Revert recent changes until we figure out deadlocks

advanceable.go

@@ -75,9 +75,6 @@ func (a *advanceable) setCurrent(now time.Time) {
 
 // register marks a subscriber to be updated when the current time changes.
 func (a *advanceable) register(subscriber subscriber) {
-	a.m.Lock()
 	a.subscribers = append(a.subscribers, subscriber)
-	a.m.Unlock()
-
 	a.cond.Broadcast()
 }

mock_clock.go

@@ -44,8 +44,9 @@ func (c *MockClock) Now() time.Time {
 // clock's internal time is at or past the supplied duration.
 func (c *MockClock) After(duration time.Duration) <-chan time.Time {
 	c.m.Lock()
+	defer c.m.Unlock()
+
 	c.afterArgs = append(c.afterArgs, duration)
-	c.m.Unlock()
 
 	if duration <= 0 {
 		return closedChan

mock_timer.go

@@ -55,14 +55,12 @@ func newMockTimerAt(
 		panic("duration cannot be 0")
 	}
 
-	advanceable.m.Lock()
 	t := &MockTimer{
 		advanceable: advanceable,
 		deadline:    advanceable.now.Add(duration),
 		ch:          make(chan time.Time),
 		f:           f,
 	}
-	advanceable.m.Unlock()
 
 	go t.process()
 	advanceable.register(t)