cleanup: Select_limit_counters rename set_unlimited to clear

The function was originally introduced by eb0804e
MDEV-18553: MDEV-16327 pre-requisits part 1: isolation of LIMIT/OFFSET handling

set_unlimited had an overloaded notion of both clearing the offset value
and the limit value. The code is used for SQL_CALC_ROWS option to
disable the limit clause after the limit is reached, while at the same
time the calling code suppreses sending of rows.

Proposed solution:
Dedicated clear method for query initialization (to ensure no garbage
remains between executions).
Dedicated set_unlimited that only alters the limit value.

sql/sql_lex.cc

@@ -2910,7 +2910,7 @@ void st_select_lex_unit::init_query()
 {
   init_query_common();
   set_linkage(GLOBAL_OPTIONS_TYPE);
-  lim.set_unlimited();
+  lim.clear();
   union_distinct= 0;
   prepared= optimized= optimized_2= executed= 0;
   bag_set_op_optimized= 0;

sql/sql_limit.h

@@ -37,8 +37,13 @@ class Select_limit_counters
    {
       offset_limit_cnt= offset;
       select_limit_cnt= limit;
-      if (select_limit_cnt + offset_limit_cnt >=
-          select_limit_cnt)
+      /*
+        Guard against an overflow condition, where limit + offset exceede
+        ha_rows value range. This case covers unreasonably large parameter
+        values that do not have any practical use so assuming in this case
+        that the query does not have a limit is fine.
+      */
+      if (select_limit_cnt + offset_limit_cnt >= select_limit_cnt)
         select_limit_cnt+= offset_limit_cnt;
       else
         select_limit_cnt= HA_POS_ERROR;
@@ -52,7 +57,16 @@ class Select_limit_counters
 
    bool is_unlimited() const
    { return select_limit_cnt == HA_POS_ERROR; }
+   /*
+      Set the limit to allow returning an unlimited number of rows. Useful
+      for cases when we want to continue execution indefinitely after the limit
+      is reached (for example for SQL_CALC_ROWS extension).
+   */
    void set_unlimited()
+   { select_limit_cnt= HA_POS_ERROR; }
+
+   /* Reset the limit entirely. */
+   void clear()
    { select_limit_cnt= HA_POS_ERROR; offset_limit_cnt= 0; }
 
    bool check_offset(ha_rows sent) const

sql/sql_parse.cc

@@ -7761,8 +7761,8 @@ void mysql_init_multi_delete(LEX *lex)
 {
   lex->sql_command=  SQLCOM_DELETE_MULTI;
   mysql_init_select(lex);
-  lex->first_select_lex()->limit_params.select_limit= 0;
-  lex->unit.lim.set_unlimited();
+  lex->first_select_lex()->limit_params.clear();
+  lex->unit.lim.clear();
   lex->first_select_lex()->table_list.
     save_and_clear(&lex->auxiliary_table_list);
   lex->query_tables= 0;

sql/sql_union.cc

@@ -1340,7 +1340,7 @@ bool st_select_lex_unit::prepare(TABLE_LIST *derived_arg,
 	else
 	{
 	  sl->join->result= result;
-          lim.set_unlimited();
+          lim.clear();
 	  if (!sl->join->procedure &&
 	      result->prepare(sl->join->fields_list, this))
 	  {