pt-online-schema change eats data on adding a unique index

Setup
=====

DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (
id tinyint unsigned auto_increment primary key,
notunique varchar(200) NOT NULL
);
INSERT INTO t1(notunique) VALUES('test01'),('test01'),('test02');
SELECT * FROM t1;

Action
=====

pt-online-schema-change --alter="ADD UNIQUE INDEX unique_1 (notunique)" --statistics --execute h=127.0.0.1,P=5711,D=test,t=t1,u=msandbox,p=msandbox

Expected/Desired result
===================

The osc aborts because the data for the unique index is not unique

Actual result
==========

On MySQL 5.6 data with duplicate entries are dropped silently
On MySQL 5.7 data with duplicate entries are dropped with a warning 1062 in the stats (probably also for MariaDB 5.5+)

Related
======

https://bugs.launchpad.net/percona-toolkit/+bug/1099836

Suggested fix
===========

• Abort on warning 1062 if possible and/or require a --force-unique flag when adding adding an unique index.
• Don't swap/drop if the original table has less rows than the original table (count inserts? stats?)

Versions
=======

• MySQL 5.7.11
• pt-online-schema-change 2.2.16 (git b1ca211)

The cause of the unexpected behavior is that pt-osc uses INSERT LOW_PRIORITY IGNORE to copy chunks, which only raises a warning instead of an error when there is a UNIQUE constraint violation (Percona and Oracle 5.6.35) so pt-osc silently ignores those rows.

example

1. pt_online_schema_change:10906 41234 INSERT LOW_PRIORITY IGNORE INTO `xy`.`_table_new` () SELECT ) WHERE ((`user_id` >= ?)) AND ((`user_id` <= ?)) LOCK IN SHARE MODE /pt-online-schema-change 41234 copy nibble/ lower boundary: 18757143 upper boundary: 18773685
2. pt_online_schema_change:10925 41234 SHOW WARNINGS

Launchpad: https://bugs.launchpad.net/percona-toolkit/+bug/1545129