MDEV-16584 SP with a cursor inside a loop wastes THD memory aggressively

Problem:

push_cursor() created sp_cursor instances on THD::main_mem_root,
which is freed only after the SP instructions loop.

Changes:
- Moving sp_cursor declaration from sp_rcontext.h to sql_class.h
- Deriving sp_instr_cpush from sp_cursor. So now sp_cursor is created
  only once (at the SP parse time) and then reused on all loop iterations
- Adding a new method reset() into sp_cursor (and its parent classes)
  to reset an sp_cursor instance before reuse.
- Moving former sp_cursor members m_fetch_count, m_row_count, m_found
  into a separate class sp_cursor_statistics. This helps to reuse
  the code in sp_cursor constructors, and in sp_cursor::reset()
- Adding a helper method sp_rcontext::pop_cursor().
- Adding "THD*" parameter to so_rcontext::pop_cursors() and pop_all_cursors()
- Removing "new" and "delete" from sp_rcontext::push_cursor() and
  sp_rconext::pop_cursor().
- Fixing sp_cursor not to derive from Sql_alloc, as it's now allocated
  only as a part of sp_instr_cpush (and not allocated separately).
- Moving lex_keeper->disable_query_cache() from sp_cursor::sp_cursor()
  to sp_instr_cpush::execute().
- Adding tests

1 files changed, 6 insertions, 4 deletions

diff --git a/sql/sp_head.cc b/sql/sp_head.cc
index 40e5a71de21..6b116cb4550 100644
--- a/sql/sp_head.cc
+++ b/sql/sp_head.cc
@@ -1406,7 +1406,7 @@ sp_head::execute(THD *thd, bool merge_da_on_success)
   /* Only pop cursors when we're done with group aggregate running. */
   if (m_chistics.agg_type != GROUP_AGGREGATE ||
       (m_chistics.agg_type == GROUP_AGGREGATE && thd->spcont->quit_func))
-    thd->spcont->pop_all_cursors(); // To avoid memory leaks after an error
+    thd->spcont->pop_all_cursors(thd); // To avoid memory leaks after an error
 
   /* Restore all saved */
   if (m_chistics.agg_type == GROUP_AGGREGATE)
@@ -4186,11 +4186,13 @@ sp_instr_cpush::execute(THD *thd, uint *nextp)
 {
   DBUG_ENTER("sp_instr_cpush::execute");
 
-  int ret= thd->spcont->push_cursor(thd, &m_lex_keeper);
+  sp_cursor::reset(thd, &m_lex_keeper);
+  m_lex_keeper.disable_query_cache();
+  thd->spcont->push_cursor(this);
 
   *nextp= m_ip+1;
 
-  DBUG_RETURN(ret);
+  DBUG_RETURN(false);
 }
 
 
@@ -4224,7 +4226,7 @@ int
 sp_instr_cpop::execute(THD *thd, uint *nextp)
 {
   DBUG_ENTER("sp_instr_cpop::execute");
-  thd->spcont->pop_cursors(m_count);
+  thd->spcont->pop_cursors(thd, m_count);
   *nextp= m_ip+1;
   DBUG_RETURN(0);
 }