remove handler::open_read_view()

ht->start_consistent_snapshot() is also not a way,
because some engines (e.g. rocksdb) only do it readonly.
instead, downgrade the lock after reading the first row
(which implicitly opens a read view).

sql/handler.h

@@ -3562,7 +3562,6 @@ class handler :public Sql_alloc
   /** to be actually called to get 'check()' functionality*/
   int ha_check(THD *thd, HA_CHECK_OPT *check_opt);
   int ha_repair(THD* thd, HA_CHECK_OPT* check_opt);
-  virtual void open_read_view(){}
   void ha_start_bulk_insert(ha_rows rows, uint flags= 0)
   {
     DBUG_ENTER("handler::ha_start_bulk_insert");

sql/sql_table.cc

@@ -11623,52 +11623,11 @@ copy_data_between_tables(THD *thd, TABLE *from, TABLE *to,
   */
   thd_progress_init(thd, MY_TEST(order) + 2 + 2 * MY_TEST(online));
 
-#ifdef HAVE_REPLICATION
-  if (online)
-  {
-    from->s->online_alter_binlog= new (thd->mem_root) Cache_flip_event_log();
-    if (!from->s->online_alter_binlog)
-      DBUG_RETURN(1);
-
-    from->s->online_alter_binlog->init_pthread_objects();
-
-    error= from->s->online_alter_binlog->open(WRITE_CACHE);
-
-    if (!error)
-    {
-      /*
-        Some engines (for example, InnoDB) might not create a read view
-        until the first row is read. We need to be sure that we won't see any
-        table changes after we enable replication and downgrade the MDL.
-        So, we force the consistent snapshot to be created now.
-      */
-      handlerton *ht= from->s->db_type();
-      if (ht->start_consistent_snapshot)
-      {
-        thd->tx_isolation= ISO_REPEATABLE_READ;
-        from->file->open_read_view();
-      }
-    }
-
-    if (error)
-    {
-      online_alter_cleanup_binlog(thd, from->s);
-      DBUG_RETURN(1);
-    }
-
-    from->mdl_ticket->downgrade_lock(MDL_SHARED_UPGRADABLE);
-    DEBUG_SYNC(thd, "alter_table_online_downgraded");
-  }
-#else
-  DBUG_ASSERT(!online);
-#endif // HAVE_REPLICATION
-
   if (!(copy= new (thd->mem_root) Copy_field[to->s->fields]))
     DBUG_RETURN(-1);
 
   if (mysql_trans_prepare_alter_copy_data(thd))
   {
-    online_alter_cleanup_binlog(thd, from->s);
     delete [] copy;
     DBUG_RETURN(-1);
   }
@@ -11676,7 +11635,6 @@ copy_data_between_tables(THD *thd, TABLE *from, TABLE *to,
   /* We need external lock before we can disable/enable keys */
   if (to->file->ha_external_lock(thd, F_WRLCK))
   {
-    online_alter_cleanup_binlog(thd, from->s);
     /* Undo call to mysql_trans_prepare_alter_copy_data() */
     ha_enable_transaction(thd, TRUE);
     delete [] copy;
@@ -11822,9 +11780,31 @@ copy_data_between_tables(THD *thd, TABLE *from, TABLE *to,
   if (!ignore) /* for now, InnoDB needs the undo log for ALTER IGNORE */
     to->file->extra(HA_EXTRA_BEGIN_ALTER_COPY);
 
-  DEBUG_SYNC(thd, "alter_table_copy_start");
+  if (!(error= info.read_record()))
+  {
+#ifdef HAVE_REPLICATION
+    if (online)
+    {
+      from->s->online_alter_binlog= new (thd->mem_root) Cache_flip_event_log();
+      if (!from->s->online_alter_binlog)
+        DBUG_RETURN(1);
+      from->s->online_alter_binlog->init_pthread_objects();
+      error= from->s->online_alter_binlog->open(WRITE_CACHE);
 
-  while (likely(!(error= info.read_record())))
+      if (error)
+      {
+        online_alter_cleanup_binlog(thd, from->s);
+        goto err;
+      }
+
+      from->mdl_ticket->downgrade_lock(MDL_SHARED_UPGRADABLE);
+      DEBUG_SYNC(thd, "alter_table_online_downgraded");
+    }
+#else
+    DBUG_ASSERT(!online);
+#endif // HAVE_REPLICATION
+
+    do
     {
       if (unlikely(thd->killed))
       {
@@ -11956,7 +11936,10 @@ copy_data_between_tables(THD *thd, TABLE *from, TABLE *to,
         mysql_stage_set_work_completed(thd->m_stage_progress_psi, found_count);
       }
       thd->get_stmt_da()->inc_current_row_for_warning();
+    } while (!(error= info.read_record()));
   }
+  else
+    online= false;
 
   DEBUG_SYNC(thd, "alter_table_copy_end");
 

storage/innobase/handler/ha_innodb.cc

@@ -21165,17 +21165,3 @@ buf_pool_size_align(
     return (size / m + 1) * m;
   }
 }
-
-void ha_innobase::open_read_view()
-{
-  trx_t *trx= m_prebuilt->trx;
-  auto thd_iso= thd_get_trx_isolation(m_user_thd);
-
-  trx->isolation_level= innobase_map_isolation_level(thd_iso);
-  ut_ad(trx->isolation_level == TRX_ISO_REPEATABLE_READ);
-  ut_ad(!trx_is_started(trx));
-
-  trx_start_if_not_started_xa(trx, false);
-
-  trx->read_view.open(m_prebuilt->trx);
-}

storage/innobase/handler/ha_innodb.h

@@ -438,7 +438,6 @@ class ha_innobase final : public handler
 			  const KEY_PART_INFO& old_part,
 			  const KEY_PART_INFO& new_part) const override;
 
-        void open_read_view() override;
 protected:
 	bool
 	can_convert_string(const Field_string* field,