redis 源代码阅读与学习笔记（二）

redis 的守护进程

redis 自身支持 4 种方式启动：

启动方式	说明
daemonize no	不作为后台程序启动
daemonize yes	作为后台程序启动
supervised upstart	做成 upstart 服务启动。 upstart ，centos7 / ubuntu 18 等已经逐渐在废弃它了
supervised systemd	做成 systemd 服务启动

相关配置：

# By default Redis does not run as a daemon. Use 'yes' if you need it.
# Note that Redis will write a pid file in /var/run/redis.pid when daemonized.
daemonize no

# If you run Redis from upstart or systemd, Redis can interact with your
# supervision tree. Options:
#   supervised no      - no supervision interaction
#   supervised upstart - signal upstart by putting Redis into SIGSTOP mode
#   supervised systemd - signal systemd by writing READY=1 to $NOTIFY_SOCKET
#   supervised auto    - detect upstart or systemd method based on
#                        UPSTART_JOB or NOTIFY_SOCKET environment variables
# Note: these supervision methods only signal "process is ready."
#       They do not enable continuous liveness pings back to your supervisor.
supervised no

本文主要介绍 systemd 相关代码，模仿这些代码，也可以使自己的服务做成 systemd 服务

源代码分析 - 启动相关

代码以 redis 5.0 为准，后续版本实现稍有不同

int main(int argc, char **argv) {

    // ... 略（无关代码略）...

    server.supervised = redisIsSupervised(server.supervised_mode);
    int background = server.daemonize && !server.supervised;
    if (background) daemonize();
	
    // ... 略（无关代码略）...

}

int redisIsSupervised(int mode) {
    if (mode == SUPERVISED_AUTODETECT) {
        // ... 略（无关代码略）...
    } else if (mode == SUPERVISED_UPSTART) {
        return redisSupervisedUpstart();
    } else if (mode == SUPERVISED_SYSTEMD) {
        return redisSupervisedSystemd();
    }

    return 0;
}

int redisSupervisedSystemd(void) {
    const char *notify_socket = getenv("NOTIFY_SOCKET");
    int fd = 1;
    struct sockaddr_un su;
    struct iovec iov;
    struct msghdr hdr;
    int sendto_flags = 0;

    if (!notify_socket) {
        serverLog(LL_WARNING,
                "systemd supervision requested, but NOTIFY_SOCKET not found");
        return 0;
    }

    if ((strchr("@/", notify_socket[0])) == NULL || strlen(notify_socket) < 2) {
        return 0;
    }

    serverLog(LL_NOTICE, "supervised by systemd, will signal readiness");
    if ((fd = socket(AF_UNIX, SOCK_DGRAM, 0)) == -1) {
        serverLog(LL_WARNING,
                "Can't connect to systemd socket %s", notify_socket);
        return 0;
    }

    memset(&su, 0, sizeof(su));
    su.sun_family = AF_UNIX;
    strncpy (su.sun_path, notify_socket, sizeof(su.sun_path) -1);
    su.sun_path[sizeof(su.sun_path) - 1] = '\0';

    if (notify_socket[0] == '@')
        su.sun_path[0] = '\0';

    memset(&iov, 0, sizeof(iov));
    iov.iov_base = "READY=1";
    iov.iov_len = strlen("READY=1");

    memset(&hdr, 0, sizeof(hdr));
    hdr.msg_name = &su;
    hdr.msg_namelen = offsetof(struct sockaddr_un, sun_path) +
        strlen(notify_socket);
    hdr.msg_iov = &iov;
    hdr.msg_iovlen = 1;

    unsetenv("NOTIFY_SOCKET");
#ifdef HAVE_MSG_NOSIGNAL
    sendto_flags |= MSG_NOSIGNAL;
#endif
    if (sendmsg(fd, &hdr, sendto_flags) < 0) {
        serverLog(LL_WARNING, "Can't send notification to systemd");
        close(fd);
        return 0;
    }
    close(fd);
    return 1;
}

分析，对 systemd 的支持，主要在 redisSupervisedSystemd 函数内，粗看下这段代码，应该是一脸的闷逼

• NOTIFY_SOCKET 环境变量怎么回事，代码中搜不到哪里设置了
• 往 NOTIFY_SOCKET 指向的本地域 socket 上发 READY=1 又是做啥

这涉及到 systemd 的一些机制的相关知识，可以参考：

• https://askubuntu.com/questions/1120023/how-to-use-systemd-notify
• https://www.freedesktop.org/software/systemd/man/systemd-notify.html

然后就可以

系统配置

代码支持后，然后就是配置实装 systemd ， 配置类似以下：

[Unit]
Description=Redis persistent key-value database
After=network.target
After=network-onlne.target
Wants=network-online.target

[service]
ExecStart=/usr/bin/redis-server /etc/redis.conf --supervised systemd
ExecStop=/usr/libexec/redis-shutdown
Type=notify
User=redis
Group=redis
RuntimeDirectory=redis
RuntimeDirectoryMode=0755

[Install]
WantedBy=multi-user.target

其他

官网 issue 中，有人提出了支持 systemd 更好的代码实现方式：
https://github.com/antirez/redis/pull/6052

该代码已经合并进最新版本，有兴趣可以对比参考之