Fix bug where timers scheduled from inside timers could end up in wrong slot

- During the timer processing the different wheels could have
  inconsistent views of time. The proper sequence is to first
  increment time in all wheels, and then process the timers from
  starting from the coarsest wheel and finishing with the most
  granular.

src/test/test_basic.cc

@@ -140,6 +140,29 @@ bool test_single_timer_hierarchy() {
     return true;
 }
 
+bool test_reschedule_from_timer() {
+    typedef std::function<void()> Callback;
+    TimerWheel timers;
+    int count = 0;
+    TimerEvent<Callback> timer([&count] () { ++count; });
+
+    // For every slot in the outermost wheel, try scheduling a timer from
+    // a timer handler 258 ticks in the future. Then reschedule it in 257
+    // ticks. It should never actually trigger.
+    for (int i = 0; i < 256; ++i) {
+        TimerEvent<Callback> rescheduler([&timers, &timer] () { timers.schedule(&timer, 258); });
+
+        timers.schedule(&rescheduler, 1);
+        timers.advance(257);
+        EXPECT_INTEQ(count, 0);
+    }
+    // But once we stop rescheduling the timer, it'll trigger as intended.
+    timers.advance(2);
+    EXPECT_INTEQ(count, 1);
+
+    return true;
+}
+
 bool test_single_timer_random() {
     typedef std::function<void()> Callback;
     TimerWheel timers;
@@ -206,8 +229,8 @@ int main(void) {
     TEST(test_single_timer_no_hierarchy);
     TEST(test_single_timer_hierarchy);
     TEST(test_single_timer_random);
+    TEST(test_reschedule_from_timer);
     TEST(test_timeout_method);
     // Test canceling timer from within timer
-    // Test rescheduling timer from within timer
     return ok ? 0 : 1;
 }

src/timer-wheel.h

@@ -127,6 +127,9 @@ public:
             now_++;
             size_t slot_index = now_ & MASK;
             auto slot = &slots_[slot_index];
+            if (slot_index == 0 && up_) {
+                up_->advance(1);
+            }
             while (slot->events()) {
                 auto event = slot->pop_event();
                 if (down_) {
@@ -136,9 +139,6 @@ public:
                     event->execute();
                 }
             }
-            if (slot_index == 0 && up_) {
-                up_->advance(1);
-            }
         }
     }