MariaDB THREADS_CONNECTED build up with php-fpm static

one text

Solution:

"I've registered a shutdown handler with PHP that checks if my DB connection is open and produces an alert. No alerts have been sent."

Regardless of whether you're getting the alert, are you certain that this is running explicitly?

DB::disconnect('yourdatabase');

If you think you've got that covered, looking at the PDO object that Laravel is using internally may help. Instead of just debugging via an alert, which may be getting lost somewhere, logging a positive assertion that the connection is closed could be helpful.

$pdo = DB::connection()->getPdo();

// My "positive assertion" comment is because this can be deceiving:
alertThatDoesNotWork();

// Instead, with odd problems like this, I prefer to do
logStateOfDatabaseConnection();

Without more information, my best guess is related to "not guilty" !== "innocent". The log-regardless-of-state approach reduces the outputs to "innocent" and "guilty" with no ambiguity.

2022-02-09 Edit: Have you checked for a persistence setting? I think this may conventionally be in a database.php or somesuch:

  'options' => [
    \PDO::ATTR_PERSISTENT => true
  ]

If you can find a use of "PTO::ATTR_PERSISTENT" in setting up the database, then that may be the culprit. If you're able to test after switching it off, it may at least clearly define the problem at the cost of some latency.

It's still a bug, but potentially one you can't do much about if the PDO object's destructor is never running. From the manual:

The connection remains active for the lifetime of that PDO object. To close the connection, you need to destroy the object by ensuring that all remaining references to it are deleted--you do this by assigning NULL to the variable that holds the object. If you don't do this explicitly, PHP will automatically close the connection when your script ends.

Perhaps the script is ending without destroying your PDO object, and the harder-stop of the process ending (and thus freeing the db resource) isn't happening because the fpm process is persistent. A hail-Mary attempt would be throwing a gc_collect_cycles(); into your shutdown function with the hope that it works around a resource leak. If there are still references to the PDO object that Laravel hasn't cleaned up, then to some extent, it's their bug. If you can hunt down references to the object, including those in your debug code, and destroy them, perhaps you can drive the ref-count to 0 so that it gets cleaned up properly. I'm not saying that we should all go back to malloc() and free(), but sometimes this "convenience features" aren't so convenient. :-/