Connection clean-up does not reset session system variables

[demurgos]

When calling get_conn on a pool, there's a risk to get a connection with session system variables in an unexpected state.
(For example variables defined like SET session innodb_lock_wait_timeout = 0;)

Both MariaDB and MySQL have support to reset the connection state, and this library exposes it as Conn::reset. However, it's required to call it manually every single time to avoid leaking state from some recycled connection.

I believe that the connection state should be reset by default in Conn::cleanup_for_pool, with potentially an option to opt-out of this behavior. Having resets enables by default is a better choice in my opinion to get predictable results.

[cloneable]

134cbf8 adds implicit non-configurable connection reset when it's returned to the pool. This kills the prepared statement cache, degrading performance, and also the conn init statements are not run after a reset, so default session settings are not kept.

[blackbeam]

@cloneable, please note, that 134cbf8 does not enforce reset for clean connections because of this condition. I would say it's broken.

I agree with @demurgos that we should reset the connection state by default, actually it might be dangerous not to do so (consider something like unique_checks).

The problem, I think, that there is no way to opt-out, so I have to add one (actually, this is the reason for a new boolean argument on GetConn::new).

and also the conn init statements are not run after a reset

This is a bug.

[cloneable]

Ok, thanks! It's very unfortunate that COM_RESET_CONNECTION clears prepared statements. I wish they would offer a reset that keeps the statement cache.

[blackbeam]

Fixed in v0.32.1