Better backwards compat/migration support

Signed-off-by: Andrew Mason <amason@slack-corp.com>

go/cmd/vtctl/vtctl.go

@@ -104,8 +104,45 @@ func main() {
 	ctx, cancel := context.WithTimeout(context.Background(), *waitTime)
 	installSignalHandlers(cancel)
 
-	if strings.EqualFold(action, "LegacyVtctlCommand") {
+	// (TODO:ajm188) <Begin backwards compatibility support>.
+	//
+	// For v12, we are going to support new commands by prefixing as:
+	//		vtctl VtctldCommand <command> <args...>
+	//
+	// Existing scripts will continue to use the legacy commands. This is the
+	// default case below.
+	//
+	// We will also support legacy commands by prefixing as:
+	//		vtctl LegacyVtctlCommand <command> <args...>
+	// This is the fallthrough to the default case.
+	//
+	// In v13, we will make the default behavior to use the new commands and
+	// drop support for the `vtctl VtctldCommand ...` prefix, and legacy
+	// commands will only by runnable with the `vtctl LegacyVtctlCommand ...`
+	// prefix.
+	//
+	// In v14, we will drop support for all legacy commands, only running new
+	// commands, without any prefixing required or supported.
+	switch {
+	case strings.EqualFold(action, "VtctldCommand"):
+		// New behavior. Strip off the prefix, and set things up to run through
+		// the vtctldclient command tree, using the localvtctldclient (in-process)
+		// client.
+		vtctld := grpcvtctldserver.NewVtctldServer(ts)
+		localvtctldclient.SetServer(vtctld)
+		command.VtctldClientProtocol = "local"
+
+		os.Args = append([]string{"vtctldclient"}, args[1:]...)
+		if err := command.Root.ExecuteContext(ctx); err != nil {
+			log.Errorf("action failed: %v %v", action, err)
+			exit.Return(255)
+		}
+	case strings.EqualFold(action, "LegacyVtctlCommand"):
+		// Strip off the prefix (being used for compatibility) and fallthrough
+		// to the legacy behavior.
 		args = args[1:]
+		fallthrough
+	default:
 		if args[0] == "--" {
 			args = args[1:]
 		}
@@ -125,35 +162,4 @@ func main() {
 			exit.Return(255)
 		}
 	}
-
-	vtctld := grpcvtctldserver.NewVtctldServer(ts)
-	localvtctldclient.SetServer(vtctld)
-	command.VtctldClientProtocol = "local"
-	for i, arg := range args {
-		if strings.HasPrefix(arg, "-") {
-			if len(arg) == 2 {
-				continue
-			}
-
-			if strings.HasPrefix(arg, "--") {
-				continue
-			}
-
-			log.Warningf("Long arg %s begins with a single dash; this will not work in future versions.", arg)
-			dashed := strings.ReplaceAll(arg, "_", "-")
-			if arg != dashed {
-				log.Warningf("Long arg %s is using underscore word separators, switching to dashes. This will not work in future versions.", arg)
-				arg = dashed
-			}
-
-			arg = "-" + arg
-			args[i] = arg
-		}
-	}
-
-	os.Args = append([]string{"vtctldclient"}, args...)
-	if err := command.Root.ExecuteContext(ctx); err != nil {
-		log.Errorf("action failed: %v %v", action, err)
-		exit.Return(255)
-	}
 }