MDEV-21540 Initialization of already inited long unique index on reorganize partition

Handler for existing partition was already index-inited at the
beginning of copy_partitions().

In the case of REORGANIZE PARTITION we fill new partition by calling
its ha_write_row() (handler is storage engine of new partition). From
that we go through the below conditions:

    if (this->inited == RND)
      table->clone_handler_for_update();
    handler *h= table->update_handler ? table->update_handler : table->file;

First, the above misses the meaning of this->inited check. Now it is
new partition and this handler is not inited. So, we assign
table->file which is ha_partition and is really not known to be inited
or not. It is supposed (this == table->file), otherwise we are
out of the logic for using update_handler. This patch adds DBUG_ASSERT
for that.

Second, we call check_duplicate_long_entries() for table->file and
that calls ha_partition::index_init() which calls index_init() for
each partition's handler. But the existing parititions' handlers was
already inited in copy_partitions() and we fail on assertion.

The fix implies that we don't need check_duplicate_long_entries()
per-partition as we've already done check_duplicate_long_entries() for
ha_partition. For REORGANIZE PARTITION that means existing row was
already checked at previous INSERT/UPDATE commands, so no need to
check it again (see NOTE in handler::ha_write_row()).

The fix also optimizes ha_update_row() so
check_duplicate_long_entries_update() is not called per-partition
considering it was already called for ha_partition. Besides,
per-partition duplicate check is not really usable.

mysql-test/main/long_unique_bugs.result

@@ -315,5 +315,24 @@ update t1,t2 set v1 = v2 , v5 = 0;
 ERROR 23000: Duplicate entry '-128' for key 'v1'
 drop table t1, t2;
 #
+# MDEV-21540 Initialization of already inited long unique index on reorganize partition
+#
+create table t1 (x int, a blob)
+partition by range (x) (
+partition p1 values less than (50),
+partition pn values less than maxvalue);
+insert into t1 values (1, 1), (100, 1);
+alter table t1 add unique key (a);
+ERROR 23000: Duplicate entry '1' for key 'a'
+update t1 set a= x;
+alter table t1 add unique key (a);
+update t1 set a= 1;
+ERROR 23000: Duplicate entry '1' for key 'a'
+update t1 set a= x + 1;
+alter table t1 reorganize partition p1 into (
+partition n0 values less than (10),
+partition n1 values less than (50));
+drop table t1;
+#
 # End of 10.4 tests
 #

mysql-test/main/long_unique_bugs.test

@@ -396,6 +396,33 @@ update t1 set v2 = 1, v3 = -128;
 update t1,t2 set v1 = v2 , v5 = 0;
 drop table t1, t2;
 
+--echo #
+--echo # MDEV-21540 Initialization of already inited long unique index on reorganize partition
+--echo #
+create table t1 (x int, a blob)
+partition by range (x) (
+  partition p1 values less than (50),
+  partition pn values less than maxvalue);
+
+insert into t1 values (1, 1), (100, 1);
+
+# a little bit of additional checks
+--error ER_DUP_ENTRY
+alter table t1 add unique key (a);
+
+update t1 set a= x;
+alter table t1 add unique key (a);
+--error ER_DUP_ENTRY
+update t1 set a= 1;
+update t1 set a= x + 1;
+
+# bug failure
+alter table t1 reorganize partition p1 into (
+  partition n0 values less than (10),
+  partition n1 values less than (50));
+
+drop table t1;
+
 --echo #
 --echo # End of 10.4 tests
 --echo #

sql/handler.cc

@@ -6782,7 +6782,18 @@ int handler::ha_write_row(const uchar *buf)
   mark_trx_read_write();
   increment_statistics(&SSV::ha_write_count);
 
-  if (table->s->long_unique_table)
+  /*
+    NOTE: this != table->file is true in 3 cases:
+
+    1. under copy_partitions() (REORGANIZE PARTITION): that does not
+       require long unique check as it does not introduce new rows or new index.
+    2. under partition's ha_write_row() (INSERT): check_duplicate_long_entries()
+       was already done by ha_partition::ha_write_row(), no need to check it
+       again for each single partition.
+    3. under ha_mroonga::wrapper_write_row()
+  */
+
+  if (table->s->long_unique_table && this == table->file)
   {
     if (this->inited == RND)
       table->clone_handler_for_update();
@@ -6830,8 +6841,17 @@ int handler::ha_update_row(const uchar *old_data, const uchar *new_data)
   MYSQL_UPDATE_ROW_START(table_share->db.str, table_share->table_name.str);
   mark_trx_read_write();
   increment_statistics(&SSV::ha_update_count);
-  if (table->s->long_unique_table &&
-          (error= check_duplicate_long_entries_update(table, table->file, (uchar *)new_data)))
+
+  /*
+    NOTE: this != table->file is true under partition's ha_update_row():
+    check_duplicate_long_entries_update() was already done by
+    ha_partition::ha_update_row(), no need to check it again for each single
+    partition. Same applies to ha_mroonga wrapper.
+  */
+
+  if (table->s->long_unique_table && this == table->file &&
+      (error= check_duplicate_long_entries_update(table, table->file,
+                                                  (uchar *)new_data)))
   {
     return error;
   }