MariaDB 10.2 and newer returns inconsistent results when query caching is enabled (query_cache_type=ON).

Steps to reproduce

1. Start a MariaDB 10.2+ database with default settings.

For example by running a docker container on port 3500:

shell> docker run --name mariadb-10.2.14 -p 3500:3306 -e MYSQL_ROOT_PASSWORD=test -e MYSQL_USER=test -e MYSQL_PASSWORD=test -e MYSQL_DATABASE=test -d mariadb:10.2.14

2. Run the attached mysqltest case MDEV-16087.mysqltest

shell> mysqltest -h 127.0.0.1 --port=3500 --user=test --password=test --result-file=MDEV-16087.record test < MDEV-16087.mysqltest

It fails with:

--- MDEV-16087.record	2018-05-15 15:42:11.495372790 +0200

+++ MDEV-16087.reject	2018-05-15 15:52:24.547984967 +0200

@@ -18,7 +18,7 @@

 1

 select count(*) from table3;

 count(*)

-1

+0

 select sql_no_cache count(*) from table3;

 count(*)

 1

mysqltest: Result content mismatch

3. Disable query caching and re-run the mysqltest case:

shell> echo "set global query_cache_type = off;" | mysql -h 127.0.0.1 --port=3500 --user=root --password=test

shell> mysqltest -h 127.0.0.1 --port=3500 --user=test --password=test --result-file=MDEV-16087.record test < MDEV-16087.mysqltest

Mysqltest succeeds.

Environment

• Ubuntu
• MariaDB 10.2.14
• Query cache settings:

  MariaDB [(none)]> show variables like '%query_cache%';

  +------------------------------+----------+

  | Variable_name | Value |

  +------------------------------+----------+

  | have_query_cache | YES |

  | query_cache_limit | 131072 |

  | query_cache_min_res_unit | 4096 |

  | query_cache_size | 67108864 |

  | query_cache_strip_comments | OFF |

  | query_cache_type | ON |

  | query_cache_wlock_invalidate | OFF |

  +------------------------------+----------+