL, &it, NULL);
}

/* Enable the software watchdog with the specified period in milliseconds. */
void enableWatchdog(int period) {
    int min_period;

    if (server.watchdog_period == 0) {
        struct sigaction act;

        /* Watchdog was actually disabled, so we have to setup the signal
         * handler. */
        sigemptyset(&act.sa_mask);
        act.sa_flags = SA_SIGINFO;
        act.sa_sigaction = watchdogSignalHandler;
        sigaction(SIGALRM, &act, NULL);
    }
    /* If the configured period is smaller than twice the timer period, it is
     * too short for the software watchdog to work reliably. Fix it now
     * if needed. */
    min_period = (1000/server.hz)*2;
    if (period < min_period) period = min_period;
    watchdogScheduleSignal(period); /* Adjust the current timer. */
    server.watchdog_period = period;
}

/* Disable the software watchdog. */
void disableWatchdog(void) {
    struct sigaction act;
    if (server.watchdog_period == 0) return; /* Already disabled. */
    watchdogScheduleSignal(0); /* Stop the current timer. */

    /* Set the signal handler to SIG_IGN, this will also remove pending
     * signals from the queue. */
    sigemptyset(&act.sa_mask);
    act.sa_flags = 0;
    act.sa_handler = SIG_IGN;
    sigaction(SIGALRM, &act, NULL);
    server.watchdog_period = 0;
}

int fun2()
{
	sleep(1);
	return 0;
}

int fun1()
{
	fun2();
	return 1+2;
}	

int main()
{
	enableWatchdog(500);

	while(1){
		usleep(100*1000);
		fun1();
		//feed dog
		if (server.watchdog_period) watchdogScheduleSignal(server.watchdog_period);
		
	}

	return 1;
}

//gcc -g -rdynamic watchdog.c -o watchdog