fix pre-1.21 docker stats

This fixes a bug introduced in #15786:

* if a pre-v1.20 client requested docker stats, the daemon
would return both an API-compatible JSON blob *and* an API-incompatible JSON
blob: see https://gist.github.com/donhcd/338a5b3681cd6a071629

Signed-off-by: Donald Huang <don.hcd@gmail.com>
(cherry picked from commit d2c04f844b8258d712da4b8feac25df7590b037c)

The commit title wrongfully mentioned API v1.22, when it meant to mention v1.21.

daemon/stats.go

@@ -75,7 +75,8 @@ func (daemon *Daemon) ContainerStats(prefixOrName string, config *ContainerStats
 				return nil
 			}
 
-			statsJSON := getStatJSON(v)
+			var statsJSON interface{}
+			statsJSONPost120 := getStatJSON(v)
 			if config.Version.LessThan("1.21") {
 				var (
 					rxBytes   uint64
@@ -87,7 +88,7 @@ func (daemon *Daemon) ContainerStats(prefixOrName string, config *ContainerStats
 					txErrors  uint64
 					txDropped uint64
 				)
-				for _, v := range statsJSON.Networks {
+				for _, v := range statsJSONPost120.Networks {
 					rxBytes += v.RxBytes
 					rxPackets += v.RxPackets
 					rxErrors += v.RxErrors
@@ -97,8 +98,8 @@ func (daemon *Daemon) ContainerStats(prefixOrName string, config *ContainerStats
 					txErrors += v.TxErrors
 					txDropped += v.TxDropped
 				}
-				statsJSONPre121 := &v1p20.StatsJSON{
-					Stats: statsJSON.Stats,
+				statsJSON = &v1p20.StatsJSON{
+					Stats: statsJSONPost120.Stats,
 					Network: types.NetworkStats{
 						RxBytes:   rxBytes,
 						RxPackets: rxPackets,
@@ -110,20 +111,8 @@ func (daemon *Daemon) ContainerStats(prefixOrName string, config *ContainerStats
 						TxDropped: txDropped,
 					},
 				}
-
-				if !config.Stream && noStreamFirstFrame {
-					// prime the cpu stats so they aren't 0 in the final output
-					noStreamFirstFrame = false
-					continue
-				}
-
-				if err := enc.Encode(statsJSONPre121); err != nil {
-					return err
-				}
-
-				if !config.Stream {
-					return nil
-				}
+			} else {
+				statsJSON = statsJSONPost120
 			}
 
 			if !config.Stream && noStreamFirstFrame {