One of the Juju tests was failing.

We assumed that doing WaitAdvance(0,0,0) was valid. However, that
triggers final before next, which means we don't see the 'everything is
ok' event. Instead put one final check during WaitAdvance that we got
what we wanted.

testclock/clock.go

@@ -130,6 +130,10 @@ func (clock *Clock) WaitAdvance(d, w time.Duration, n int) error {
 	for {
 		select {
 		case <-finalTimeout:
+                        if clock.hasNWaiters(n) {
+                            clock.Advance(d)
+                            return nil
+                        }
 			clock.mu.Lock()
 			got := len(clock.waiting)
 			var stacks string

testclock/clock_test.go

@@ -57,8 +57,12 @@ func (*clockSuite) TestWaitAdvance(c *gc.C) {
 	t0 := time.Now()
 	cl := testclock.NewClock(t0)
 
+        // It is legal to just say 'nothing is waiting'
+	err := cl.WaitAdvance(0, 0, 0)
+	c.Check(err, jc.ErrorIsNil)
+
 	// Test that no timers errors out.
-	err := cl.WaitAdvance(time.Millisecond, 10*time.Millisecond, 1)
+	err = cl.WaitAdvance(time.Millisecond, 10*time.Millisecond, 1)
 	c.Check(err, gc.ErrorMatches, "got 0 timers added after waiting 10ms: wanted 1, stacks:\n")
 
 	// Test that a timer doesn't error.