MDEV-12136 SELECT COUNT(DISTINCT) returns the wrong value when tmp_table_size is limited

Same MDEV, second bug.
Merge buffer must fit at least MERGEBUFF2 (that is, 15) key values.
Because merge_index() can merge that many buffers, and
merge_many_buff() leaves that many buffers unmerged.

mysql-test/r/count_distinct.result

@@ -100,5 +100,9 @@ set @@tmp_table_size = 1024;
 select count(distinct user_id) from t1;
 count(distinct user_id)
 17
+alter table t1 modify user_id char(128) character set utf8;
+select count(distinct user_id) from t1;
+count(distinct user_id)
+17
 drop table t1;
 set @@tmp_table_size = default;

mysql-test/t/count_distinct.test

@@ -115,6 +115,8 @@ create table t1 (user_id char(64) character set utf8);
 insert t1 values(1),(2),(3),(4),(5),(6),(7),(8),(9),(10),(11),(12),(13),(14),(15),(16),(17);
 set @@tmp_table_size = 1024;
 select count(distinct user_id) from t1;
+alter table t1 modify user_id char(128) character set utf8;
+select count(distinct user_id) from t1;
 drop table t1;
 set @@tmp_table_size = default;
 

sql/uniques.cc

@@ -607,11 +607,15 @@ bool Unique::walk(TABLE *table, tree_walk_action action, void *walk_action_arg)
     return 1;
   if (flush_io_cache(&file) || reinit_io_cache(&file, READ_CACHE, 0L, 0, 0))
     return 1;
-  size_t buff_sz= (max_in_memory_size / full_size + 1) * full_size;
+  /*
+    merge_buffer must fit at least MERGEBUFF2 keys, because
+    merge_index() can merge that many BUFFPEKs at once.
+  */
+  size_t buff_sz= max(MERGEBUFF2, max_in_memory_size/full_size+1) * full_size;
   if (!(merge_buffer = (uchar *)my_malloc(buff_sz, MYF(MY_WME))))
     return 1;
   if (buff_sz < (ulong) (full_size * (file_ptrs.elements + 1)))
-    res= merge(table, merge_buffer, buff_sz >= full_size * MERGEBUFF2) ;
+    res= merge(table, merge_buffer, true) ;
   
   if (!res)
   {