Allow cascading into the current tick

Summary:
Currently, when cascading, we are unable to use bucket 0. This means that timeouts that are already expired would need to wait another tick before being fired.

A simple example is when we're at tick 0 and scheduling for tick 256, such timeout would go into cascading logic and should fall into bucket 0 of next wheel epoch. However, the existing logic would cascade it to bucket 1.

This diff fixes that, by reordering draining of current bucket and cascading timeouts.

Reviewed By: yfeldblum

Differential Revision: D13541506

fbshipit-source-id: 1284fca18612ae91f96538192bfad75e27cd816c

folly/io/async/HHWheelTimer.cpp

@@ -210,6 +210,14 @@ void HHWheelTimer::timeoutExpired() noexcept {
   while (lastTick_ < nextTick) {
     int idx = lastTick_ & WHEEL_MASK;
 
+    if (idx == 0) {
+      // Cascade timers
+      if (cascadeTimers(1, (lastTick_ >> WHEEL_BITS) & WHEEL_MASK) &&
+          cascadeTimers(2, (lastTick_ >> (2 * WHEEL_BITS)) & WHEEL_MASK)) {
+        cascadeTimers(3, (lastTick_ >> (3 * WHEEL_BITS)) & WHEEL_MASK);
+      }
+    }
+
     auto bi = makeBitIterator(bitmap_.begin());
     *(bi + idx) = false;
 
@@ -220,14 +228,6 @@ void HHWheelTimer::timeoutExpired() noexcept {
       cbs->pop_front();
       timeoutsToRunNow_.push_back(*cb);
     }
-
-    if (idx == 0) {
-      // Cascade timers
-      if (cascadeTimers(1, (lastTick_ >> WHEEL_BITS) & WHEEL_MASK) &&
-          cascadeTimers(2, (lastTick_ >> (2 * WHEEL_BITS)) & WHEEL_MASK)) {
-        cascadeTimers(3, (lastTick_ >> (3 * WHEEL_BITS)) & WHEEL_MASK);
-      }
-    }
   }
 
   while (!timeoutsToRunNow_.empty()) {