unix: match kqueue and epoll code

Match the implementation for linux.c to kqueue.c in the code around the
calls to kevent and epoll.

In linux.c the code was made more DRY by moving the nfds check up
(including a comment of why it's possible) and combining two if checks
into one.

In kqueue.c the EINTR check was moved to match linux.c and an assert
that was added in 42cc412 has been moved to be called directly after
the EINTR check. Since it should always be true regardless.

Ref: libuv#3893
Ref: nodejs/node#48490

src/unix/kqueue.c

@@ -260,9 +260,6 @@ void uv__io_poll(uv_loop_t* loop, int timeout) {
                   ARRAY_SIZE(events),
                   timeout == -1 ? NULL : &spec);
 
-    if (nfds == -1)
-      assert(errno == EINTR);
-
     if (pset != NULL)
       pthread_sigmask(SIG_UNBLOCK, pset, NULL);
 
@@ -272,6 +269,12 @@ void uv__io_poll(uv_loop_t* loop, int timeout) {
      */
     uv__update_time(loop);
 
+    if (nfds == -1)
+      assert(errno == EINTR);
+    else if (nfds == 0)
+      /* Unlimited timeout should only return with events or signal. */
+      assert(timeout != -1);
+
     if (nfds == 0 || nfds == -1) {
       /* If kqueue is empty or interrupted, we might still have children ready
        * to reap immediately. */
@@ -285,13 +288,13 @@ void uv__io_poll(uv_loop_t* loop, int timeout) {
       if (reset_timeout != 0) {
         timeout = user_timeout;
         reset_timeout = 0;
-      } else if (nfds == 0) {
-        /* Reached the user timeout value. */
-        assert(timeout != -1);
-        return;
       }
 
-      /* Interrupted by a signal. Update timeout and poll again. */
+      /* If nfds == 0 then we may have been inside the system call for longer
+       * than |timeout| milliseconds so we need to update the timestamp to
+       * avoid drift.
+       * If nfds == -1 then it was interrupted by a signal. Update timeout and
+       * poll again. */
       goto update_timeout;
     }
 

src/unix/linux.c

@@ -1364,42 +1364,23 @@ void uv__io_poll(uv_loop_t* loop, int timeout) {
      */
     SAVE_ERRNO(uv__update_time(loop));
 
-    if (nfds == 0) {
+    if (nfds == -1)
+      assert(errno == EINTR);
+    else if (nfds == 0)
+      /* Unlimited timeout should only return with events or signal. */
       assert(timeout != -1);
 
+    if (nfds == 0 || nfds == -1) {
       if (reset_timeout != 0) {
         timeout = user_timeout;
         reset_timeout = 0;
       }
 
-      if (timeout == -1)
-        continue;
-
-      if (timeout == 0)
-        break;
-
-      /* We may have been inside the system call for longer than |timeout|
-       * milliseconds so we need to update the timestamp to avoid drift.
-       */
-      goto update_timeout;
-    }
-
-    if (nfds == -1) {
-      if (errno != EINTR)
-        abort();
-
-      if (reset_timeout != 0) {
-        timeout = user_timeout;
-        reset_timeout = 0;
-      }
-
-      if (timeout == -1)
-        continue;
-
-      if (timeout == 0)
-        break;
-
-      /* Interrupted by a signal. Update timeout and poll again. */
+      /* If nfds == 0 then we may have been inside the system call for longer
+       * than |timeout| milliseconds so we need to update the timestamp to
+       * avoid drift.
+       * If nfds == -1 then it was interrupted by a signal. Update timeout and
+       * poll again. */
       goto update_timeout;
     }
 
@@ -1509,13 +1490,13 @@ void uv__io_poll(uv_loop_t* loop, int timeout) {
       break;
     }
 
+update_timeout:
     if (timeout == 0)
       break;
 
     if (timeout == -1)
       continue;
 
-update_timeout:
     assert(timeout > 0);
 
     real_timeout -= (loop->time - base);