Client::new() returns error -50 after 6 calls about coremidi

[chris-zen]

Hi @sourcebox, thanks for the report.

I've been investigating it a bit. Apparently the Drop for Client is working fine and the client is disposed at CoreMIDI level.

With this code:

impl Drop for Client {
    fn drop(&mut self) {
        println!("Dropping client ...");
        let status = unsafe { MIDIClientDispose(self.object.0) };
        println!("{}", status);
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test() {
        let mut counter = 0;
        loop {
            let client = Client::new("test midi client");
            counter += 1;

            match client.as_ref() {
                Ok(client) => {
                    println!("{}, {:?}", counter, client);
                }
                Err(error) => {
                    println!("{:?}", error);
                    break;
                }
            }

            std::thread::sleep(std::time::Duration::from_millis(1000));
        }
    }
}

I see this:

1, Client { object: Object(bc29041), callback: BoxedCallback(0x0) }
Dropping client ...
0
2, Client { object: Object(bc29042), callback: BoxedCallback(0x0) }
Dropping client ...
0
3, Client { object: Object(bc29043), callback: BoxedCallback(0x0) }
Dropping client ...
0
4, Client { object: Object(bc29044), callback: BoxedCallback(0x0) }
Dropping client ...
0
5, Client { object: Object(bc29045), callback: BoxedCallback(0x0) }
Dropping client ...
0
6, Client { object: Object(bc29046), callback: BoxedCallback(0x0) }
Dropping client ...
0
7, Client { object: Object(bc29047), callback: BoxedCallback(0x0) }
Dropping client ...
0
8, Client { object: Object(bc29048), callback: BoxedCallback(0x0) }
Dropping client ...
0
-50

But reading the docs I just found a possible cause for the problem that it would be out of my control:

https://developer.apple.com/documentation/coremidi/1495335-midiclientdispose#discussion

Don’t explicitly dispose of your client; the system automatically disposes all clients when an app terminates.
However, if you call this method to dispose the last or only client owned by an app,
the MIDI server may exit if there are no other clients remaining in the system.
If this occurs, all subsequent calls by your app to MIDIClientCreate(_:_:_:_:) and MIDIClientCreateWithBlock(_:_:_:) fail.

So apparently, clients are not supposed to be created and destroyed that frequently, but instead live for the whole lifecycle of your app.