time: fix timer stop

Due to data structure corruption,
some timers could not be removed.
Fixes #2495.

R=golang-dev, adg
CC=golang-dev, mdbrown
https://golang.org/cl/5437060

src/pkg/runtime/time.goc

@@ -133,9 +133,16 @@ deltimer(Timer *t)
 		return false;
 	}
 
-	timers.t[i] = timers.t[--timers.len];
+	timers.len--;
+	if(i == timers.len) {
+		timers.t[i] = nil;
+	} else {
+		timers.t[i] = timers.t[timers.len];
+		timers.t[timers.len] = nil;
+		timers.t[i]->i = i;
 		siftup(i);
 		siftdown(i);
+	}
 	runtime·unlock(&timers);
 	return true;
 }

src/pkg/time/sleep_test.go

@@ -205,3 +205,19 @@ func testAfterQueuing(t *testing.T) error {
 	}
 	return nil
 }
+
+func TestTimerStopStress(t *testing.T) {
+	if testing.Short() {
+		return
+	}
+	for i := 0; i < 100; i++ {
+		go func(i int) {
+			timer := AfterFunc(2e9, func() {
+				t.Fatalf("timer %d was not stopped", i)
+			})
+			Sleep(1e9)
+			timer.Stop()
+		}(i)
+	}
+	Sleep(3e9)
+}