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  1. MariaDB Server
  2. MDEV-26712

row events never reset thd->mem_root

Details

    Description

      row event execution starts from Rows_log_event::do_apply_event which calls open_and_lock_tables() which calls lock_tables(), which does

      if (!(ptr=start=(TABLE**) thd->alloc(sizeof(TABLE*)*count)))

      thus allocating an array of points of the thd's memroot. This is just few bytes, 8 in the test below. But this memory is never freed, not by row events at least. So, by creating a transaction with a huge number of row events we can consume any amount of memory.

      source include/have_innodb.inc;
      		 
      source include/have_binlog_format_row.inc;
      		 
      source include/master-slave.inc;
      		 
      create table t1 (a int) engine=innodb;
      		 
      insert t1 values (1);
      		 
       
      		 
      sync_slave_with_master;
      		 
      stop slave;
      		 
      set global max_session_mem_used=10*1024*1024;
      		 
      start slave;
      		 
       
      		 
      connection master;
      		 
      let $a=1000000;
      		 
      start transaction;
      		 
      disable_query_log;
      		 
      while ($a) {
      		 
        update t1 set a=a+1;
      		 
        dec $a;
      		 
      }
      		 
      enable_query_log;
      		 
      commit;
      		 
      drop table t1;
      		 
      sync_slave_with_master;
      		 
      set global max_session_mem_used=default;
      		 
      source include/rpl_end.inc;

      To fix that, rbr events must do

      free_root(thd->mem_root, MYF(MY_KEEP_PREALLOC));

      somewhere. Where it's safe, because it'll destroy anything allocated on the thd memroot.
      This patch appears to be working fine, it frees thd memroot after closing all opened tables:

      --- a/sql/rpl_rli.cc
      		 
      +++ b/sql/rpl_rli.cc
      		 
      @@ -2326,6 +2326,7 @@ void rpl_group_info::cleanup_context(THD *thd, bool error)
      		 
         */
      		 
         reset_row_stmt_start_timestamp();
      		 
         unset_long_find_row_note_printed();
      		 
      +  free_root(thd->mem_root, MYF(MY_KEEP_PREALLOC));
      		 
       
         DBUG_EXECUTE_IF("inject_sleep_gtid_100_x_x", {
      		 
             if (current_gtid.domain_id == 100)

      Attachments

        Issue Links

          relates to

          Bug - A problem which impairs or prevents the functions of the product. MDEV-20455 memory leak in 10.4 series

          • Critical - Crashes, loss of data, severe memory leak.
          • Closed
          mentioned in

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          Activity

            Transition Time In Source Status Execution Times
            Sergei Golubchik made transition -
            Open Confirmed
            		12h 9m 1
            Andrei Elkin made transition -
            Confirmed In Progress
            		9h 16m 1
            Andrei Elkin made transition -
            In Progress In Review
            		2m 29s 1
            Sergei Golubchik made transition -
            In Review Stalled
            		2d 4h 57m 1
            Andrei Elkin made transition -
            Stalled Closed
            		11d 21h 46m 1

            People

              Elkin Andrei Elkin
              serg Sergei Golubchik
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              2 Vote for this issue
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              Dates

                Created:
                Updated:
                Resolved:

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