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

sql/sp_rcontext.cc

@@ -425,28 +425,26 @@ bool sp_rcontext::set_return_value(THD *thd, Item **return_value_item)
 }
 
 
-bool sp_rcontext::push_cursor(THD *thd, sp_lex_keeper *lex_keeper)
+void sp_rcontext::push_cursor(sp_cursor *c)
 {
-  /*
-    We should create cursors in the callers arena, as
-    it could be (and usually is) used in several instructions.
-  */
-  sp_cursor *c= new (callers_arena->mem_root) sp_cursor(thd, lex_keeper);
-
-  if (c == NULL)
-    return true;
-
   m_cstack[m_ccount++]= c;
-  return false;
 }
 
 
-void sp_rcontext::pop_cursors(size_t count)
+void sp_rcontext::pop_cursor(THD *thd)
+{
+  DBUG_ASSERT(m_ccount > 0);
+  if (m_cstack[m_ccount - 1]->is_open())
+    m_cstack[m_ccount - 1]->close(thd);
+  m_ccount--;
+}
+
+
+void sp_rcontext::pop_cursors(THD *thd, size_t count)
 {
   DBUG_ASSERT(m_ccount >= count);
-
   while (count--)
-    delete m_cstack[--m_ccount];
+    pop_cursor(thd);
 }
 
 
@@ -733,22 +731,6 @@ bool sp_rcontext::set_case_expr(THD *thd, int case_expr_id,
 ///////////////////////////////////////////////////////////////////////////
 
 
-sp_cursor::sp_cursor(THD *thd_arg, sp_lex_keeper *lex_keeper):
-   result(thd_arg),
-   m_lex_keeper(lex_keeper),
-   server_side_cursor(NULL),
-   m_fetch_count(0),
-   m_row_count(0),
-   m_found(false)
-{
-  /*
-    currsor can't be stored in QC, so we should prevent opening QC for
-    try to write results which are absent.
-  */
-  lex_keeper->disable_query_cache();
-}
-
-
 /*
   Open an SP cursor
 
@@ -811,8 +793,7 @@ int sp_cursor::close(THD *thd)
                MYF(0));
     return -1;
   }
-  m_row_count= m_fetch_count= 0;
-  m_found= false;
+  sp_cursor_statistics::reset();
   destroy();
   return 0;
 }