How to downcast connection_limits::Exceeded ?

[nathanielc]

I am trying to upgrade to libp2p 0.51.0, specifically to use the new connection_limits::Behaviour however I am unable to downcast the error cause as explained in the docs.

The ListenError::Denied and respectively the DialError::Denied variant contain a ConnectionDenied type that can be downcast to Exceeded error if (and only if) this behaviour denied the connection.

Here is my code snippet

// Inside impl NetworkBehaviour ...

fn on_swarm_event(&mut self, event: libp2p::swarm::FromSwarm<Self::ConnectionHandler>) {
    match event {
        // ...
        libp2p::swarm::FromSwarm::DialFailure(event) => {
            if let DialError::Denied { cause } = event.error {
                if let Ok(connection_limits::Exceeded { .. }) = cause.downcast() {
                    // Do something about the connection limit error
                }
            }
        } 
        // ...
    }
}

I am unable to downcast as I only have a reference to the cause. Downcast needs to move the cause value but its behind a reference. I can't clone the value either as it does not implement Clone. Am I missing something?

Happy to contribute a fix if one is needed.

[thomaseizinger]

Ah yes, the downcast was intended to be used when you receive the event via polling the Swarm.

We can't give ownership of the error to all behaviours because errors are seldomly clonable.

[mxinden]

@nathanielc can you post your NetworkBehaviour hierarchy here? Are you wrapping the connection_limits::Behaviour?

[nathanielc]

Yeah makes sense why Clone isn't available.

Here is the WIP branch I have for more context https://github.com/nathanielc/beetle/blob/custom-behaviour/iroh-bitswap/src/lib.rs#L426

This is in a stripped down fork of beetle specifically in the bitswap implementation.

Here is the behaviour itself https://github.com/nathanielc/beetle/blob/custom-behaviour/iroh-p2p/src/behaviour.rs#L37-L52 Its mostly flat with exception of the custom_behavior that allows us to layer in more behaviours below the beetle behaviours (i.e. ceramicnetwork/rust-ceramic#52)

[thomaseizinger]

There is not reason we cannot also expose downcast_ref. Mind opening a PR?

[nathanielc]

That was simple, let me know if you'd like a different/better test #4020