Description:
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Mysql close is NOT synchronous - it does NOT wait until the connection is closed/released by the database server which causes more simultaneous connections than needed from a script than connect/disconnect from the same database often.

An example of such usecase is unit testing.

I have tested also PostgreSQL. This PDO driver handles close synchronously/ok (SQL to limit connections: ALTER ROLE user CONNECTION LIMIT 1).

MySQL driver needs to be fixed as even server limit of 2 is not enough, currently limit of like 5 is needed even if there is no more than 1 simultaneous client connection.

Test script:
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# 1. limit connection limit to 1
php -r '(new PDO("mysql:host=mysql", "user", "pass"))->exec("ALTER USER '"'"'user'"'"'@'"'"'%'"'"' WITH MAX_USER_CONNECTIONS 1");'

# 2. connect/disconnect often
for ($i = 0; $i < 10000; $i++) {
    (new PDO("mysql:host=mysql", "user", "pass"))->exec("SELECT 1");
}

Expected result:
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no error

Actual result:
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connection limit is exhausted about every 1000th connection