從本篇開始，介紹Nginx中HTTP相關。Nginx做為Web Server那麼HTTP必然是重中之重。本篇不打算介紹太深入，本篇最主要內容是如何與事件驅動關聯起來。

一、監聽埠

Nginx預設監聽埠是80埠，那麼Nginx是如何將80埠的listening事件註冊到事件驅動（epoll）中呢？這裡簡單回顧一下，具體內容可參考《菜鳥學習nginx之核心模組ngx_events_module》。

worker程序的程序主函式是ngx_worker_process_cycle。進入該函式會先呼叫函式ngx_worker_process_init進行初始化流程，當ngx_worker_process_init返回之後則進入無限迴圈即投入服務。那麼在ngx_worker_process_init函式就會把listening socket加入到事件驅動epoll中。然而真正加入到epoll物件中是函式ngx_event_process_init函式，部分程式碼如下：

    /**
     * for each listening socket 
     * 迴圈遍歷listening 將listen socket 與connection物件進行繫結
     */
    ls = cycle->listening.elts;
    for (i = 0; i < cycle->listening.nelts; i++)
    {

#if (NGX_HAVE_REUSEPORT)
        if (ls[i].reuseport && ls[i].worker != ngx_worker)
        {
            continue;
        }
#endif
        /* 返回可用連線物件 這裡listening socket也會佔用一個connection物件 */
        c = ngx_get_connection(ls[i].fd, cycle->log);

        if (c == NULL)
        {
            return NGX_ERROR;
        }

        c->type = ls[i].type;
        c->log = &ls[i].log;

        c->listening = &ls[i];
        ls[i].connection = c;

        rev = c->read;

        rev->log = c->log;
        rev->accept = 1; /* 表示當前讀事件是accept事件 用於區別正常資料報文讀取還是Accept事件讀取 */

#if (NGX_HAVE_DEFERRED_ACCEPT)
        rev->deferred_accept = ls[i].deferred_accept;
#endif

        if (!(ngx_event_flags & NGX_USE_IOCP_EVENT))
        {
            if (ls[i].previous)
            {

                /*
                 * delete the old accept events that were bound to
                 * the old cycle read events array
                 */

                old = ls[i].previous->connection;

                if (ngx_del_event(old->read, NGX_READ_EVENT, NGX_CLOSE_EVENT) == NGX_ERROR)
                {
                    return NGX_ERROR;
                }

                old->fd = (ngx_socket_t)-1;
            }
        }

#if (NGX_WIN32)

        ...

#else

        rev->handler = (c->type == SOCK_STREAM) ? ngx_event_accept
                                                : ngx_event_recvmsg;

#if (NGX_HAVE_REUSEPORT)

        if (ls[i].reuseport)
        {
            if (ngx_add_event(rev, NGX_READ_EVENT, 0) == NGX_ERROR)
            {
                return NGX_ERROR;
            }

            continue;
        }

#endif

        if (ngx_use_accept_mutex)
        {
            continue;
        }

        //註冊讀事件 主要是用於listen監聽
        if (ngx_add_event(rev, NGX_READ_EVENT, 0) == NGX_ERROR)
        {
            return NGX_ERROR;
        }

#endif
    }
 

迴圈遍歷listenings陣列，通過ngx_add_event函式，逐個將listening socket加入到epoll物件中且設定事件型別為READ事件。

至此Nginx完成了listening socket註冊到epoll事件驅動中。當客戶端發來請求時就會呼叫回撥函式--ngx_event_accept。

二、Accept事件

註冊listening socket到epoll的流程中有這樣一行程式碼，用於設定回撥函式，具體如下：

/* TCP連線的回撥函式是ngx_event_accept，其他的回撥函式則是ngx_event_recv_msg */
rev->handler = (c->type == SOCK_STREAM) ? ngx_event_accept
                                                : ngx_event_recvmsg;
 

這裡使用的協議是TCP，所以當客戶端的連線請求到來時，會觸發回撥函式ngx_event_accept函式執行，具體如何執行可參考《菜鳥學習nginx之事件模組epoll（1）》。

2.1、流程圖

從流程圖中可以看出，這部分程式碼邏輯還是比較繁瑣，下面我們分片進行分析解讀。

2.2、超時處理 

/**
 * 處理Accept事件
 * @param ev 待讀事件
 */
void
ngx_event_accept(ngx_event_t *ev)
{
    socklen_t          socklen;
    ngx_err_t          err;
    ngx_log_t         *log;
    ngx_uint_t         level;
    ngx_socket_t       s;
    ngx_event_t       *rev, *wev;
    ngx_sockaddr_t     sa;
    ngx_listening_t   *ls;
    ngx_connection_t  *c, *lc;
    ngx_event_conf_t  *ecf;
#if (NGX_HAVE_ACCEPT4)
    static ngx_uint_t  use_accept4 = 1;
#endif

    if (ev->timedout) {//表示超時 重新新增listen socket到事件驅動中
        if (ngx_enable_accept_events((ngx_cycle_t *) ngx_cycle) != NGX_OK) {
            return;
        }

        ev->timedout = 0;
    }
    //獲取event配置項
    ecf = ngx_event_get_conf(ngx_cycle->conf_ctx, ngx_event_core_module);

    if (!(ngx_event_flags & NGX_USE_KQUEUE_EVENT)) {
        ev->available = ecf->multi_accept;
    }
    //從使用者私有資料中獲取當前事件所對應的connection以及listening物件
    lc = ev->data;
    ls = lc->listening;
    ev->ready = 0;

    ngx_log_debug2(NGX_LOG_DEBUG_EVENT, ev->log, 0,
                   "accept on %V, ready: %d", &ls->addr_text, ev->available);

這程式碼主要用於判斷當前事件是否為超時事件。超時事件的產生條件是在定時器週期內沒有新的請求到來，此時需要重新註冊listening socket到epoll中。下面進入主流程do while迴圈。

2.3、呼叫accept接收使用者請求

    do {
        socklen = sizeof(ngx_sockaddr_t);
        /* 接收客戶端連線建立請求 */
#if (NGX_HAVE_ACCEPT4)
        if (use_accept4) {
            s = accept4(lc->fd, &sa.sockaddr, &socklen, SOCK_NONBLOCK);//預設建立的socket是非阻塞socket
        } else {
            s = accept(lc->fd, &sa.sockaddr, &socklen);
        }
#else
        s = accept(lc->fd, &sa.sockaddr, &socklen);
#endif
        /* 連線建立失敗 */
        if (s == (ngx_socket_t) -1) {
            err = ngx_socket_errno;

            if (err == NGX_EAGAIN) {
                ngx_log_debug0(NGX_LOG_DEBUG_EVENT, ev->log, err,
                               "accept() not ready");
                return;
            }

            level = NGX_LOG_ALERT;

            if (err == NGX_ECONNABORTED) {
                level = NGX_LOG_ERR;

            } else if (err == NGX_EMFILE || err == NGX_ENFILE) {
                level = NGX_LOG_CRIT;
            }

#if (NGX_HAVE_ACCEPT4)
            ngx_log_error(level, ev->log, err,
                          use_accept4 ? "accept4() failed" : "accept() failed");

            if (use_accept4 && err == NGX_ENOSYS) {
                use_accept4 = 0;
                ngx_inherited_nonblocking = 0;//嘗試用accept介面處理新請求
                continue;
            }
#else
            ngx_log_error(level, ev->log, err, "accept() failed");
#endif

            if (err == NGX_ECONNABORTED) {
                if (ngx_event_flags & NGX_USE_KQUEUE_EVENT) {
                    ev->available--;
                }

                if (ev->available) {
                    continue;
                }
            }

            if (err == NGX_EMFILE || err == NGX_ENFILE) {
                if (ngx_disable_accept_events((ngx_cycle_t *) ngx_cycle, 1)
                    != NGX_OK)
                {
                    return;
                }

                if (ngx_use_accept_mutex) {
                    if (ngx_accept_mutex_held) {
                        ngx_shmtx_unlock(&ngx_accept_mutex);
                        ngx_accept_mutex_held = 0;
                    }

                    ngx_accept_disabled = 1;

                } else {
                    ngx_add_timer(ev, ecf->accept_mutex_delay);
                }
            }

            return;
        }

根據不同的錯誤碼，進行特殊處理，大部分場景最後都是return操作。

2.4、建立connection物件

當accept返回成功，則表新的tcp連線已經建立成功，Nginx需要對其進行封裝成connection物件並對connection結構成員進行初始化操作，例如socket操作設定等，具體程式碼如下：

        /* 負數 負載均衡*/       
        ngx_accept_disabled = ngx_cycle->connection_n / 8
                              - ngx_cycle->free_connection_n;

        c = ngx_get_connection(s, ev->log);//獲取新的connection物件

        if (c == NULL) {
            if (ngx_close_socket(s) == -1) {
                ngx_log_error(NGX_LOG_ALERT, ev->log, ngx_socket_errno,
                              ngx_close_socket_n " failed");
            }

            return;
        }

        c->type = SOCK_STREAM;

#if (NGX_STAT_STUB)
        (void) ngx_atomic_fetch_add(ngx_stat_active, 1);
#endif
        /* 建立連線級記憶體池 */
        c->pool = ngx_create_pool(ls->pool_size, ev->log);
        if (c->pool == NULL) {
            ngx_close_accepted_connection(c);
            return;
        }

        if (socklen > (socklen_t) sizeof(ngx_sockaddr_t)) {
            socklen = sizeof(ngx_sockaddr_t);
        }

        c->sockaddr = ngx_palloc(c->pool, socklen);
        if (c->sockaddr == NULL) {
            ngx_close_accepted_connection(c);
            return;
        }

        ngx_memcpy(c->sockaddr, &sa, socklen);

        log = ngx_palloc(c->pool, sizeof(ngx_log_t));
        if (log == NULL) {
            ngx_close_accepted_connection(c);
            return;
        }

        /* set a blocking mode for iocp and non-blocking mode for others */

        if (ngx_inherited_nonblocking) {
            if (ngx_event_flags & NGX_USE_IOCP_EVENT) {
                if (ngx_blocking(s) == -1) {
                    ngx_log_error(NGX_LOG_ALERT, ev->log, ngx_socket_errno,
                                  ngx_blocking_n " failed");
                    ngx_close_accepted_connection(c);
                    return;
                }
            }

        } else {
            if (!(ngx_event_flags & NGX_USE_IOCP_EVENT)) {
                if (ngx_nonblocking(s) == -1) {
                    ngx_log_error(NGX_LOG_ALERT, ev->log, ngx_socket_errno,
                                  ngx_nonblocking_n " failed");
                    ngx_close_accepted_connection(c);
                    return;
                }
            }
        }

        *log = ls->log;
        /* 初始化connection物件 參考ngx_linux_io */
        c->recv = ngx_recv;
        c->send = ngx_send;
        c->recv_chain = ngx_recv_chain;
        c->send_chain = ngx_send_chain;

        c->log = log;
        c->pool->log = log;

        c->socklen = socklen;
        c->listening = ls;
        c->local_sockaddr = ls->sockaddr;
        c->local_socklen = ls->socklen;

#if (NGX_HAVE_UNIX_DOMAIN)
        if (c->sockaddr->sa_family == AF_UNIX) {
            c->tcp_nopush = NGX_TCP_NOPUSH_DISABLED;
            c->tcp_nodelay = NGX_TCP_NODELAY_DISABLED;
#if (NGX_SOLARIS)
            /* Solaris's sendfilev() supports AF_NCA, AF_INET, and AF_INET6 */
            c->sendfile = 0;
#endif
        }
#endif

        rev = c->read;
        wev = c->write;

        wev->ready = 1;

        if (ngx_event_flags & NGX_USE_IOCP_EVENT) {
            rev->ready = 1;
        }

        if (ev->deferred_accept) {
            rev->ready = 1;
#if (NGX_HAVE_KQUEUE || NGX_HAVE_EPOLLRDHUP)
            rev->available = 1;
#endif
        }

        rev->log = log;
        wev->log = log;

        /*
         * TODO: MT: - ngx_atomic_fetch_add()
         *             or protection by critical section or light mutex
         *
         * TODO: MP: - allocated in a shared memory
         *           - ngx_atomic_fetch_add()
         *             or protection by critical section or light mutex
         */
        c->number = ngx_atomic_fetch_add(ngx_connection_counter, 1);

#if (NGX_STAT_STUB)
        (void) ngx_atomic_fetch_add(ngx_stat_handled, 1);
#endif

        if (ls->addr_ntop) {
            c->addr_text.data = ngx_pnalloc(c->pool, ls->addr_text_max_len);
            if (c->addr_text.data == NULL) {
                ngx_close_accepted_connection(c);
                return;
            }

            c->addr_text.len = ngx_sock_ntop(c->sockaddr, c->socklen,
                                             c->addr_text.data,
                                             ls->addr_text_max_len, 0);
            if (c->addr_text.len == 0) {
                ngx_close_accepted_connection(c);
                return;
            }
        }

2.5、HTTP框架初始化connection物件

下面這段程式碼最主要功能是對connection物件初始化流程，如下：

        /**
         * ngx_add_conn不空且沒有設定NGX_USE_EPOLL_EVENT標誌位 
         * epoll模型不會進入此分支
         */
        if (ngx_add_conn && (ngx_event_flags & NGX_USE_EPOLL_EVENT) == 0) {
            if (ngx_add_conn(c) == NGX_ERROR) {
                ngx_close_accepted_connection(c);
                return;
            }
        }

        log->data = NULL;
        log->handler = NULL;
        
        /*
         * 此回撥函式用於處理新的連線 handler賦值由HTTP框架設定
         * ngx_http_init_connection 這個函式主要功能是將當前socket註冊到事件驅動中
         */
        ls->handler(c);

        if (ngx_event_flags & NGX_USE_KQUEUE_EVENT) {
            ev->available--;
        }

    } while (ev->available);

三、註冊事件驅動

上面章節，Nginx是如何處理Accept事件，但是有一點疑問是accept函式返回的socket是如何加入到epoll中呢？listening socket是如何再次加入到epoll中呢（每次accept事件之後需要重新加入）?

3.1、新socket加入epoll事件驅動

在上一節中有這一行程式碼：

        /*
         * 此回撥函式用於處理新的連線 handler賦值由HTTP框架設定
         * ngx_http_init_connection 這個函式主要功能是將當前socket註冊到事件驅動中
         */
        ls->handler(c);

該程式碼是HTTP框架用於初始化連線 ，回撥函式是ngx_http_init_connection，該函式用於將客戶端與服務端通訊的socket加入到事件驅動中：

/**
 * 初始化http連線
 * @param c TCP連線
 */
void ngx_http_init_connection(ngx_connection_t *c)
{
    ngx_uint_t i;
    ngx_event_t *rev;
    struct sockaddr_in *sin;
    ngx_http_port_t *port;
    ngx_http_in_addr_t *addr;
    ngx_http_log_ctx_t *ctx;
    ngx_http_connection_t *hc;
#if (NGX_HAVE_INET6)
    struct sockaddr_in6 *sin6;
    ngx_http_in6_addr_t *addr6;
#endif
    /* 在記憶體池中申請http connection物件 */
    hc = ngx_pcalloc(c->pool, sizeof(ngx_http_connection_t));
    if (hc == NULL)
    {
        ngx_http_close_connection(c);
        return;
    }

    c->data = hc;//save 

    /* find the server configuration for the address:port */

    port = c->listening->servers;
    /* 獲取監聽地址 如果有多個地址 則迴圈遍歷 */
    if (port->naddrs > 1)
    {

        /*
         * there are several addresses on this port and one of them
         * is an "*:port" wildcard so getsockname() in ngx_http_server_addr()
         * is required to determine a server address
         */

        if (ngx_connection_local_sockaddr(c, NULL, 0) != NGX_OK)
        {
            ngx_http_close_connection(c);
            return;
        }

        switch (c->local_sockaddr->sa_family)
        {

#if (NGX_HAVE_INET6)
        case AF_INET6:
            sin6 = (struct sockaddr_in6 *)c->local_sockaddr;

            addr6 = port->addrs;

            /* the last address is "*" */

            for (i = 0; i < port->naddrs - 1; i++)
            {
                if (ngx_memcmp(&addr6[i].addr6, &sin6->sin6_addr, 16) == 0)
                {
                    break;
                }
            }

            hc->addr_conf = &addr6[i].conf;

            break;
#endif

        default: /* AF_INET */
            sin = (struct sockaddr_in *)c->local_sockaddr;

            addr = port->addrs;

            /* the last address is "*" */

            for (i = 0; i < port->naddrs - 1; i++)
            {
                if (addr[i].addr == sin->sin_addr.s_addr)
                {
                    break;
                }
            }

            hc->addr_conf = &addr[i].conf;

            break;
        }
    }
    else
    {

        switch (c->local_sockaddr->sa_family)
        {

#if (NGX_HAVE_INET6)
        case AF_INET6:
            addr6 = port->addrs;
            hc->addr_conf = &addr6[0].conf;
            break;
#endif

        default: /* AF_INET */
            addr = port->addrs;
            hc->addr_conf = &addr[0].conf;
            break;
        }
    }

    /* the default server configuration for the address:port */
    hc->conf_ctx = hc->addr_conf->default_server->ctx;

    ctx = ngx_palloc(c->pool, sizeof(ngx_http_log_ctx_t));
    if (ctx == NULL)
    {
        ngx_http_close_connection(c);
        return;
    }

    ctx->connection = c;
    ctx->request = NULL;
    ctx->current_request = NULL;

    c->log->connection = c->number;
    c->log->handler = ngx_http_log_error;
    c->log->data = ctx;
    c->log->action = "waiting for request";

    c->log_error = NGX_ERROR_INFO;
    /* 設定讀寫事件handler回撥函式 */
    rev = c->read;
    rev->handler = ngx_http_wait_request_handler; /* 讀事件回撥函式 */
    c->write->handler = ngx_http_empty_handler; /* 寫事件回撥函式 再未收到client請求不會主動傳送資料 */

#if (NGX_HTTP_V2)
    if (hc->addr_conf->http2)
    {
        rev->handler = ngx_http_v2_init;
    }
#endif

#if (NGX_HTTP_SSL)
    {
        ngx_http_ssl_srv_conf_t *sscf;

        sscf = ngx_http_get_module_srv_conf(hc->conf_ctx, ngx_http_ssl_module);

        if (sscf->enable || hc->addr_conf->ssl)
        {

            c->log->action = "SSL handshaking";

            if (hc->addr_conf->ssl && sscf->ssl.ctx == NULL)
            {
                ngx_log_error(NGX_LOG_ERR, c->log, 0,
                              "no \"ssl_certificate\" is defined "
                              "in server listening on SSL port");
                ngx_http_close_connection(c);
                return;
            }

            hc->ssl = 1;

            rev->handler = ngx_http_ssl_handshake;
        }
    }
#endif

    if (hc->addr_conf->proxy_protocol)
    {
        hc->proxy_protocol = 1;
        c->log->action = "reading PROXY protocol";
    }

    if (rev->ready)
    {
        /* the deferred accept(), iocp */

        if (ngx_use_accept_mutex)
        {
            ngx_post_event(rev, &ngx_posted_events);
            return;
        }

        rev->handler(rev);
        return;
    }
    /*
     * 將事件新增到定時器和epoll事件驅動中
     * 回撥函式都是ngx_http_wait_request_handler
     */
    ngx_add_timer(rev, c->listening->post_accept_timeout);
    ngx_reusable_connection(c, 1);
    if (ngx_handle_read_event(rev, 0) != NGX_OK)
    {
        ngx_http_close_connection(c);
        return;
    }
}

通過上述程式碼可知：

1、讀事件處理函式為ngx_http_wait_request_handler，寫事件處理函式為ngx_http_empty_handler。

2、將當前socket加入到epoll以及定時器中。

四、總結

至此，HTTP會話流程已經建立完成。HTTP會話主要是初始化connection物件以及將通訊socket加入到事件驅動中即可。