Need a way to set some state while rate limiting

[sukunrt]

I'm limiting concurrent quic connections from the same IP in go-libp2p. The way we implement the limiting is to create a resource manager scope that tracks all the connection resource usage throughout the lifetime of the connection.

I want to:

1. On receiving ClientHello, create a scope and reject the connection without handshaking if there are too many connections from the same IP.
2. On accepting a ClientHello, when I eventually get the quic connection from Server.Accept retrieve the scope created in 1.

I can use Config.GetConfigForClient to rate limit based on IP, but there's no way to set store the scope that I can later get back after the handshake.
I can use Server.ConnContext to store the scope in the context, but the ConnContext callback has no info about remote IP.

If we add the connection context to ClientHelloInfo it'll be enough for my usecase. This change is also backwards compatible. See #5014

[marten-seemann]

Can you elaborate what information the application would store and retrieve at which point during the handshake?

[sukunrt]

I want to store the resource manager scope and retrieve it later after handshake completes and libp2p accepts the connection(Server.Accept) using conn.Context().Value("scope")

My proposed solution doesn't work as I need to modify the context used for the connection with:
context.WithValue(ctx, "scope", scope)

If the ConnContext were something like http server, I could do:

httpServer.ConnContext = func (ctx context.Context, conn net.Conn) context.Context {
    scope, err := rcmgr.OpenConnection(conn.RemoteAddr())
    if err != nil {
        conn.Close() // drop the connection rate limited
        return ctx
    }
    ctx = context.WithValue(ctx, "scope", scope)
    context.AfterFunc(ctx, func() {scope.Done()}) // arrange for scope to be closed whenever conn closes.  
    return ctx
}

[sukunrt]

Actually #5014 gets me half way through.

See the scope handling here:
If the context were mutable, I'd store the scope on the context there.
https://github.com/libp2p/go-libp2p/blob/296faf59a6be6591ef3c5c93110024e9ef1d5dd8/p2p/transport/quic/transport.go#L318-L338

And then extract the scope here:
https://github.com/libp2p/go-libp2p/blob/296faf59a6be6591ef3c5c93110024e9ef1d5dd8/p2p/transport/quic/listener.go#L53-L60