How would you make an idiomatic idle loop?

[bamthomas]

An email client will most of the time use an IDLE loop to get the new messages/deleted messages events in real time.
What would be the best way to do this with async-imap as the session is borrowed by idle?

I have stored a session in a struct:

pub struct ImapClient {
    session: async_imap::Session<async_native_tls::TlsStream<TcpStream>>
}

That session is initialized with a try_new method.

Then I have a method that makes an idle wait. It returns the number of fetched messages.

   pub async fn wait_for_new_messages(&mut self) -> Result<usize> {
        self.session.select("INBOX").await.unwrap();
        let mut idle = self.session.idle(); // boom: self.session moved due to this method call
        idle.init().await?
       // code for idle/fetch EXISTS
    }

cf this file

The caller is responsible to loop like:

loop {
    match imap_client.wait_for_new_messages().await {
        Ok(nb) => debug!("received {} messages", nb),
        Err(err) => error!("{:?}", err)
    }
}

I understand that the session moves to the idle Handler, to return it when idle is done.

How could I use the session instance with idle?
(@hpk42 I will push a doc PR with my findings, I just want to reach a working example)
Thank you for your answers.

What I tried:

• encapsulate idle handler in the struct ImapClient but when creating the session, same issue is happening with self
• encapsulate the Client and recreating a session each time but that means re-authenticate at each idle loop step thus having an unnecessary network overhead
• encapsulate the Client and making the idle loop inside the ImapClient. But same issue happens with the borrow in the loop

[bamthomas]

For now I made a workaround by overriding the session variable in a loop with the idle returned instance. I'm going to try to refactor to make the code cleaner and see if I can override the struct instance with the idle.done() instance.

https://github.com/iroco-co/mailtempest/blob/4bbf011bf388cb589aaa578d5aee7406f07e48b3/src/mail_reader.rs#L42-L56

[felinira]

I'd really prefer if the IDLE handle would only take a mutable borrow and have a lifetime associated that mutably borrows the session for that long. I store my sessions in a mutex for easy reuse and also box them with a trait for connection type agnosticity and this makes this it nearly impossible to use IDLE.

[link2xt]

In Delta Chat session is stored inside an Option:
https://github.com/deltachat/deltachat-core-rust/blob/061d091c971ac8cb860f92e6e81c298dcffa8f26/src/imap.rs#L93

This allows to .take() it instead of borrowing while you are using it, then return it back.

[felinira]

I suppose that works :) I guess not using mutex is for the better anyways. Just mostly didn't expect to suddenly need an owned version of it, as everything else works fine without.