MDEV-16024 transaction_registry.begin_timestamp is wrong for explicit transactions

Store transaction start time in thd->transaction.start_time.
THD::transaction_time() wraps over transaction.start_time taking into
account current status of BEGIN.

mysql-test/suite/versioning/r/trx_id.result

@@ -166,5 +166,20 @@ select * from t1 for system_time as of timestamp'1990-1-1 00:00';
 x
 select * from t1 for system_time as of NULL;
 x
+# MDEV-16024 transaction_registry.begin_timestamp is wrong for explicit transactions
+create or replace table t1 (
+x int(11) default null,
+row_start bigint(20) unsigned generated always as row start invisible,
+row_end bigint(20) unsigned generated always as row end invisible,
+period for system_time (row_start, row_end)
+) engine=innodb with system versioning;
+begin;
+set @ts1= now(6);
+insert into t1 values (1);
+commit;
+select row_start from t1 into @trx_id;
+select trt_begin_ts(@trx_id) <= @ts1 as BEGIN_TS_GOOD;
+BEGIN_TS_GOOD
+1
 drop database test;
 create database test;

mysql-test/suite/versioning/t/trx_id.opt

+--plugin-load-add=test_versioning

mysql-test/suite/versioning/t/trx_id.test

@@ -153,5 +153,21 @@ delete from t1;
 select * from t1 for system_time as of timestamp'1990-1-1 00:00';
 select * from t1 for system_time as of NULL;
 
+--echo # MDEV-16024 transaction_registry.begin_timestamp is wrong for explicit transactions
+create or replace table t1 (
+    x int(11) default null,
+    row_start bigint(20) unsigned generated always as row start invisible,
+    row_end bigint(20) unsigned generated always as row end invisible,
+    period for system_time (row_start, row_end)
+) engine=innodb with system versioning;
+begin;
+set @ts1= now(6);
+--sleep 0.01
+insert into t1 values (1);
+commit;
+
+select row_start from t1 into @trx_id;
+select trt_begin_ts(@trx_id) <= @ts1 as BEGIN_TS_GOOD;
+
 drop database test;
 create database test;

sql/sql_class.h

@@ -2627,6 +2627,15 @@ class THD :public Statement,
     WT_THD wt;                          ///< for deadlock detection
     Rows_log_event *m_pending_rows_event;
 
+    struct st_trans_time : public timeval
+    {
+      void reset(THD *thd)
+      {
+        tv_sec= thd->query_start();
+        tv_usec= (long) thd->query_start_sec_part();
+      }
+    } start_time;
+
     /*
        Tables changed in transaction (that must be invalidated in query cache).
        List contain only transactional tables, that not invalidated in query
@@ -3470,6 +3479,13 @@ class THD :public Statement,
   }
 
 public:
+  timeval transaction_time()
+  {
+    if (!in_multi_stmt_transaction_mode())
+      transaction.start_time.reset(this);
+    return transaction.start_time;
+  }
+
   inline void set_start_time()
   {
     if (user_time.val)

sql/table.cc

@@ -8612,12 +8612,10 @@ bool TR_table::update(ulonglong start_id, ulonglong end_id)
   if (!table && open())
     return true;
 
-  timeval start_time= {thd->query_start(), long(thd->query_start_sec_part())};
-  thd->set_start_time();
+  store(FLD_BEGIN_TS, thd->transaction_time());
   timeval end_time= {thd->query_start(), long(thd->query_start_sec_part())};
   store(FLD_TRX_ID, start_id);
   store(FLD_COMMIT_ID, end_id);
-  store(FLD_BEGIN_TS, start_time);
   store(FLD_COMMIT_TS, end_time);
   store_iso_level(thd->tx_isolation);
 

sql/transaction.cc

@@ -203,6 +203,7 @@ bool trans_begin(THD *thd, uint flags)
   thd->transaction.all.reset();
   thd->has_waiter= false;
   thd->waiting_on_group_commit= false;
+  thd->transaction.start_time.reset(thd);
 
   if (res)
     DBUG_RETURN(TRUE);