Drupal\Core\Database\Database::closeConnection() does not actually close connection.
Currently the Database::closeConnection() works like follows.
unset(self::$connections[$key]);
// Force garbage collection to run. This ensures that client connection
// objects and results in the connection being closed are destroyed.
gc_collect_cycles();
I think the intention was to trigger connection destructor which should close the actual DB connection by removing PDO object. However this does not work as PHP won't call a destructor if there are any references to the object. In this case references likely come from services that depend on @database because they keep reference to Drupal\Core\Database\Connection in their class properties.
Drupal\Core\Database\Database::closeConnection();
sleep(60);
While the Drupal is "sleeping" check open connections to your database with tools like pg_stat_activity.
Remove Database::closeConnection() as it basically useless.
Add \Drupal\Core\Database\Connection::closeClientConnection() and \Drupal\Core\Database\Connection::openClientConnection() methods.
Those can look like follows.
public function openClientConnection(): void {
$this->connection = self::open($this->connectionOptions);
}
public function closeClientConnection(): void {
$this->connection = NULL;
}
The use case for closing and reopening connections is to keep their number under limit. The limit can be easily exceeded when you have many PHP processes that are doing slow operations, i.e. HTTP requests to external servers.
So theoretically you could deal with this like follows.
$this->database->closeClientConnection();
$this->httpClient->get('https://some.very/slow/service');
$this->database->openClientConnection();