It seems that FLUSH PRIVILEGES can cause simultaeous connection attempts to fail.

We noticed this in a Kubernetes cluster with mariadb-operator, which frequently calls FLUSH PRIVILEGES after reconciling users/grants. This causes sporadic connection failures on all clients (mariadb-operator itself, the metrics pod spawned by the operator, and our PHP app).

The problem is however not limited to mariadb-operator (see MRE below).

The server side log message is:

[Warning] Aborted connection <conn id> to db: 'unconnected' user: 'unauthenticated' host: '<ip address>' (This connection closed normally without authentication)

The PHP client error message is:

Fatal error: Uncaught mysqli_sql_exception: Host '<ip address>' is not allowed to connect to this MariaDB server

MRE in a self-contained docker compose file:

services:

  db:

    image: mariadb:11.4

    entrypoint: >

      bash -c '

      docker-entrypoint.sh --character-set-server=utf8mb4 & 

      sleep 10 ;

      for (( n=0; n<10000; n++ )) ; do mariadb -u root -e "FLUSH PRIVILEGES;" ; done

      '

    environment:

      - MARIADB_DATABASE=foo

      - MARIADB_ALLOW_EMPTY_ROOT_PASSWORD=true

  php:

    image: php:8.4

    entrypoint: >

      bash -c "

      docker-php-ext-install mysqli ;

      sleep 5 ;

      for (( n=0; n<10000; n++ )) ; do php -r $'mysqli_connect(\\'db\\', \\'root\\', \\'\\', \\'foo\\');' ; done

      "

How to run: save as compose.yaml, then run docker compose up in the same directory
What it does: runs a mariadb container, which runs FLUSH PRIVILEGES 10,000 times, and a php container, which tries to connect to the mariadb 10,000 times.
What I expect: no output ( = no errors)
What I see instead: sporadic connection failures, for example:

php-1  | Warning: mysqli_connect(): (HY000/1130): Host '172.22.0.2' is not allowed to connect to this MariaDB server in Command line code on line 1

db-1   | 2024-12-16 21:49:01 1124 [Warning] Aborted connection 1124 to db: 'unconnected' user: 'unauthenticated' host: '172.22.0.2' (This connection closed normally without authentication)

I have tested this with previous MariaDB image versions back to 10.2 (and PHP versions back to 7.0), all of which exhibit the same problem.
As the mariadb-operator is written in go and runs into the same problem, I'm confident that this is not a PHP issue.

I think these mariadb-operator issues are caused by this bug:

https://github.com/mariadb-operator/mariadb-operator/issues/686
https://github.com/mariadb-operator/mariadb-operator/issues/1005