Address re-entrancy of Tick

This defers execution of the ticked function until after the lock has
been released, allowing it to make other calls without a deadlock.

clock.go

@@ -284,16 +284,20 @@ type internalTimer Timer
 
 func (t *internalTimer) Next() time.Time { return t.next }
 func (t *internalTimer) Tick(now time.Time) {
+	// a gosched() after ticking, to allow any consequences of the
+	// tick to complete
+	defer gosched()
+
 	t.mock.mu.Lock()
 	if t.fn != nil {
-		t.fn()
+		// defer function execution until the lock is released, and
+		defer t.fn()
 	} else {
 		t.c <- now
 	}
 	t.mock.removeClockTimer((*internalTimer)(t))
 	t.stopped = true
 	t.mock.mu.Unlock()
-	gosched()
 }
 
 // Ticker holds a channel that receives "ticks" at regular intervals.

clock_test.go

@@ -613,5 +613,20 @@ func ExampleMock_Timer() {
 	// Count is 1 after 10 seconds
 }
 
+func TestMock_ReentrantDeadlock(t *testing.T) {
+	mockedClock := NewMock()
+	timer20 := mockedClock.Timer(20 * time.Second)
+	go func() {
+		v := <-timer20.C
+		panic(fmt.Sprintf("timer should not have ticked: %v", v))
+	}()
+	mockedClock.AfterFunc(10*time.Second, func() {
+		timer20.Stop()
+	})
+
+	mockedClock.Add(15 * time.Second)
+	mockedClock.Add(15 * time.Second)
+}
+
 func warn(v ...interface{})              { fmt.Fprintln(os.Stderr, v...) }
 func warnf(msg string, v ...interface{}) { fmt.Fprintf(os.Stderr, msg+"\n", v...) }