Function clusterLockConfig

app/redis-6.2.6/src/cluster.c:395–441

Lock the cluster config using flock(), and leaks the file descriptor used to * acquire the lock so that the file will be locked forever. * * This works because we always update nodes.conf with a new version * in-place, reopening the file, and writing to it in place (later adjusting * the length with ftruncate()). * * On success C_OK is returned, otherwise an error is logged and * the funct

 * On success C_OK is returned, otherwise an error is logged and
 * the function returns C_ERR to signal a lock was not acquired. */
int clusterLockConfig(char *filename) {
/* flock() does not exist on Solaris
 * and a fcntl-based solution won't help, as we constantly re-open that file,
 * which will release _all_ locks anyway
 */
#if !defined(__sun)
    /* To lock it, we need to open the file in a way it is created if
     * it does not exist, otherwise there is a race condition with other
     * processes. */
    int fd = open(filename,O_WRONLY|O_CREAT|O_CLOEXEC,0644);
    if (fd == -1) {
        serverLog(LL_WARNING,
            "Can't open %s in order to acquire a lock: %s",
            filename, strerror(errno));
        return C_ERR;
    }

    if (flock(fd,LOCK_EX|LOCK_NB) == -1) {
        if (errno == EWOULDBLOCK) {
            serverLog(LL_WARNING,
                 "Sorry, the cluster configuration file %s is already used "
                 "by a different Redis Cluster node. Please make sure that "
                 "different nodes use different cluster configuration "
                 "files.", filename);
        } else {
            serverLog(LL_WARNING,
                "Impossible to lock %s: %s", filename, strerror(errno));
        }
        close(fd);
        return C_ERR;
    }
    /* Lock acquired: leak the 'fd' by not closing it, so that we'll retain the
     * lock to the file as long as the process exists.
     *
     * After fork, the child process will get the fd opened by the parent process,
     * we need save `fd` to `cluster_config_file_lock_fd`, so that in redisFork(),
     * it will be closed in the child process.
     * If it is not closed, when the main process is killed -9, but the child process
     * (redis-aof-rewrite) is still alive, the fd(lock) will still be held by the
     * child process, and the main process will fail to get lock, means fail to start. */
    server.cluster_config_file_lock_fd = fd;
#else
    UNUSED(filename);
#endif /* __sun */

    return C_OK;
}

/* Derives our ports to be announced in the cluster bus. */
void deriveAnnouncedPorts(int *announced_port, int *announced_pport,