MDEV-21665: Server crashes in my_qsort2 / Filesort_buffer::sort_buffer

Allocation should use ALIGN_SIZE when we need to make sure that we
have atleast enough space to store one record in MERGEBUFF2 buffers

mysql-test/main/order_by.result

@@ -3466,3 +3466,35 @@ Note	1003	select `test`.`t4`.`a` AS `a`,`test`.`t4`.`b` AS `b`,`test`.`t4`.`c` A
 set histogram_size=@tmp_h, histogram_type=@tmp_ht, use_stat_tables=@tmp_u,
 optimizer_use_condition_selectivity=@tmp_o;
 drop table t1,t2,t3,t4;
+#
+# MDEV-21655: Server crashes in my_qsort2 / Filesort_buffer::sort_buffer
+#
+set @save_sql_mode= @@sql_mode;
+set sql_mode= 'PAD_CHAR_TO_FULL_LENGTH';
+CREATE TABLE t1 ( a CHAR(255) charset utf8,
+b CHAR(255) charset utf8, c TEXT);
+INSERT INTO t1 VALUES
+('1','a', 'a'), ('2','b', 'b'), ('3','c', 'c'), ('4','d','d'),
+('5','e', 'e'), ('6','f', 'f'), ('7','g','g'), ('8','h','h'),
+('9','i', 'i'), ('10','j','j'), ('11','k','k'), ('12','l','l'),
+('13','m','m'), ('14','n','n'), ('15','o','o');
+set sort_buffer_size=517*30;
+select c from t1 order by a,b;
+c
+a
+j
+k
+l
+m
+n
+o
+b
+c
+d
+e
+f
+g
+h
+i
+set sql_mode= @save_sql_mode;
+drop table t1;

mysql-test/main/order_by.test

@@ -2292,3 +2292,24 @@ set histogram_size=@tmp_h, histogram_type=@tmp_ht, use_stat_tables=@tmp_u,
     optimizer_use_condition_selectivity=@tmp_o; 
 
 drop table t1,t2,t3,t4;
+
+--echo #
+--echo # MDEV-21655: Server crashes in my_qsort2 / Filesort_buffer::sort_buffer
+--echo #
+
+set @save_sql_mode= @@sql_mode;
+set sql_mode= 'PAD_CHAR_TO_FULL_LENGTH';
+CREATE TABLE t1 ( a CHAR(255) charset utf8,
+                  b CHAR(255) charset utf8, c TEXT);
+INSERT INTO t1 VALUES
+('1','a', 'a'), ('2','b', 'b'), ('3','c', 'c'), ('4','d','d'),
+('5','e', 'e'), ('6','f', 'f'), ('7','g','g'), ('8','h','h'),
+('9','i', 'i'), ('10','j','j'), ('11','k','k'), ('12','l','l'),
+('13','m','m'), ('14','n','n'), ('15','o','o');
+
+set sort_buffer_size=517*30;
+select c from t1 order by a,b;
+
+set sql_mode= @save_sql_mode;
+
+drop table t1;

sql/filesort_utils.cc

@@ -111,7 +111,8 @@ uchar *Filesort_buffer::alloc_sort_buffer(uint num_records,
     one key.
     TODO varun: move this to the place where min_sort_memory is used.
   */
-  set_if_bigger(buff_size, (record_length +sizeof(uchar*)) * MERGEBUFF2);
+  set_if_bigger(buff_size,
+               ALIGN_SIZE((record_length +sizeof(uchar*)) * MERGEBUFF2));
 
   if (m_rawmem)
   {