Fix bug with timer rescheduling

- Not enough to just redefine the relink() method, need to
  also use it...

src/test/test_basic.cc

@@ -43,22 +43,28 @@ bool test_single_timer_no_hierarchy() {
     int count = 0;
     TimerEvent<Callback> timer([&count] () { ++count; });
 
+    // Unscheduled timer does nothing.
     timers.advance(10);
     EXPECT_INTEQ(count, 0);
     EXPECT(!timer.active());
 
+    // Schedule timer, should trigger at right time.
     timers.schedule(&timer, 5);
     EXPECT(timer.active());
-    timers.advance(10);
+    timers.advance(5);
     EXPECT_INTEQ(count, 1);
 
-    timers.advance(10);
+    // Only trigger once, not repeatedly (even if wheel wraps
+    // around).
+    timers.advance(256);
     EXPECT_INTEQ(count, 1);
 
+    // ... unless, of course, the timer gets scheduled again.
     timers.schedule(&timer, 5);
-    timers.advance(10);
+    timers.advance(5);
     EXPECT_INTEQ(count, 2);
 
+    // Canceled timers don't run.
     timers.schedule(&timer, 5);
     timer.cancel();
     EXPECT(!timer.active());
@@ -71,6 +77,15 @@ bool test_single_timer_no_hierarchy() {
     timers.advance(10);
     EXPECT_INTEQ(count, 3);
 
+    // Timers that are scheduled multiple times only run at the last
+    // scheduled tick.
+    timers.schedule(&timer, 5);
+    timers.schedule(&timer, 10);
+    timers.advance(5);
+    EXPECT_INTEQ(count, 3);
+    timers.advance(5);
+    EXPECT_INTEQ(count, 4);
+
     return true;
 }
 

src/timer-wheel.h

@@ -153,8 +153,18 @@ public:
 
         size_t slot_index = (now_ + delta) & MASK;
         auto slot = &slots_[slot_index];
+        if (event->active()) {
+            event->relink(slot);
+        } else {
             slot->push_event(event);
         }
+    }
+
+    // Return the current tick value. Note that if the timers advance by
+    // multiple ticks during a single call to advance(), the value of now()
+    // will be the tick on which the timer was first run. Not the tick that
+    // the timer eventually will advance to.
+    const Tick& now() const { return now_; }
 
 private:
     TimerWheel(const TimerWheel& other) = delete;