Return a connection object from a function?

[Ziris85]

Hello all,

I'm fairly new to rust and dbus (working through various tutorials and walkthroughs as I go along), and I've written up this bit of code that won't compile due to ownership/lifetime issues, and I was hoping someone here might be able to point me in the right direction on how to overcome:

enum DbusInterface {
    Timedate,
    Systemd,
}

impl DbusInterface {
    
    fn connect(&self) -> Result<Proxy<'_, &dbus::blocking::Connection>, dbus::Error> {
        let c = Connection::new_system()?;
       match &self {
            DbusInterface::Timedate => Ok{c.with_proxy("org.freedesktop.timedate1", "/org/freedesktop/timedate1", Duration::from_millis(5000))),
            DbusInterface::Systemd => Ok(c.with_proxy("org.freedesktop.systemd", "/org/freedesktop/systemd", Duration::from_millis(5000))),
        }
    }
//    fn get() {}
//    fn get_all() {}

}

The end goal is to be able to return the connection to be used in other functions to interact with dbus in various ways. However, Rust won't compile this with:

error[E0515]: cannot return value referencing local variable `c`
  --> src/main.rs:18:40
   |
18 | ...imedate => Ok(c.with_proxy("org.freedesktop.timedate1", "/org/freedesktop/timedate1", Duration::from_millis(5000))),
   |               ^^^-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |               |  |
   |               |  `c` is borrowed here
   |               returns a value referencing data owned by the current function

Attempting to dereference also just gives me an "lol no" error message. I'm sure I'm missing something obvious here, and perhaps I'm not asking in the right location (maybe stackoverflow?), but I thought I'd start here. I would be greatly appreciative of any guidance. Thank you!

[diwic]

I think there isn't one right answer to this question; there are several designs. Probably the easiest one is to create a struct like this:

struct MyConnection {
    conn: Connection,
    intf: DBusInterface // I e, your existing enum
}

And then in every method of MyConnection, construct the proxy and then call whatever methods you need on that.

The other option would be to create a struct that contains both a Connection and a Proxy and that struct then has to use a crate like rental because the Proxy references the Connection. I guess this is more complicated, but I don't know because I never used rental myself.

[Ziris85]

diwic,

Thanks again for the reply! I gave your new suggestion a try as-is and it complained about the Result value being present instead:

DBusInterface::Timedate => Proxy::new("org.freedesktop.timedate1", "/org/freedesktop/timedate1", Duration::from_millis(5000), Connection::new_system()),

error[E0308]: mismatched types
  --> src/main.rs:71:137
   |
71 | ...rom_millis(5000), Connection::new_system())),
   |                      ^^^^^^^^^^^^^^^^^^^^^^^^ expected `&dbus::blocking::Connection`, found enum `std::result::Result`
   |
   = note: expected reference `&dbus::blocking::Connection`
                   found enum `std::result::Result<dbus::blocking::Connection, dbus::Error>`

Adding a simple unwrap to that gets to something else that was hinted at earlier - that it expects a borrowed value to be present there:

DBusInterface::Timedate => Proxy::new("org.freedesktop.timedate1", "/org/freedesktop/timedate1", Duration::from_millis(5000), Connection::new_system().unwrap()),

error[E0308]: mismatched types
  --> src/main.rs:70:139
   |
70 | ...s(5000), Connection::new_system().unwrap()),
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |             |
   |             expected `&dbus::blocking::Connection`, found struct `dbus::blocking::Connection`
   |             help: consider borrowing here: `&Connection::new_system().unwrap()`

And of course, making that a borrowed value takes us back to the original problem:

DBusInterface::Timedate => Proxy::new("org.freedesktop.timedate1", "/org/freedesktop/timedate1", Duration::from_millis(5000), &Connection::new_system().unwrap()),

error[E0515]: cannot return value referencing temporary value
  --> src/main.rs:70:40
   |
70 | ...=> Proxy::new("org.freedesktop.timedate1", "/org/freedesktop/timedate1", Duration::from_millis(5000), &Connection::new_system().unwrap(...
   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^---------------------------------^
   |       |                                                                                                   |
   |       |                                                                                                   temporary value created here
   |       returns a value referencing data owned by the current function

I appreciate any other ideas or suggestions you might have here! I did manage to come up with a solution that involves a messy struct and such, but being able to do something like what you mentioned would be FAR cleaner and simpler code indeed 😄

[diwic]

Make sure there is no & in the return value of the function, it should look like this:

fn connect(&self) -> Result<Proxy<'static, dbus::blocking::Connection>, dbus::Error>

Now you can add a ? after new_system() to have it handle errors more gracefully.

[Ziris85]

Ah yes, thank you! That gets me a bit closer to making it work. It does successfully allow me to return that value now, but I'm having a hard time actually doing anything with it due to trait bounds. Here's what I have at this point (forgive the poor quality of the code):

pub enum DBusInterface {
    Timedate,
}

struct DBusRequest<'a> {
    conn: Proxy<'a, dbus::blocking::Connection>,
    url: String
}

impl DBusInterface {
    fn connect(&self) -> Result<DBusRequest, dbus::Error> {
        match &self {
            DBusInterface::Timedate => Ok(
                DBusRequest {
                    conn: Proxy::new("org.freedesktop.timedate1", "/org/freedesktop/timedate1", Duration::from_millis(5000), Connection::new_system()?),
                    url: String::from("org.freedesktop.timedate1")
                } 
            )
        }
    }
    pub fn get(&self, req: String) -> Box<dyn arg::RefArg> {
        let p = self.connect().unwrap();
        p.conn.get(p.url, &req).unwrap()
    }
}

Compiling returns:

error[E0599]: no method named `get` found for struct `Proxy<'_, dbus::blocking::Connection>` in the current scope
   --> src/lib.rs:94:16
    |
94  |         p.conn.get(p.url, &req).unwrap()
    |                ^^^ method not found in `Proxy<'_, dbus::blocking::Connection>`
    | 
   ::: /root/.cargo/registry/src/github.com-1ecc6299db9ec823/dbus-0.9.3/src/blocking.rs:117:1
    |
117 | pub struct Connection {
    | ---------------------
    | |
    | doesn't satisfy `<dbus::blocking::Connection as Deref>::Target = _`
    | doesn't satisfy `dbus::blocking::Connection: Deref`
...
315 | pub struct Proxy<'a, C> {
    | ----------------------- doesn't satisfy `_: dbus::blocking::stdintf::org_freedesktop_dbus::Properties`
    |
    = note: the method `get` exists but the following trait bounds were not satisfied:
            `<dbus::blocking::Connection as Deref>::Target = _`
            which is required by `Proxy<'_, dbus::blocking::Connection>: dbus::blocking::stdintf::org_freedesktop_dbus::Properties`
            `dbus::blocking::Connection: Deref`
            which is required by `Proxy<'_, dbus::blocking::Connection>: dbus::blocking::stdintf::org_freedesktop_dbus::Properties`

I'm a little confused as to why this is happening, since as I understand it, the "with_proxy" method is just a convenience wrapper that returns a Proxy struct that I'm just constructing by hand, so I'm not quite sure how/why this is different. Traits are still something of a confusing topic for me, so that's probably why I'm struggling here. I've read through the Rust docs on them and they seem really neat/useful! Just not quite sure how to work with them yet heh.

[diwic]

Hmm, so Deref is not implemented for itself, sorry. Easiest (but slightly annoying) is to just wrap it in a box:

struct DBusRequest {
    conn: Proxy<'static, Box<dbus::blocking::Connection>>,
    url: String
}

And when creating the proxy:

Proxy::new("org.freedesktop.timedate1", "/org/freedesktop/timedate1", Duration::from_millis(5000), Box::new(Connection::new_system()?))

[Ziris85]

I forgot to follow up on this! This worked beautifully, thank you for your help!