unix: update events from pevents between polls

Watchers could be stopped between two `kevent()`/`epoll_wait()` calls
(which may happen in the same loop in `uv__io_poll()`), in such cases
`watcher->events` could be stale and won't be updated to
`watcher->pevents`.

Try to use and rely on `watcher->pevents` instead of blindly expecting
`watcher->events` to be always correct.

src/unix/kqueue.c

@@ -191,7 +191,7 @@ void uv__io_poll(uv_loop_t* loop, int timeout) {
       revents = 0;
 
       if (ev->filter == EVFILT_READ) {
-        if (w->events & UV__POLLIN) {
+        if (w->pevents & UV__POLLIN) {
           revents |= UV__POLLIN;
           w->rcount = ev->data;
         } else {
@@ -205,7 +205,7 @@ void uv__io_poll(uv_loop_t* loop, int timeout) {
       }
 
       if (ev->filter == EVFILT_WRITE) {
-        if (w->events & UV__POLLOUT) {
+        if (w->pevents & UV__POLLOUT) {
           revents |= UV__POLLOUT;
           w->wcount = ev->data;
         } else {

src/unix/linux-core.c

@@ -105,6 +105,7 @@ void uv__io_poll(uv_loop_t* loop, int timeout) {
   uv__io_t* w;
   uint64_t base;
   uint64_t diff;
+  unsigned int masked_events;
   int nevents;
   int count;
   int nfds;
@@ -208,7 +209,15 @@ void uv__io_poll(uv_loop_t* loop, int timeout) {
         continue;
       }
 
-      w->cb(loop, w, pe->events);
+      /*
+       * Give users only events they're interested in. Prevents spurious
+       * callbacks when previous callback invocation in this loop has stopped
+       * the current watcher. Also, filters out events that users has not
+       * requested us to watch.
+       */
+      masked_events = pe->events & w->pevents;
+      if (masked_events != 0)
+        w->cb(loop, w, masked_events);
       nevents++;
     }