This warning should appear only in debug version and as it doesn't occur any corruption the msg in misleading. Below is the test case. As the history list length limit is 2000000, we have apply patch to mysqld and change it to 0x1234 = 4660 so we can test it bit faster.

Step:1 Create table like "CREATE TABLE t ENGINE=InnoDB SELECT 1 a;”
Step:2 Start session 1 with `START TRANSACTION WITH CONSISTENT SNAPSHOT;`
Step:3 Run update for more than 30K like,
[nil@centos68 ~]$ yes 'UPDATE t SET a=a+1;'|mysql -uroot -pmsandbox --socket=/tmp/mysql_sandbox10212.sock test
check history list length with this command.
mysql -uroot -pmsandbox --socket=/tmp/mysql_sandbox10212.sock -e "SHOW ENGINE INNODB STATUS \G" | grep "History list length"

Step:4 After 30K update, start one more session 2 with `START TRANSACTION WITH CONSISTENT SNAPSHOT;`
Step:5 Run commit on session 1
Step:6 Run commit on session 2

Got warnings in error log like,

2018-01-10 20:23:13 140297300334336 [Warning] InnoDB: Purge reached the head of the history list, but its length is still reported as 22612! Make a detailed bug report, and submit it to https://jira.mariadb.org/

2018-01-10 20:23:13 140297300334336 [Warning] InnoDB: Purge reached the head of the history list, but its length is still reported as 22612! Make a detailed bug report, and submit it to https://jira.mariadb.org/

2018-01-10 20:23:13 140297300334336 [Warning] InnoDB: Purge reached the head of the history list, but its length is still reported as 22612! Make a detailed bug report, and submit it to https://jira.mariadb.org/

2018-01-10 20:23:13 140297300334336 [Warning] InnoDB: Purge reached the head of the history list, but its length is still reported as 22612! Make a detailed bug report, and submit it to https://jira.mariadb.org/