Add a way to set a maximum number of event loop turns before we poll for input.

c++/src/kj/async.c++

@@ -436,10 +436,16 @@ void waitImpl(Own<_::PromiseNode>&& node, _::ExceptionOrValue& result, WaitScope
   loop.running = true;
   KJ_DEFER(loop.running = false);
 
+  uint counter = 0;
   while (!doneEvent.fired) {
     if (!loop.turn()) {
       // No events in the queue.  Wait for callback.
+      counter = 0;
       loop.port.wait();
+    } else if (++counter > waitScope.busyPollInterval) {
+      // Note: It's intentional that if busyPollInterval is kj::maxValue, we never poll.
+      counter = 0;
+      loop.port.poll();
     }
   }
 

c++/src/kj/async.h

@@ -798,8 +798,13 @@ public:
   void poll();
   // Pumps the event queue and polls for I/O until there's nothing left to do (without blocking).
 
+  void setBusyPollInterval(uint count) { busyPollInterval = count; }
+  // Set the maximum number of events to run in a row before calling poll() on the EventPort to
+  // check for new I/O.
+
 private:
   EventLoop& loop;
+  uint busyPollInterval = kj::maxValue;
   friend class EventLoop;
   friend void _::waitImpl(Own<_::PromiseNode>&& node, _::ExceptionOrValue& result,
                           WaitScope& waitScope);