BIND9的架构与机制笔记1

　BIND9采用的是事件驱动的机制来工作，而事件的源头则是IO，IO在linux使用的EPOLL的边缘触发模式。

　　本篇说的是epoll，BIND9如果创建了watcher线程（宏USE_WATCHER_THREAD控制），这里就讨论有线程的情况，实际上即使不创建

线程干的也都是一样的活。在lib/isc/socket.c中setup_watcher函数：（所有的代码都是截取的epoll下的片段，因为还有kqueue，devpoll，select等的实现代码，太多了）

#elif defined(USE_EPOLL)    manager->nevents = ISC_SOCKET_MAXEVENTS;    manager->events = isc_mem_get(mctx, sizeof(struct epoll_event) *                      manager->nevents);    if (manager->events == NULL)        return (ISC_R_NOMEMORY);    manager->epoll_fd = epoll_create(manager->nevents);    if (manager->epoll_fd == -1) {        result = isc__errno2result(errno);        isc__strerror(errno, strbuf, sizeof(strbuf));        UNEXPECTED_ERROR(__FILE__, __LINE__,                 "epoll_create %s: %s",                 isc_msgcat_get(isc_msgcat, ISC_MSGSET_GENERAL,                        ISC_MSG_FAILED, "failed"),                 strbuf);        isc_mem_put(mctx, manager->events,                sizeof(struct epoll_event) * manager->nevents);        return (result);    }#ifdef USE_WATCHER_THREAD    result = watch_fd(manager, manager->pipe_fds[0], SELECT_POKE_READ);    if (result != ISC_R_SUCCESS) {        close(manager->epoll_fd);        isc_mem_put(mctx, manager->events,                sizeof(struct epoll_event) * manager->nevents);        return (result);    }#endif    /* USE_WATCHER_THREAD */

先是创建了要监视的最大socket fd数目（manager->nevents）对应的epoll_event结构体数组，然后调用epoll_create函数创建一个epoll fd，参数则是指定监视的socket fd

最大数目。我的内核版本是3.13，man一下epoll_create发现它是这样说的：epoll_create()  creates  an  epoll(7) instance.  Since Linux 2.6.8, thesize argument is ignored, but must be  greater  than  zero。这个函数在2.6.8内核以后就忽略参数size了，但是传递的参数值一定要大于0。后来找了一下资料，网上的高手的博客说的就很清楚了http://www.cnblogs.com/apprentice89/p/3234677.html。继续往下说，后面的watch_fd实在创建线程的情况下才有，就是将pipe_fds[0]这个管道描述符，也就是一个可读的流，而上述的socket fd都是可以归为流。watch_fd的实现代码：

#elif defined(USE_EPOLL)        struct epoll_event event;        if (msg == SELECT_POKE_READ)                event.events = EPOLLIN;        else                event.events = EPOLLOUT;        memset(&event.data, 0, sizeof(event.data));        event.data.fd = fd;        if (epoll_ctl(manager->epoll_fd, EPOLL_CTL_ADD, fd, &event) == -1 &&            errno != EEXIST) {                result = isc__errno2result(errno);        }        return (result);

这是将pipe_fds[0]加入epoll_fd的监听队列，EPOLL_CTL_ADD是操作类型，注册该fd到epoll_fd上。这个管道的目的是接收管理该线程的消息，比如线程退出。

那么进入线程看：

static isc_threadresult_twatcher(void *uap) {    isc__socketmgr_t *manager = uap;    isc_boolean_t done;    int ctlfd;    int cc;#ifdef USE_KQUEUE    const char *fnname = "kevent()";#elif defined (USE_EPOLL)    const char *fnname = "epoll_wait()";#elif defined(USE_DEVPOLL)    const char *fnname = "ioctl(DP_POLL)";    struct dvpoll dvp;#elif defined (USE_SELECT)    const char *fnname = "select()";    int maxfd;#endif    char strbuf[ISC_STRERRORSIZE];#ifdef ISC_SOCKET_USE_POLLWATCH    pollstate_t pollstate = poll_idle;#endif    /*     * Get the control fd here.  This will never change.     */    ctlfd = manager->pipe_fds[0];    done = ISC_FALSE;    while (!done) {        do {#ifdef USE_KQUEUE            cc = kevent(manager->kqueue_fd, NULL, 0,                    manager->events, manager->nevents, NULL);#elif defined(USE_EPOLL)            cc = epoll_wait(manager->epoll_fd, manager->events,                    manager->nevents, -1);#elif defined(USE_DEVPOLL)            dvp.dp_fds = manager->events;            dvp.dp_nfds = manager->nevents;#ifndef ISC_SOCKET_USE_POLLWATCH            dvp.dp_timeout = -1;#else            if (pollstate == poll_idle)                dvp.dp_timeout = -1;            else                dvp.dp_timeout = ISC_SOCKET_POLLWATCH_TIMEOUT;#endif    /* ISC_SOCKET_USE_POLLWATCH */            cc = ioctl(manager->devpoll_fd, DP_POLL, &dvp);#elif defined(USE_SELECT)            LOCK(&manager->lock);            memcpy(manager->read_fds_copy, manager->read_fds,                   manager->fd_bufsize);            memcpy(manager->write_fds_copy, manager->write_fds,                   manager->fd_bufsize);            maxfd = manager->maxfd + 1;            UNLOCK(&manager->lock);            cc = select(maxfd, manager->read_fds_copy,                    manager->write_fds_copy, NULL, NULL);#endif    /* USE_KQUEUE */            if (cc < 0 && !SOFT_ERROR(errno)) {                isc__strerror(errno, strbuf, sizeof(strbuf));                FATAL_ERROR(__FILE__, __LINE__,                        "%s %s: %s", fnname,                        isc_msgcat_get(isc_msgcat,                               ISC_MSGSET_GENERAL,                               ISC_MSG_FAILED,                               "failed"), strbuf);            }#if defined(USE_DEVPOLL) && defined(ISC_SOCKET_USE_POLLWATCH)            if (cc == 0) {                if (pollstate == poll_active)                    pollstate = poll_checking;                else if (pollstate == poll_checking)                    pollstate = poll_idle;            } else if (cc > 0) {                if (pollstate == poll_checking) {                    /*                     * XXX: We'd like to use a more                     * verbose log level as it's actually an                     * unexpected event, but the kernel bug                     * reportedly happens pretty frequently                     * (and it can also be a false positive)                     * so it would be just too noisy.                     */                    manager_log(manager,                            ISC_LOGCATEGORY_GENERAL,                            ISC_LOGMODULE_SOCKET,                            ISC_LOG_DEBUG(1),                            "unexpected POLL timeout");                }                pollstate = poll_active;            }#endif        } while (cc < 0);#if defined(USE_KQUEUE) || defined (USE_EPOLL) || defined (USE_DEVPOLL)        done = process_fds(manager, manager->events, cc);#elif defined(USE_SELECT)        process_fds(manager, maxfd, manager->read_fds_copy,                manager->write_fds_copy);        /*         * Process reads on internal, control fd.         */        if (FD_ISSET(ctlfd, manager->read_fds_copy))            done = process_ctlfd(manager);#endif    }    manager_log(manager, TRACE, "%s",            isc_msgcat_get(isc_msgcat, ISC_MSGSET_GENERAL,                   ISC_MSG_EXITING, "watcher exiting"));    return ((isc_threadresult_t)0);}

无限循环，epoll_wait当监听的epoll_fd队列上有IO事件发生时，将对应的socket fd和事件放入events数组中，并且将这些注册在epoll_fd上的socket fd对应事件清空。

process_fds遍历数组，找到对应的socket fd，并判断该fd是不是线程控制管道，如果是则会在执行完其他socket fd上的对应事件后再处理管道中的控制消息。

static isc_boolean_tprocess_fds(isc__socketmgr_t *manager, struct epoll_event *events, int nevents){    int i;    isc_boolean_t done = ISC_FALSE;#ifdef USE_WATCHER_THREAD    isc_boolean_t have_ctlevent = ISC_FALSE;#endif    if (nevents == manager->nevents) {        manager_log(manager, ISC_LOGCATEGORY_GENERAL,                ISC_LOGMODULE_SOCKET, ISC_LOG_INFO,                "maximum number of FD events (%d) received",                nevents);    }    for (i = 0; i < nevents; i++) {        REQUIRE(events[i].data.fd < (int)manager->maxsocks);#ifdef USE_WATCHER_THREAD        if (events[i].data.fd == manager->pipe_fds[0]) {            have_ctlevent = ISC_TRUE;            continue;        }#endif        if ((events[i].events & EPOLLERR) != 0 ||            (events[i].events & EPOLLHUP) != 0) {            /*             * epoll does not set IN/OUT bits on an erroneous             * condition, so we need to try both anyway.  This is a             * bit inefficient, but should be okay for such rare             * events.  Note also that the read or write attempt             * won't block because we use non-blocking sockets.             */            events[i].events |= (EPOLLIN | EPOLLOUT);        }        process_fd(manager, events[i].data.fd,               (events[i].events & EPOLLIN) != 0,               (events[i].events & EPOLLOUT) != 0);    }#ifdef USE_WATCHER_THREAD    if (have_ctlevent)        done = process_ctlfd(manager);#endif    return (done);}

 待续