Allow canceling inactive timers

- Mostly for convenience; it's common to cancel timers even if
  you don't know they are running. Without this, such code would
  need a lot of (fragile) checks.

src/test/test_basic.cc

@@ -86,6 +86,14 @@ bool test_single_timer_no_hierarchy() {
     timers.advance(5);
     EXPECT_INTEQ(count, 4);
 
+    // Timer can safely be canceled multiple times.
+    timers.schedule(&timer, 5);
+    timer.cancel();
+    timer.cancel();
+    EXPECT(!timer.active());
+    timers.advance(10);
+    EXPECT_INTEQ(count, 4);
+
     return true;
 }
 

src/timer-wheel.h

@@ -206,6 +206,11 @@ private:
 };
 
 void TimerEventInterface::cancel() {
+    // It's ok to cancel a timer that's not running.
+    if (!slot_) {
+        return;
+    }
+
     if (this == slot_->events()) {
         slot_->pop_event();
     } else {