MDEV-22830: SQL_CALC_FOUND_ROWS not working properly for single SELECT for DUAL

In case of SELECT without tables which returns either 0 or 1 rows,
JOIN::exec_inner() did not check if the flag representing SQL_CALC_FOUND_ROWS
is set or not and send_records was direclty assigned 0. So SELECT FOUND_ROWS()
was giving 0 in the output. Now it checks if the flag is set, if it is set
send_record=1 else 0. 1 is the number of rows that could have been sent
to the client if the SELECT query had SQL_CALC_FOUND_ROWS.
It is 0 when no rows were sent because the SELECT query did not have
SQL_CALC_FOUND_ROWS.

mysql-test/r/select_found.result

@@ -362,4 +362,9 @@ c1
 select found_rows();
 found_rows()
 5
+SELECT SQL_CALC_FOUND_ROWS 1 FROM DUAL WHERE 0;
+1
+SELECT FOUND_ROWS();
+FOUND_ROWS()
+1
 drop table t1;

mysql-test/t/select_found.test

@@ -286,4 +286,11 @@ select * from t1 order by c1 limit 2,1;
 select found_rows();
 select sql_calc_found_rows * from t1 order by c1 limit 2,1;
 select found_rows();
+
+#
+# MDEV-22830 SQL_CALC_FOUND_ROWS not working properly for single SELECT for DUAL
+#
+SELECT SQL_CALC_FOUND_ROWS 1 FROM DUAL WHERE 0;
+SELECT FOUND_ROWS();
+
 drop table t1;

sql/sql_select.cc

@@ -2647,7 +2647,7 @@ void JOIN::exec_inner()
                          thd->get_sent_row_count());
       }
       else
-        send_records= 0;
+        send_records= (select_options & OPTION_FOUND_ROWS) ? 1 : 0;
       if (!error)
       {
         join_free();                      // Unlock all cursors