Remove duplicate keys in labels of docker info

This fix tries to address the issue raised in 24392 where
labels with duplicate keys exist in `docker info`, which
contradicts with the specifications in the docs.

The reason for duplicate keys is that labels are stored as
slice of strings in the format of `A=B` (and the input/output).

This fix tries to address this issue by checking conflict
labels when daemon started, and remove duplicate labels (K-V).

The existing `/info` API has not been changed.

An additional integration test has been added to cover the
changes in this fix.

This fix fixes 24392.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
Upstream-commit: 805f669512
Component: cli

components/cli/command/system/info.go

@@ -223,6 +223,21 @@ func prettyPrintInfo(dockerCli *command.DockerCli, info types.Info) error {
 		for _, attribute := range info.Labels {
 			fmt.Fprintf(dockerCli.Out(), " %s\n", attribute)
 		}
+		// TODO: Engine labels with duplicate keys has been deprecated in 1.13 and will be error out
+		// after 3 release cycles (1.16). For now, a WARNING will be generated. The following will
+		// be removed eventually.
+		labelMap := map[string]string{}
+		for _, label := range info.Labels {
+			stringSlice := strings.SplitN(label, "=", 2)
+			if len(stringSlice) > 1 {
+				// If there is a conflict we will throw out an warning
+				if v, ok := labelMap[stringSlice[0]]; ok && v != stringSlice[1] {
+					fmt.Fprintln(dockerCli.Err(), "WARNING: labels with duplicate keys and conflicting values have been deprecated")
+					break
+				}
+				labelMap[stringSlice[0]] = stringSlice[1]
+			}
+		}
 	}
 
 	ioutils.FprintfIfTrue(dockerCli.Out(), "Experimental: %v\n", info.ExperimentalBuild)