RecoveryConfig in AdvancedSubscriber needs to be refined

[YuanYuYuan]

Describe the release item

We expect the RecoveryConfig to be configured as either the heartbeat or the periodic_queries in our current design. To achieve this, we use the following magic trick and introduce some condition checks in the runtime.

#[derive(Default, Clone, Copy)]
pub struct Unconfigured;
#[derive(Default, Clone, Copy)]
pub struct Configured;

#[zenoh_macros::unstable]
pub struct RecoveryConfig<T> {
    periodic_queries: Option<Duration>,
    heartbeat: Option<()>,
    phantom: PhantomData<T>,
}
#[zenoh_macros::unstable]
impl RecoveryConfig<Unconfigured> {
    pub fn periodic_queries(self, period: Duration) -> RecoveryConfig<Configured> {
        RecoveryConfig {
            periodic_queries: Some(period),
            heartbeat: None,
            phantom: PhantomData::<Configured>,
        }
    }
    pub fn heartbeat(self) -> RecoveryConfig<Configured> {
        RecoveryConfig {
            periodic_queries: None,
            heartbeat: Some(()),
            phantom: PhantomData::<Configured>,
        }
    }
}

However, users could construct an empty config by RecoveryConfig::default surprisingly marked as Configured. For instance,

let subscriber = session
    .declare_subscriber(key_expr)
    .history(HistoryConfig::default().detect_late_publishers())
    .recovery(RecoveryConfig::default())  // <-- with two default None values inside and resolved as Configured
    .subscriber_detection()
    .await
    .unwrap();

this means recovery is used but does nothing.

Moreover, it doesn't align with the c/cpp behavior. https://github.com/eclipse-zenoh/zenoh-c/blob/cdc53527c88c5d27b35fa5d4552560a06f287e44/src/advanced_subscriber.rs#L91-L105
The default recovery option is set to heartbeat and users can't configure it explicitly since it's not exposed in c/cpp due to the generic type used in Rust.

I would suggest using the following isomorphic enum to avoid unnecessary complexity and solve the above API misalignment issue effortlessly.

enum RecoveryConfig {
   PeriodicQueries(Option<Duration>),
   #[default]
   Heartbeat
}

[OlivierHecart]

RecoveryConfig does not need to be configured as either the heartbeat or the periodic_queries.

Recovery by default detects misses through the sequence_numbers and so don't need heartbeat or periodic_queries. This is what you get with .recovery(RecoveryConfig::default()). This works well for periodic publications, but for sporadic publications you may miss the last sample and not be able to detect it. We provide 2 ways of detecting last sample miss: heartbeat or periodic_queries that we want to be mutually exclusive (we don't want to allow users to activate both).

So to summarize: we currently have 3 recovery modes:

• recovery(RecoveryConfig::default()): sample miss detection based on sequence_numbers only.
• recovery(RecoveryConfig::heartbeat()): sample miss detection based on sequence_numbers and heartbeat.
• recovery(RecoveryConfig:: periodic_queries()): sample miss detection based on sequence_numbers and periodic queries.