cmd/gomote: add support for groups to ping

For golang/go#53956.

Change-Id: I3195cf01b56fd090611b065a486b2444863fdbd9
Reviewed-on: https://go-review.googlesource.com/c/build/+/418783
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

cmd/gomote/ping.go

@@ -48,23 +48,39 @@ func legacyPing(args []string) error {
 }
 
 func ping(args []string) error {
-	if activeGroup != nil {
-		return fmt.Errorf("command does not yet support groups")
-	}
-
 	fs := flag.NewFlagSet("ping", flag.ContinueOnError)
 	fs.Usage = func() {
-		fmt.Fprintln(os.Stderr, "ping usage: gomote ping [--status] <instance>")
+		fmt.Fprintln(os.Stderr, "ping usage: gomote ping [instance]")
+		fmt.Fprintln(os.Stderr, "")
+		fmt.Fprintln(os.Stderr, "Instance name is optional if a group is specified.")
 		fs.PrintDefaults()
 		os.Exit(1)
 	}
 	fs.Parse(args)
 
-	if fs.NArg() != 1 {
+	var pingSet []string
+	if fs.NArg() == 1 {
+		pingSet = []string{fs.Arg(0)}
+	} else if fs.NArg() == 0 && activeGroup != nil {
+		for _, inst := range activeGroup.Instances {
+			pingSet = append(pingSet, inst)
+		}
+	} else {
 		fs.Usage()
 	}
-	name := fs.Arg(0)
+
 	ctx := context.Background()
+	for _, inst := range pingSet {
+		if err := doPing(ctx, inst); err != nil {
+			fmt.Fprintf(os.Stderr, "%s: %v\n", inst, err)
+		} else {
+			fmt.Fprintf(os.Stderr, "%s: alive\n", inst)
+		}
+	}
+	return nil
+}
+
+func doPing(ctx context.Context, name string) error {
 	client := gomoteServerClient(ctx)
 	_, err := client.InstanceAlive(ctx, &protos.InstanceAliveRequest{
 		GomoteId: name,