Fix mysql_upgrade during Restore.

1) mysqld needs to be running to use mysql_upgrade.
2) mysql_upgrade needs to be told how to connect to mysqld.

go/vt/mysqlctl/backup.go

@@ -552,12 +552,25 @@ func Restore(ctx context.Context, mysqld MysqlDaemon, bucket string, restoreConc
 		return proto.ReplicationPosition{}, err
 	}
 
-	log.Infof("Restore: running mysql_upgrade if necessary")
-	if err := mysqld.RunMysqlUpgrade(); err != nil {
+	// mysqld needs to be running in order for mysql_upgrade to work.
+	log.Infof("Restore: starting mysqld for mysql_upgrade")
+	err = mysqld.Start(ctx)
+	if err != nil {
 		return proto.ReplicationPosition{}, err
 	}
 
-	log.Infof("Restore: restart mysqld")
+	log.Infof("Restore: running mysql_upgrade")
+	if err := mysqld.RunMysqlUpgrade(); err != nil {
+		return proto.ReplicationPosition{}, fmt.Errorf("mysql_upgrade failed: %v", err)
+	}
+
+	// The MySQL manual recommends restarting mysqld after running mysql_upgrade,
+	// so that any changes made to system tables take effect.
+	log.Infof("Restore: restarting mysqld after mysql_upgrade")
+	err = mysqld.Shutdown(ctx, true)
+	if err != nil {
+		return proto.ReplicationPosition{}, err
+	}
 	err = mysqld.Start(ctx)
 	if err != nil {
 		return proto.ReplicationPosition{}, err

go/vt/mysqlctl/mysqld.go

@@ -143,7 +143,17 @@ func (mysqld *Mysqld) RunMysqlUpgrade() error {
 	}
 
 	// run the program, if it fails, we fail
-	cmd := exec.Command(name, "--defaults-file="+mysqld.config.path)
+	args := []string{
+		// --defaults-file=* must be the first arg.
+		"--defaults-file=" + mysqld.config.path,
+		"--socket", mysqld.config.SocketFile,
+		"--user", mysqld.dba.Uname,
+	}
+	if mysqld.dba.Pass != "" {
+		// --password must be omitted entirely if empty, or else it will prompt.
+		args = append(args, "--password", mysqld.dba.Pass)
+	}
+	cmd := exec.Command(name, args...)
 	cmd.Env = []string{os.ExpandEnv("LD_LIBRARY_PATH=$VT_MYSQL_ROOT/lib/mysql")}
 	stderr, err := cmd.StderrPipe()
 	if err != nil {