Zero out stats values in the cli

Based on some feedback, when you have a container via the cli that you
are monitoring for stats, if you stop the container it will stay in the
display but report the last datapoint that was received.

This PR changes the display to zero out the values for containers where
an update has not been received within a specified duration, i.e. 2
seconds.  This signals the user that the container has stopped as it
reports cpu and memory usage of 0.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

api/client/commands.go

@@ -2641,13 +2641,13 @@ func (s *containerStats) Collect(stream io.ReadCloser) {
 		previousSystem uint64
 		start          = true
 		dec            = json.NewDecoder(stream)
+		u              = make(chan error, 1)
 	)
+	go func() {
 		for {
 			var v *stats.Stats
 			if err := dec.Decode(&v); err != nil {
-			s.mu.Lock()
-			s.err = err
-			s.mu.Unlock()
+				u <- err
 				return
 			}
 			var (
@@ -2666,9 +2666,29 @@ func (s *containerStats) Collect(stream io.ReadCloser) {
 			s.NetworkRx = float64(v.Network.RxBytes)
 			s.NetworkTx = float64(v.Network.TxBytes)
 			s.mu.Unlock()
-
 			previousCpu = v.CpuStats.CpuUsage.TotalUsage
 			previousSystem = v.CpuStats.SystemUsage
+			u <- nil
+		}
+	}()
+	for {
+		select {
+		case <-time.After(2 * time.Second):
+			// zero out the values if we have not received an update within
+			// the specified duration.
+			s.mu.Lock()
+			s.CpuPercentage = 0
+			s.Memory = 0
+			s.MemoryPercentage = 0
+			s.mu.Unlock()
+		case err := <-u:
+			if err != nil {
+				s.mu.Lock()
+				s.err = err
+				s.mu.Unlock()
+				return
+			}
+		}
 	}
 }