immediately fire a timer set to a negative duration

clock.go

@@ -219,7 +219,6 @@ func (m *Mock) Ticker(d time.Duration) *Ticker {
 // Timer creates a new instance of Timer.
 func (m *Mock) Timer(d time.Duration) *Timer {
 	m.mu.Lock()
-	defer m.mu.Unlock()
 	ch := make(chan time.Time, 1)
 	t := &Timer{
 		C:       ch,
@@ -229,6 +228,8 @@ func (m *Mock) Timer(d time.Duration) *Timer {
 		stopped: false,
 	}
 	m.timers = append(m.timers, (*internalTimer)(t))
+	m.mu.Unlock()
+	m.runNextTimer(m.now)
 	return t
 }
 

clock_test.go

@@ -175,6 +175,16 @@ func TestClock_Timer_Reset(t *testing.T) {
 	}
 }
 
+func TestClock_NegativeDuration(t *testing.T) {
+	clock := NewMock()
+	timer := clock.Timer(-time.Second)
+	select {
+	case <-timer.C:
+	default:
+		t.Fatal("timer should have fired immediately")
+	}
+}
+
 // Ensure reset can be called immediately after reading channel
 func TestClock_Timer_Reset_Unlock(t *testing.T) {
 	clock := NewMock()