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 != -1)
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) == -1)
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 = -1;
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 0;
case BEV_CTRL_GET_FD:
data->fd = event_get_fd(&bev->ev_read);
return 0;
case BEV_CTRL_GET_UNDERLYING:
case BEV_CTRL_CANCEL_ALL:
default:
return -1;
}
}
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 >= 0)
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 (-1);
if ((req->remote_host = mm_strdup(evcon->address)) == NULL) {
event_warn("%s: strdup", __func__);
evhttp_request_free(req);
return (-1);
}
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 = 1;
TAILQ_INSERT_TAIL(&evcon->requests, req, next);
req->kind = EVHTTP_REQUEST;
evhttp_start_read(evcon);
return (0);
}
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。
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