libevent(十三)evhttp事件处理流程

在libevent(六)http server中，作为一个单线程http server，不仅要监听每个连接的到来，还要监听每个连接上的I/O事件。

查看源码可知，在evhttp_bind_socket中设置了accept的回调函数:accept_socket_cb。

/* Listener callback when a connection arrives at a server. */
static void
accept_socket_cb(struct evconnlistener *listener, evutil_socket_t nfd, struct sockaddr *peer_sa, int peer_socklen, void *arg)
{
    struct evhttp *http = arg;

    evhttp_get_request(http, nfd, peer_sa, peer_socklen);
}

static void
evhttp_get_request(struct evhttp *http, evutil_socket_t fd,
    struct sockaddr *sa, ev_socklen_t salen)
{
    struct evhttp_connection *evcon;

    evcon = evhttp_get_request_connection(http, fd, sa, salen);
    if (evcon == NULL) {
        event_sock_warn(fd, "%s: cannot get connection on "EV_SOCK_FMT,
            __func__, EV_SOCK_ARG(fd));
        evutil_closesocket(fd);
        return;
    }

    /* the timeout can be used by the server to close idle connections */
    if (http->timeout != -)
        evhttp_connection_set_timeout(evcon, http->timeout);

    /*
     * if we want to accept more than one request on a connection,
     * we need to know which http server it belongs to.
     */
    evcon->http_server = http;
    TAILQ_INSERT_TAIL(&http->connections, evcon, next);

    if (evhttp_associate_new_request_with_connection(evcon) == -)
        evhttp_connection_free(evcon);
}

两个重要函数: evhttp_get_request_connection、evhttp_associate_new_request_with_connection。

1. evhttp_get_request_connection

/*
 * Takes a file descriptor to read a request from.
 * The callback is executed once the whole request has been read.
 */

static struct evhttp_connection*
evhttp_get_request_connection(
    struct evhttp* http,
    evutil_socket_t fd, struct sockaddr *sa, ev_socklen_t salen)
{
    struct evhttp_connection *evcon;
    char *hostname = NULL, *portname = NULL;

    name_from_addr(sa, salen, &hostname, &portname);
    if (hostname == NULL || portname == NULL) {
        if (hostname) mm_free(hostname);
        if (portname) mm_free(portname);
        return (NULL);
    }

    event_debug(("%s: new request from %s:%s on "EV_SOCK_FMT"\n",
        __func__, hostname, portname, EV_SOCK_ARG(fd)));

    /* we need a connection object to put the http request on */
    evcon = evhttp_connection_base_new(
        http->base, NULL, hostname, atoi(portname));
    mm_free(hostname);
    mm_free(portname);
    if (evcon == NULL)
        return (NULL);

    evcon->max_headers_size = http->default_max_headers_size;
    evcon->max_body_size = http->default_max_body_size;

    evcon->flags |= EVHTTP_CON_INCOMING;
    evcon->state = EVCON_READING_FIRSTLINE;

    evcon->fd = fd;

    bufferevent_setfd(evcon->bufev, fd);

    return (evcon);
}

注意点:

1. 通过evhttp_connection_base_new设置了bufferevent的readcd: evhttp_read_cb，

　　writecb: evhttp_write_cb。

2. 调用bufferevent_setfd

bufferevent_setfd代码如下:

int
bufferevent_setfd(struct bufferevent *bev, evutil_socket_t fd)
{
    union bufferevent_ctrl_data d;
    int res = -;
    d.fd = fd;
    BEV_LOCK(bev);
    if (bev->be_ops->ctrl)
        res = bev->be_ops->ctrl(bev, BEV_CTRL_SET_FD, &d);
    BEV_UNLOCK(bev);
    return res;
}

static int
be_socket_ctrl(struct bufferevent *bev, enum bufferevent_ctrl_op op,
    union bufferevent_ctrl_data *data)
{
    switch (op) {
    case BEV_CTRL_SET_FD:
        be_socket_setfd(bev, data->fd);
        return ;
    case BEV_CTRL_GET_FD:
        data->fd = event_get_fd(&bev->ev_read);
        return ;
    case BEV_CTRL_GET_UNDERLYING:
    case BEV_CTRL_CANCEL_ALL:
    default:
        return -;
    }
}

static void
be_socket_setfd(struct bufferevent *bufev, evutil_socket_t fd)
{
    BEV_LOCK(bufev);
    EVUTIL_ASSERT(bufev->be_ops == &bufferevent_ops_socket);

    event_del(&bufev->ev_read);
    event_del(&bufev->ev_write);

    event_assign(&bufev->ev_read, bufev->ev_base, fd,
        EV_READ|EV_PERSIST, bufferevent_readcb, bufev);
    event_assign(&bufev->ev_write, bufev->ev_base, fd,
        EV_WRITE|EV_PERSIST, bufferevent_writecb, bufev);

    if (fd >= )
        bufferevent_enable(bufev, bufev->enabled);

    BEV_UNLOCK(bufev);
}

这里主要设置fd的读事件回调bufferevent_readcb，写事件回调bufferevent_writecb。

(这里的bufferevent_enable可以不用在意，后面会重置。)

2. evhttp_associate_new_request_with_connection

static int
evhttp_associate_new_request_with_connection(struct evhttp_connection *evcon)
{
    struct evhttp *http = evcon->http_server;
    struct evhttp_request *req;
    if ((req = evhttp_request_new(evhttp_handle_request, http)) == NULL)
        return (-);

    if ((req->remote_host = mm_strdup(evcon->address)) == NULL) {
        event_warn("%s: strdup", __func__);
        evhttp_request_free(req);
        return (-);
    }
    req->remote_port = evcon->port;

    req->evcon = evcon;    /* the request ends up owning the connection */
    req->flags |= EVHTTP_REQ_OWN_CONNECTION;

    /* We did not present the request to the user user yet, so treat it as
     * if the user was done with the request.  This allows us to free the
     * request on a persistent connection if the client drops it without
     * sending a request.
     */
    req->userdone = ;

    TAILQ_INSERT_TAIL(&evcon->requests, req, next);

    req->kind = EVHTTP_REQUEST;

    evhttp_start_read(evcon);

    return ();
}

第一步设置evhttp_request的回调函数evhttp_handle_request，第二步调用evhttp_start_read:

/*
 * Reads data from file descriptor into request structure
 * Request structure needs to be set up correctly.
 */

void
evhttp_start_read(struct evhttp_connection *evcon)
{
    /* Set up an event to read the headers */
    bufferevent_disable(evcon->bufev, EV_WRITE);
    bufferevent_enable(evcon->bufev, EV_READ);
    evcon->state = EVCON_READING_FIRSTLINE;
    /* Reset the bufferevent callbacks */
    bufferevent_setcb(evcon->bufev,
        evhttp_read_cb,
        evhttp_write_cb,
        evhttp_error_cb,
        evcon);

    /* If there's still data pending, process it next time through the
     * loop.  Don't do it now; that could get recusive. */
    if (evbuffer_get_length(bufferevent_get_input(evcon->bufev))) {
        event_deferred_cb_schedule(get_deferred_queue(evcon),
            &evcon->read_more_deferred_cb);
    }
}

可以看到，这里将fd的读事件添加到了事件循环中。

最后梳理下读事件调用流程:

1. fd上有读事件发生

2. bufferevent_readcb

3. evhttp_read_cb

4. evhttp_connection_done

5. evhttp_handle_request

6. 调用用户定义的evhttp回调函数

关于数据的流向

当fd上有读事件发生时，首先将fd上的数据读到evhttp_connection的bufferevent中，然后将bufferevent中的数据读到evhttp_request的输入缓冲中。

当我们使用evhttp_send_reply发送数据时，首先将数据写入evhttp_request的输出缓冲中，然后写入evhttp_connection的bufferevent中，最后写入到fd的输出缓冲。