unix: match kqueue and epoll code (libuv#4091)

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 assert to check the timeout when nfds == 0 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

@@ -262,6 +262,9 @@ void uv__io_poll(uv_loop_t* loop, int timeout) {
 
     if (nfds == -1)
       assert(errno == EINTR);
+    else if (nfds == 0)
+      /* Unlimited timeout should only return with events or signal. */
+      assert(timeout != -1);
 
     if (pset != NULL)
       pthread_sigmask(SIG_UNBLOCK, pset, NULL);
@@ -286,8 +289,6 @@ void uv__io_poll(uv_loop_t* loop, int timeout) {
         timeout = user_timeout;
         reset_timeout = 0;
       } else if (nfds == 0) {
-        /* Reached the user timeout value. */
-        assert(timeout != -1);
         return;
       }
 

src/unix/linux.c

@@ -1395,41 +1395,20 @@ 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;
+      } else if (nfds == 0) {
+        return;
       }
 
-      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. */
       goto update_timeout;
     }
@@ -1540,13 +1519,13 @@ 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);