MDEV-18889 Long unique on virtual fields crashes server

Use table->record[0] for ha_index_read_map so that vfield gets automatically
be updated.

mysql-test/main/long_unique_bugs.result

@@ -158,3 +158,16 @@ insert into t1 values(1,1);
 ERROR 23000: Duplicate entry '1-1' for key 'a'
 alter table t1 add column c int;
 drop table t1;
+create table t1(a blob , b blob as (a) unique);
+insert into t1 values(1, default);
+insert into t1 values(1, default);
+ERROR 23000: Duplicate entry '1' for key 'b'
+drop table t1;
+create table t1(a blob, b blob, c blob as (left(a, 5000)) virtual, d blob as (left(b, 5000)) persistent, unique(a,b(4000)));
+insert into t1(a,b) values(10,11);
+insert into t1(a,b) values(10,11);
+ERROR 23000: Duplicate entry '10-11' for key 'a'
+insert into t1(a,b) values(2,2);
+insert into t1(a,b) values(2,3);
+insert into t1(a,b) values(3,2);
+drop table t1;

mysql-test/main/long_unique_bugs.test

@@ -178,3 +178,21 @@ insert into t1 values(1,1);
 insert into t1 values(1,1);
 alter table t1 add column c int;
 drop table t1;
+
+#
+# MDEV-18889 Long unique on virtual fields crashes server
+#
+create table t1(a blob , b blob as (a) unique);
+insert into t1 values(1, default);
+--error ER_DUP_ENTRY
+insert into t1 values(1, default);
+drop table t1;
+
+create table t1(a blob, b blob, c blob as (left(a, 5000)) virtual, d blob as (left(b, 5000)) persistent, unique(a,b(4000)));
+insert into t1(a,b) values(10,11);
+--error ER_DUP_ENTRY
+insert into t1(a,b) values(10,11);
+insert into t1(a,b) values(2,2);
+insert into t1(a,b) values(2,3);
+insert into t1(a,b) values(3,2);
+drop table t1;

sql/handler.cc

@@ -6547,7 +6547,8 @@ static int check_duplicate_long_entry_key(TABLE *table, handler *h,
   result= h->ha_index_init(key_no, 0);
   if (result)
     return result;
-  result= h->ha_index_read_map(table->check_unique_buf,
+  store_record(table, check_unique_buf);
+  result= h->ha_index_read_map(table->record[0],
                                ptr, HA_WHOLE_KEY, HA_READ_KEY_EXACT);
   if (!result)
   {
@@ -6583,7 +6584,7 @@ static int check_duplicate_long_entry_key(TABLE *table, handler *h,
         }
       }
     }
-    while (!is_same && !(result= h->ha_index_next_same(table->check_unique_buf,
+    while (!is_same && !(result= h->ha_index_next_same(table->record[0],
                          ptr, key_info->key_length)));
     if (is_same)
       error= HA_ERR_FOUND_DUPP_KEY;
@@ -6597,10 +6598,11 @@ static int check_duplicate_long_entry_key(TABLE *table, handler *h,
     table->file->errkey= key_no;
     if (h->ha_table_flags() & HA_DUPLICATE_POS)
     {
-      h->position(table->check_unique_buf);
+      h->position(table->record[0]);
       memcpy(table->file->dup_ref, h->ref, h->ref_length);
     }
   }
+  restore_record(table, check_unique_buf);
   h->ha_index_end();
   return error;
 }