Can you rollback a query in state 'committing alter table to storage engine' We've got an InnoDB table with 70 million rows, and we have been trying to run an alter table statement to modify and add a couple of columns. The query seems to have altered the table, and is now in the state of 'committing alter table to storage engine'. 

START TRANSACTION;
ALTER TABLE table
  MODIFY COLUMN column1 int(11) NOT NULL DEFAULT 0,
  MODIFY COLUMN column2 tinyint(1) NOT NULL DEFAULT 1,
  ADD COLUMN column3 int(11),
  ADD COLUMN column4 int(11) NOT NULL DEFAULT 1,
  ADD COLUMN column5 varchar(255);
COMMIT;

This has been running overnight, and is at 19 hours at the current time. We do not have the performance schema enabled so cannot look at an estimated time of completion. My concern lies as to what the query is doing and whether the query will rollback if killed. I have seen other questions relate to queries that are stuck in copying to tmp tables, or awaiting a table lock. However I cannot find anything about being stuck while the alter table is committing.

Is it safe to kill a query in this state, and if the query is killed, will it rollback successfully?

The server is running MariaDB 10.2
I am sorry to have to inform you, but ALTER TABLE cannot be rolled back. In fact, ALTER TABLE triggers an implicit commit.

According to Page 418 Paragraph 3 of the Book



the following commands can and will trigger an implicit commit


ALTER TABLE
BEGIN
CREATE INDEX
DROP DATABASE
DROP INDEX
DROP TABLE
RENAME TABLE
TRUNCATE TABLE
LOCK TABLES
UNLOCK TABLES
SET AUTOCOMMIT = 1
START TRANSACTION


I have mentioned this before


Aug 21, 2015 : Transactional DDL workflow for MySQL
Feb 12, 2014 : row locking within ACID transaction innodb
Mar 15, 2013 : MySQL backup InnoDB


In your case, the ALTER TABLE will either finish or you will have a temp table left hanging around. In either case, it will not be a rollback in a transactional sense.