rMWa4da386e6a8a: installer: Check MariaDB version in updater/installer was new in 1.41 for T237898: Add MariaDB version check in wikimedia/rdbms (separate from MySQL) . Not guaranteed to be at fault, but seems possibly related.

What is the full string your setup gives for SELECT VERSION(); or similar?

It wouldn't surprise me if the checking now looking for MariaDB is giving a false positive or something here..

	public static function meetsMinimumRequirement( IDatabase $conn ) {
		$type = str_contains( $conn->getSoftwareLink(), 'MariaDB' ) ? 'MariaDB' : 'MySQL';
		self::$minimumVersion = self::MIN_VERSIONS[$type];
		// Used messages: config-mysql-old, config-mariadb-old
		self::$notMinimumVersionMessage = 'config-' . strtolower( $type ) . '-old';
		return parent::meetsMinimumRequirement( $conn );
	}

https://gerrit.wikimedia.org/r/q/Ie4853e90a7c8ea655feff2b7bd6402d587c08d50 was backported to older releases, so I think if that would have been the cause, I would have noticed it on an earlier version.

So rMWebadcf1cca74: rdbms: Avoid making a db query when getting MySQL/MariaDB version seems possibly relevant then, as it wasn't backported.

Per the extended commit message

To get the version, you don't actually need to call "SELECT VERSION()"

The PDO provides that value for free and from reading the source code,
It's provided during the handshake:
https://github.com/php/php-src/blob/cb927e0fc043cb454ee58b7485cbce191df5d512/ext/mysqlnd/mysqlnd.h#L136

Which would explain the behaviour, as Server version: 5.6.47.0 MariaDB Server is in the connection header you posted.

If the proxy is returning a different version and still calling it "Server version"... Is this necessarily a MW bug? Is the proxy supposed to be changing that version? I could understand it if it was inserting its own prefix/suffix, or even a "Proxy version"... Or is the MySQL client somewhere picking that up incorrectly from some header it is presented with?

https://learn.microsoft.com/en-us/azure/mysql/single-server/concepts-connect-to-a-gateway-node

In the Single Server deployment option, a gateway is used to redirect the connections to server instances. After the connection is established, the MySQL client displays the version of MySQL set in the gateway, not the actual version running on your MySQL server instance. To determine the version of your MySQL server instance, use the SELECT VERSION(); command at the MySQL prompt.

It's working as MS intends, sure..

As Azure Database for MySQL supports major version v5.7 and v8.0, the default port 3306 to connect to Azure Database for MySQL runs MySQL client version 5.6 (least common denominator) to support connections to servers of all 2 supported major versions. However, if your application has a requirement to connect to specific major version say v5.7 or v8.0, you can do so by changing the port in your server connection string.

In Azure Database for MySQL Service, gateway nodes listens on port 3308 for v5.7 clients and port 3309 for v8.0 clients. In other words, if you would like to connect to v5.7 gateway client, you should use your fully qualified server name and port 3308 to connect to your server from client application. Similarly, if you would like to connect to v8.0 gateway client, you can use your fully qualified server name and port 3309 to connect to your server. Check the following example for further clarity.

I’ve an idea for a fallback/partial revert. Will post a patch later