libevent中事件添加源碼分析

// struct event 的實現：

struct event {

	// 保存回調函數的相關參數
	struct event_callback ev_evcallback;

	/* for managing timeouts */
	// 僅用於當前事件是定時器的時候
	union {
		TAILQ_ENTRY(event) ev_next_with_common_timeout;
		int min_heap_idx;  // 指明瞭當前event在堆中的位置
	} ev_timeout_pos;
	// 對一個event如果綁定的是信號，那麼ev_fd就是信號的值
	// 如果綁定的是IO事件，ev_fd是文件描述符fd
	evutil_socket_t ev_fd;
	// event創建的時候事件屬性，比如EV_READ, EV_WRITE等
	short ev_events;
	// 經過回調函數返回的參數
	short ev_res;		/* result passed to event callback */
	// 保存該事件的所處的base
	struct event_base *ev_base;

	union {
		/* used for io events */
		struct {
			LIST_ENTRY (event) ev_io_next;
			struct timeval ev_timeout;
		} ev_io;

		/* used by signal events */
		struct {
			LIST_ENTRY (event) ev_signal_next;
			//事件就緒執行時,將要調用ev_callback 的次數,通常爲1
			short ev_ncalls;
			/* Allows deletes in callback */
			short *ev_pncalls;
		} ev_signal;
	} ev_;

	// 事件的絕對超時時間
	struct timeval ev_timeout;
};

// 保存了event回調函數的相關信息
struct event_callback {
	TAILQ_ENTRY(event_callback) evcb_active_next;
	// 當前event的狀態，用於把當前event歸類到某一個鏈表中
	short evcb_flags;
	// 一個任務的優先級，數字越小優先級越高
	ev_uint8_t evcb_pri;	/* smaller numbers are higher priority */
	ev_uint8_t evcb_closure;
	/* allows us to adopt for different types of events */
	union {
		
		void (*evcb_callback)(evutil_socket_t, short, void *);// 回調函數
		void (*evcb_selfcb)(struct event_callback *, void *);
		void (*evcb_evfinalize)(struct event *, void *);
		void (*evcb_cbfinalize)(struct event_callback *, void *);
	} evcb_cb_union;
	void *evcb_arg;  // 回調函數的參數
};




// 創建一個event
struct event *
event_new(struct event_base *base, evutil_socket_t fd, short events, void (*cb)(evutil_socket_t, short, void *), void *arg)
{
	struct event *ev;
	ev = mm_malloc(sizeof(struct event));
	if (ev == NULL)
		return (NULL);
	if (event_assign(ev, base, fd, events, cb, arg) < 0) {
		mm_free(ev);
		return (NULL);
	}

	return (ev);
}


int
event_assign(struct event *ev, struct event_base *base, evutil_socket_t fd, short events, void (*callback)(evutil_socket_t, short, void *), void *arg)
{
	if (!base)
		base = current_base;
	// 外部給回調函數傳遞參數是 event_self_cbarg() ，那麼說明要把創建的event傳遞進去
	if (arg == &event_self_cbarg_ptr_)
		arg = ev;

	if (!(events & EV_SIGNAL))
		event_debug_assert_socket_nonblocking_(fd);
	event_debug_assert_not_added_(ev);

	ev->ev_base = base;				// 設置event所屬的base

	ev->ev_callback = callback;		// 設置event的回調函數
	ev->ev_arg = arg;				// 設置回調函數的參數
	ev->ev_fd = fd;					// 當前event監聽的文件描述符
	ev->ev_events = events;			// 當前event具有的屬性
	ev->ev_res = 0;					// 經過回調函數返回的參數
	ev->ev_flags = EVLIST_INIT;     // event當前的狀態是EVLIST_INIT
	ev->ev_ncalls = 0;
	ev->ev_pncalls = NULL;

	if (events & EV_SIGNAL) {
		// 如果設置了EV_SIGNAL並且設置了(EV_READ|EV_WRITE|EV_CLOSED)中的一個，就出錯返回
		if ((events & (EV_READ|EV_WRITE|EV_CLOSED)) != 0) {
			event_warnx("%s: EV_SIGNAL is not compatible with "
			    "EV_READ, EV_WRITE or EV_CLOSED", __func__);
			return -1;
		}
		ev->ev_closure = EV_CLOSURE_EVENT_SIGNAL;
	} else {
		if (events & EV_PERSIST) {
			evutil_timerclear(&ev->ev_io_timeout);
			// 如果添加超時有EV_PERSIST標誌，就設置EV_CLOSURE_EVENT_PERSIST
			ev->ev_closure = EV_CLOSURE_EVENT_PERSIST;
		} else {
			ev->ev_closure = EV_CLOSURE_EVENT;
		}
	}
	// 初始化event在堆中的index
	min_heap_elem_init_(ev);

	if (base != NULL) {
		/* by default, we put new events into the middle priority */
		// 默認情況下event的優先級是base->nactivequeues / 2;
		ev->ev_pri = base->nactivequeues / 2;
	}

	event_debug_note_setup_(ev);

	return 0;
}



int
event_add(struct event *ev, const struct timeval *tv)
{
	int res;

	// 如果這個event所處的base是空的或者沒有設置，則出錯返回
	if (EVUTIL_FAILURE_CHECK(!ev->ev_base)) {
		event_warnx("%s: event has no event_base set.", __func__);
		return -1;
	}
	// 給base上鎖
	EVBASE_ACQUIRE_LOCK(ev->ev_base, th_base_lock);

	// 添加event，第三個參數表明提供的超時時間是絕對時間還是相對時間
	res = event_add_nolock_(ev, tv, 0);
	// 給base解鎖
	EVBASE_RELEASE_LOCK(ev->ev_base, th_base_lock);

	return (res);
}



// 第三個參數表明提供的超時時間是絕對時間還是相對時間
int
event_add_nolock_(struct event *ev, const struct timeval *tv,
    int tv_is_absolute)
{
	// 獲取當前event的base
	struct event_base *base = ev->ev_base;
	int res = 0;
	int notify = 0;

	EVENT_BASE_ASSERT_LOCKED(base);
	event_debug_assert_is_setup_(ev);

	event_debug((
		 "event_add: event: %p (fd "EV_SOCK_FMT"), %s%s%s%scall %p",
		 ev,
		 EV_SOCK_ARG(ev->ev_fd),
		 ev->ev_events & EV_READ ? "EV_READ " : " ",
		 ev->ev_events & EV_WRITE ? "EV_WRITE " : " ",
		 ev->ev_events & EV_CLOSED ? "EV_CLOSED " : " ",
		 tv ? "EV_TIMEOUT " : " ",
		 ev->ev_callback));

	EVUTIL_ASSERT(!(ev->ev_flags & ~EVLIST_ALL));

	if (ev->ev_flags & EVLIST_FINALIZING) {
		/* XXXX debug */
		return (-1);
	}

	/*
	 * prepare for timeout insertion further below, if we get a
	 * failure on any step, we should not change any state.
	 */

	if (tv != NULL && !(ev->ev_flags & EVLIST_TIMEOUT)) {
		if (min_heap_reserve_(&base->timeheap,
			1 + min_heap_size_(&base->timeheap)) == -1)
			return (-1);  /* ENOMEM == errno */
	}

	/* If the main thread is currently executing a signal event's
	 * callback, and we are not the main thread, then we want to wait
	 * until the callback is done before we mess with the event, or else
	 * we can race on ev_ncalls and ev_pncalls below. */
#ifndef EVENT__DISABLE_THREAD_SUPPORT
	if (base->current_event == event_to_event_callback(ev) &&
	    (ev->ev_events & EV_SIGNAL)
	    && !EVBASE_IN_THREAD(base)) {
		++base->current_event_waiters;
		EVTHREAD_COND_WAIT(base->current_event_cond, base->th_base_lock);
	}
#endif

	if ((ev->ev_events & (EV_READ|EV_WRITE|EV_CLOSED|EV_SIGNAL)) &&
	    !(ev->ev_flags & (EVLIST_INSERTED|EVLIST_ACTIVE|EVLIST_ACTIVE_LATER))) {
		if (ev->ev_events & (EV_READ|EV_WRITE|EV_CLOSED))
			res = evmap_io_add_(base, ev->ev_fd, ev);
		else if (ev->ev_events & EV_SIGNAL)
			res = evmap_signal_add_(base, (int)ev->ev_fd, ev);
		if (res != -1)
			event_queue_insert_inserted(base, ev);
		if (res == 1) {
			/* evmap says we need to notify the main thread. */
			notify = 1;
			res = 0;
		}
	}

	/*
	 * we should change the timeout state only if the previous event
	 * addition succeeded.
	 */
	// 如果提供了超時時間纔會進入
	if (res != -1 && tv != NULL) {
		struct timeval now;
		int common_timeout;
#ifdef USE_REINSERT_TIMEOUT
		int was_common;
		int old_timeout_idx;
#endif

		/*
		 * for persistent timeout events, we remember the
		 * timeout value and re-add the event.
		 *
		 * If tv_is_absolute, this was already set.
		 */
		// 如果new事件的時候設置了EV_PERSIST，就變成了永久事件
		// 所以需要把超時時間保存下來
		if (ev->ev_closure == EV_CLOSURE_EVENT_PERSIST && !tv_is_absolute)
			ev->ev_io_timeout = *tv;

#ifndef USE_REINSERT_TIMEOUT
		// 如果對於一個event，多次add，並且每次都設置了不同的超時值
		// 那麼只會保存最後一個
		if (ev->ev_flags & EVLIST_TIMEOUT) {
			// 如果之前有從超時值，就先刪除之前
			event_queue_remove_timeout(base, ev);
		}
#endif

		/* Check if it is active due to a timeout.  Rescheduling
		 * this timeout before the callback can be executed
		 * removes it from the active list. */
		// 如果當前的event處於分發狀態。 並且觸發事件的原因是timeout
		// 那麼就從active中移除這個event
		if ((ev->ev_flags & EVLIST_ACTIVE) &&
		    (ev->ev_res & EV_TIMEOUT)) {
			if (ev->ev_events & EV_SIGNAL) {
				/* See if we are just active executing
				 * this event in a loop
				 */
				if (ev->ev_ncalls && ev->ev_pncalls) {
					/* Abort loop */
					*ev->ev_pncalls = 0;
				}
			}

			event_queue_remove_active(base, event_to_event_callback(ev));
		}

		gettime(base, &now);

		common_timeout = is_common_timeout(tv, base);
#ifdef USE_REINSERT_TIMEOUT
		was_common = is_common_timeout(&ev->ev_timeout, base);
		old_timeout_idx = COMMON_TIMEOUT_IDX(&ev->ev_timeout);
#endif
		
		if (tv_is_absolute) {
			// 如果tv_is_absolute被設置說明傳入的就是一個絕對時間
			ev->ev_timeout = *tv;
		} else if (common_timeout) {
			struct timeval tmp = *tv;
			tmp.tv_usec &= MICROSECONDS_MASK;
			evutil_timeradd(&now, &tmp, &ev->ev_timeout);
			ev->ev_timeout.tv_usec |=
			    (tv->tv_usec & ~MICROSECONDS_MASK);
		} else {
			// 如果是相對時間，那麼就從當前時間now，加上相對時間tv
			// 保存到ev_timeout中
			evutil_timeradd(&now, tv, &ev->ev_timeout);
		}

		event_debug((
			 "event_add: event %p, timeout in %d seconds %d useconds, call %p",
			 ev, (int)tv->tv_sec, (int)tv->tv_usec, ev->ev_callback));

#ifdef USE_REINSERT_TIMEOUT
		event_queue_reinsert_timeout(base, ev, was_common, common_timeout, old_timeout_idx);
#else
		// 把超時事件加入到小根堆中
		event_queue_insert_timeout(base, ev);
#endif

		if (common_timeout) {
			struct common_timeout_list *ctl =
			    get_common_timeout_list(base, &ev->ev_timeout);
			if (ev == TAILQ_FIRST(&ctl->events)) {
				common_timeout_schedule(ctl, &now, ev);
			}
		} else {
			struct event* top = NULL;
			/* See if the earliest timeout is now earlier than it
			 * was before: if so, we will need to tell the main
			 * thread to wake up earlier than it would otherwise.
			 * We double check the timeout of the top element to
			 * handle time distortions due to system suspension.
			 */
			// 如果當前加入的event的超時事件是最小的
			// 就需要立刻通知主線程
			if (min_heap_elt_is_top_(ev))
				notify = 1;
			else if ((top = min_heap_top_(&base->timeheap)) != NULL &&
					 evutil_timercmp(&top->ev_timeout, &now, <))
				notify = 1;
		}
	}

	/* if we are not in the right thread, we need to wake up the loop */
	// EVBASE_NEED_NOTIFY 這個宏判斷當前的線程是否不等於主線程
	// 如果是的話，就要調用evthread_notify_base
	if (res != -1 && notify && EVBASE_NEED_NOTIFY(base))
		evthread_notify_base(base);

	event_debug_note_add_(ev);

	return (res);
}