MDEV-13564: Refuse MLOG_TRUNCATE in mariabackup

The MySQL 5.7 TRUNCATE TABLE is inherently incompatible
with hot backup, because it is creating and deleting a separate
log file, and it is not writing redo log for all changes of the
InnoDB data dictionary tables. Refuse to create a corrupted backup
if the unsafe form of TRUNCATE was executed.

Note: Undo log tablespace truncation cannot be detected easily.
Also it is incompatible with backup, for similar reasons.

xtrabackup_backup_func(): "Subscribe to" the log events before
the first invocation of xtrabackup_copy_logfile().

recv_parse_or_apply_log_rec_body(): If the function pointer
log_truncate is set, invoke it to report MLOG_TRUNCATE.

mysql-test/suite/mariabackup/truncate_during_backup.test

@@ -0,0 +1,19 @@
+--source include/have_debug.inc
+let $targetdir=$MYSQLTEST_VARDIR/tmp/backup;
+mkdir $targetdir;
+
+CREATE TABLE t1 ENGINE=InnoDB SELECT 1;
+
+--let after_load_tablespaces=TRUNCATE test.t1
+
+--disable_result_log
+--error 1
+exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf  --backup  --target-dir=$targetdir  --dbug=+d,mariabackup_events;
+--enable_result_log
+
+--let after_load_tablespaces=
+
+DROP TABLE t1;
+SET GLOBAL innodb_log_checkpoint_now=1;
+SET GLOBAL innodb_log_checkpoint_now=DEFAULT;
+rmdir $targetdir;