MDEV-7219 SQL_CALC_FOUND_ROWS yields wrong result

revert the code in filesort that conditionally updated 'found_rows',
rely on filesort_limit_arg instead.

mysql-test/r/select_found.result

@@ -332,3 +332,19 @@ select found_rows() as count;
 count
 2
 drop table t1, t2;
+create table t1 (i int, v varchar(64), key (i));
+select sql_calc_found_rows * from t1 where i = 0 order by v limit 59,2;
+i	v
+0	foo
+0	foo
+select found_rows();
+found_rows()
+75
+select sql_calc_found_rows * from t1 ignore index (i) where i = 0 order by v limit 59,2;
+i	v
+0	foo
+0	foo
+select found_rows();
+found_rows()
+75
+drop table t1;

mysql-test/t/select_found.test

@@ -257,3 +257,23 @@ select sql_calc_found_rows 1 as res from t1 left join t2 on i1 = i2 where v2 = 5
 select found_rows() as count;
 drop table t1, t2;
 
+#
+# MDEV-7219 SQL_CALC_FOUND_ROWS yields wrong result
+#
+create table t1 (i int, v varchar(64), key (i));
+
+--disable_query_log
+let $1=150;
+while ($1) 
+{
+  eval insert into t1 values ($1 % 2, 'foo');
+  dec $1;
+}
+--enable_query_log
+
+select sql_calc_found_rows * from t1 where i = 0 order by v limit 59,2;
+select found_rows();
+select sql_calc_found_rows * from t1 ignore index (i) where i = 0 order by v limit 59,2;
+select found_rows();
+drop table t1;
+

sql/filesort.cc

@@ -166,8 +166,6 @@ ha_rows filesort(THD *thd, TABLE *table, SORT_FIELD *sortorder, uint s_length,
   TABLE_LIST *tab= table->pos_in_table_list;
   Item_subselect *subselect= tab ? tab->containing_subselect() : 0;
 
-  *found_rows= HA_POS_ERROR;
-
   MYSQL_FILESORT_START(table->s->db.str, table->s->table_name.str);
   DEBUG_SYNC(thd, "filesort_start");
 
@@ -190,6 +188,7 @@ ha_rows filesort(THD *thd, TABLE *table, SORT_FIELD *sortorder, uint s_length,
   my_b_clear(&buffpek_pointers);
   buffpek=0;
   error= 1;
+  *found_rows= HA_POS_ERROR;
 
   param.init_for_filesort(sortlength(thd, sortorder, s_length,
                                      &multi_byte_charset),
@@ -690,7 +689,6 @@ static ha_rows find_all_keys(Sort_param *param, SQL_SELECT *select,
   ref_pos= ref_buff;
   quick_select=select && select->quick;
   record=0;
-  if (pq) // don't count unless pq is used
   *found_rows= 0;
   flag= ((file->ha_table_flags() & HA_REC_NOT_IN_SEQ) || quick_select);
   if (flag)
@@ -814,14 +812,9 @@ static ha_rows find_all_keys(Sort_param *param, SQL_SELECT *select,
 
     if (write_record)
     {
+       ++(*found_rows);
       if (pq)
       {
-        /*
-          only count rows when pq is used - otherwise there might be
-          other filters *after* the filesort, we don't know the final row
-          count here
-        */
-        (*found_rows)++;
         pq->push(ref_pos);
         idx= pq->num_elements();
       }

sql/sql_select.cc

@@ -3032,6 +3032,7 @@ void JOIN::exec_inner()
       const ha_rows select_limit_arg=
         select_options & OPTION_FOUND_ROWS
         ? HA_POS_ERROR : unit->select_limit_cnt;
+      curr_join->filesort_found_rows= filesort_limit_arg != HA_POS_ERROR;
 
       DBUG_PRINT("info", ("has_group_by %d "
                           "curr_join->table_count %d "
@@ -3079,7 +3080,8 @@ void JOIN::exec_inner()
                                    Protocol::SEND_NUM_ROWS | Protocol::SEND_EOF);
   error= do_select(curr_join, curr_fields_list, NULL, procedure);
   thd->limit_found_rows= curr_join->send_records;
-  if (curr_join->order && curr_join->filesort_found_rows)
+  if (curr_join->order && curr_join->sortorder &&
+      curr_join->filesort_found_rows)
   {
     /* Use info provided by filesort. */
     DBUG_ASSERT(curr_join->table_count > curr_join->const_tables);
@@ -18900,7 +18902,8 @@ end_send(JOIN *join, JOIN_TAB *join_tab __attribute__((unused)),
         records are read. Because of optimization in some cases it can
         provide only select_limit_cnt+1 records.
       */
-      if (join->order && join->filesort_found_rows &&
+      if (join->order && join->sortorder &&
+          join->filesort_found_rows &&
           join->select_options & OPTION_FOUND_ROWS)
       {
         DBUG_PRINT("info", ("filesort NESTED_LOOP_QUERY_LIMIT"));
@@ -18923,7 +18926,8 @@ end_send(JOIN *join, JOIN_TAB *join_tab __attribute__((unused)),
 	  TABLE *table=jt->table;
 
 	  join->select_options ^= OPTION_FOUND_ROWS;
-          if (join->filesort_found_rows)
+	  if (table->sort.record_pointers ||
+	      (table->sort.io_cache && my_b_inited(table->sort.io_cache)))
 	  {
 	    /* Using filesort */
 	    join->send_records= table->sort.found_records;
@@ -20754,11 +20758,7 @@ create_sort_index(THD *thd, JOIN *join, ORDER *order,
                             select, filesort_limit, 0,
                             &examined_rows, &found_rows);
   table->sort.found_records= filesort_retval;
-  if (found_rows != HA_POS_ERROR)
-  {
   tab->records= found_rows;                     // For SQL_CALC_ROWS
-    join->filesort_found_rows= true;
-  }
 
   if (quick_created)
   {

sql/sql_select.h

@@ -1341,7 +1341,6 @@ public:
     emb_sjm_nest= NULL;
     sjm_lookup_tables= 0;
 
-    filesort_found_rows= false;
     exec_saved_explain= false;
     /* 
       The following is needed because JOIN::cleanup(true) may be called for