restore, kv: detect if SwitchMode exists instead of check version

TiKV and PD's versions are exposed before 2.1, so we can't rely on their
APIs to verify for 2.0.4+. Since the only reason we need 2.0.4+ is the
SwitchMode gRPC interface, we instead inspect if SwitchMode is
unimplemented and display a user-friendly message on stderr.

lightning/kv/kv-deliver.go

@@ -1,7 +1,10 @@
 package kv
 
 import (
+	"fmt"
 	"math"
+	"os"
+	"strings"
 	"sync"
 	"time"
 
@@ -639,6 +642,9 @@ func (c *KVDeliverClient) Switch(mode sstpb.SwitchMode) error {
 	}
 	_, err := c.cli.SwitchMode(c.ctx, req)
 	if err != nil {
+		if strings.Contains(err.Error(), "status: Unimplemented") {
+			fmt.Fprintln(os.Stderr, "Error: The TiKV instance does not support mode switching. Please make sure the TiKV version is 2.0.4 or above.")
+		}
 		return errors.Trace(err)
 	}
 	common.AppLogger.Infof("switch to tikv %s mode takes %v", mode, time.Since(timer))

lightning/restore/restore.go

@@ -399,11 +399,14 @@ func (rc *RestoreController) checkRequirements(_ context.Context) error {
 	if err := rc.checkTiDBVersion(client); err != nil {
 		return errors.Trace(err)
 	}
+	// TODO: Reenable the PD/TiKV version check after we upgrade the dependency to 2.1.
 	if err := rc.checkPDVersion(client); err != nil {
-		return errors.Trace(err)
+		// return errors.Trace(err)
+		common.AppLog.Infof("PD version check failed: %v", err)
 	}
 	if err := rc.checkTiKVVersion(client); err != nil {
-		return errors.Trace(err)
+		// return errors.Trace(err)
+		common.AppLog.Infof("TiKV version check failed: %v", err)
 	}
 
 	return nil