motorcortex-rust 0.5.0

//! Per-group subscription handle.
//!
//! Clone + Send + Sync — all clones share the same
//! `Arc<SubscriptionInner>`. Alias and `fdiv` are fixed for the life of
//! the subscription; `id` + the parameter layout are swappable so
//! [`Subscribe::resubscribe`] can replace the server-side group (which
//! gets a fresh id) without invalidating outstanding handles. The
//! payload buffer is stored in a `tokio::sync::watch` channel so
//! callers can read the latest value synchronously *and* await the
//! next update, both without touching a `RwLock`. The `notify`
//! callback is the only mutable piece, and it lives behind a separate
//! `RwLock`.
//!
//! [`Subscribe::resubscribe`]: crate::core::Subscribe::resubscribe

use std::sync::atomic::{AtomicU32, Ordering};
use std::sync::{Arc, Mutex, RwLock};

use futures::Stream;
use futures::stream::unfold;
use tokio::sync::{broadcast, watch};

use crate::TimeSpec;
use crate::error::{MotorcortexError, Result};
use crate::msg::{DataType, GroupStatusMsg};
use crate::parameter_value::{
    GetParameterTuple, GetParameterValue, decode_parameter_value,
};

type Callback = Arc<dyn Fn(&Subscription) + Send + Sync + 'static>;

/// Emitted by [`Subscription::stream`] when the consumer falls behind
/// the bounded broadcast ring. The inner `u64` is the number of
/// samples the ring overwrote before the consumer got back to it —
/// callers can use it to decide whether to reset derived state or
/// just keep going.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Missed(pub u64);

impl std::fmt::Display for Missed {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "stream consumer missed {} samples", self.0)
    }
}

impl std::error::Error for Missed {}

/// Parameter layout for a subscription. `Subscribe::resubscribe`
/// replaces this wholesale when the server returns a fresh group
/// descriptor.
struct GroupLayout {
    description: GroupStatusMsg,
    data_types: Vec<u32>,
}

impl GroupLayout {
    fn from_group_msg(description: GroupStatusMsg) -> Self {
        let data_types = description
            .params
            .iter()
            .map(|p| {
                DataType::try_from(p.info.data_type as i32)
                    .unwrap_or(DataType::Undefined) as u32
            })
            .collect();
        Self {
            description,
            data_types,
        }
    }
}

struct SubscriptionInner {
    /// Group id the server most recently assigned. Stored atomically
    /// so the subscribe receive loop can read it without taking the
    /// layout lock; [`rebind`](Subscription::rebind) swaps it in
    /// place on resubscribe.
    id: AtomicU32,
    alias: String,
    /// Frequency divider passed to `subscribe()`. Remembered so
    /// `resubscribe` can hand it back to `create_group`.
    fdiv: u32,
    /// Parameter layout — may change on resubscribe if the server
    /// reshuffled offsets / ids. Held behind a `RwLock` so decoders
    /// see a consistent snapshot while the driver swaps it.
    layout: RwLock<GroupLayout>,
    /// Latest raw payload from the receive thread. Seeded with an
    /// empty `Vec` — readers interpret empty as "no payload yet".
    buffer: watch::Sender<Vec<u8>>,
    callback: RwLock<Option<Callback>>,
    /// Lazy broadcast sink — created on the first `stream()` call.
    /// Every payload the receive thread hands in is forwarded to the
    /// sender (if any), so `stream()` consumers see every sample
    /// the bounded ring can hold.
    broadcast: Mutex<Option<broadcast::Sender<Vec<u8>>>>,
}

/// Async-side subscription handle. Returned by
/// [`crate::core::Subscribe::subscribe`].
pub struct Subscription {
    inner: Arc<SubscriptionInner>,
}

impl Subscription {
    /// Build a Subscription from a freshly-minted group descriptor.
    /// Only `Subscribe::subscribe` should call this.
    pub(crate) fn new(group_msg: GroupStatusMsg, fdiv: u32) -> Self {
        let id = AtomicU32::new(group_msg.id);
        let alias = group_msg.alias.clone();
        let layout = RwLock::new(GroupLayout::from_group_msg(group_msg));
        let (buffer, _) = watch::channel(Vec::new());
        Self {
            inner: Arc::new(SubscriptionInner {
                id,
                alias,
                fdiv,
                layout,
                buffer,
                callback: RwLock::new(None),
                broadcast: Mutex::new(None),
            }),
        }
    }

    /// Group id most recently assigned by the server. Updated by
    /// [`Subscribe::resubscribe`] if the server hands out a fresh id.
    ///
    /// [`Subscribe::resubscribe`]: crate::core::Subscribe::resubscribe
    pub fn id(&self) -> u32 {
        self.inner.id.load(Ordering::Acquire)
    }

    /// Group alias (the name passed to `subscribe`). Fixed for the
    /// life of the subscription.
    pub fn name(&self) -> &str {
        &self.inner.alias
    }

    /// Frequency divider the subscription was created with.
    pub fn fdiv(&self) -> u32 {
        self.inner.fdiv
    }

    /// Parameter paths the subscription was created against, in order.
    /// Extracted from the current server layout; on resubscribe the
    /// server may hand back fresh offsets, but paths themselves don't
    /// change.
    pub fn paths(&self) -> Vec<String> {
        self.inner
            .layout
            .read()
            .unwrap()
            .description
            .params
            .iter()
            .map(|p| p.info.path.clone())
            .collect()
    }

    /// Replace the server-side group descriptor in-place. Called by
    /// the subscribe driver after `Subscribe::resubscribe` receives a
    /// fresh `GroupStatusMsg`. Swaps the id + layout atomically with
    /// respect to decoders, which acquire a read lock on the layout.
    pub(crate) fn rebind(&self, new_group: GroupStatusMsg) {
        let new_id = new_group.id;
        let new_layout = GroupLayout::from_group_msg(new_group);
        // Install the new layout under the write lock so any decoder
        // sees either the old layout or the new layout, never a torn
        // mix. Then bump the id atomically so the subscribe receive
        // loop routes incoming frames to this handle under the new id.
        {
            let mut guard = self.inner.layout.write().unwrap();
            *guard = new_layout;
        }
        self.inner.id.store(new_id, Ordering::Release);
    }

    /// Install a fire-and-forget callback invoked every time a new
    /// payload arrives. The callback runs on the subscribe receive
    /// thread — don't block it.
    ///
    /// ```no_run
    /// # fn demo(subscription: motorcortex_rust::core::Subscription) {
    /// subscription.notify(|s| {
    ///     if let Some((_ts, value)) = s.read::<f64>() {
    ///         println!("got {value}");
    ///     }
    /// });
    /// # }
    /// ```
    pub fn notify<F>(&self, cb: F)
    where
        F: Fn(&Subscription) + Send + Sync + 'static,
    {
        *self.inner.callback.write().unwrap() = Some(Arc::new(cb));
    }

    /// Decode the most recent payload into a tuple matching the
    /// subscription's parameter shape. Returns `None` if no payload
    /// has arrived yet.
    pub fn read<V>(&self) -> Option<(TimeSpec, V)>
    where
        V: GetParameterTuple,
    {
        let rx = self.inner.buffer.subscribe();
        let buffer = rx.borrow().clone();
        let layout = self.inner.layout.read().unwrap();
        decode_tuple::<V>(&layout, &buffer)
    }

    /// Decode the most recent payload into a flat `Vec<V>` — every
    /// scalar element of every subscribed parameter, in order.
    pub fn read_all<V>(&self) -> Option<(TimeSpec, Vec<V>)>
    where
        V: GetParameterValue + Default,
    {
        let rx = self.inner.buffer.subscribe();
        let buffer = rx.borrow().clone();
        let layout = self.inner.layout.read().unwrap();
        decode_flat::<V>(&layout, &buffer)
    }

    /// Await the latest payload. Resolves immediately if any payload
    /// has already arrived; otherwise waits for the first one.
    ///
    /// The underlying channel is lossy by design — if many payloads
    /// arrive between calls, you only see the most recent. For "give
    /// me every sample" semantics, see [`stream`](Self::stream).
    ///
    /// ```no_run
    /// # async fn demo(subscription: motorcortex_rust::core::Subscription)
    /// #   -> motorcortex_rust::Result<()> {
    /// let (_ts, value): (_, f64) = subscription.latest().await?;
    /// println!("latest = {value}");
    /// # Ok(()) }
    /// ```
    pub async fn latest<V>(&self) -> Result<(TimeSpec, V)>
    where
        V: GetParameterTuple,
    {
        let mut rx = self.inner.buffer.subscribe();
        loop {
            // Snapshot the current value; if a payload is already
            // present, decode and return without awaiting.
            let buffer = rx.borrow().clone();
            if !buffer.is_empty() {
                let layout = self.inner.layout.read().unwrap();
                return decode_tuple::<V>(&layout, &buffer).ok_or_else(|| {
                    MotorcortexError::Decode(
                        "subscription payload used an unsupported protocol version".into(),
                    )
                });
            }
            // No payload yet — wait for the next publish.
            rx.changed().await.map_err(|_| {
                MotorcortexError::Subscription(
                    "subscription watch channel closed before any payload arrived".into(),
                )
            })?;
        }
    }

    /// Subscribe to every payload via a bounded ring buffer.
    ///
    /// ```no_run
    /// # async fn demo(subscription: motorcortex_rust::core::Subscription)
    /// #   -> motorcortex_rust::Result<()> {
    /// use futures::StreamExt;
    /// use motorcortex_rust::core::Missed;
    ///
    /// let mut stream = Box::pin(subscription.stream::<f64>(256));
    /// while let Some(item) = stream.next().await {
    ///     match item {
    ///         Ok((_ts, v))       => println!("{v}"),
    ///         Err(Missed(n))     => eprintln!("dropped {n} samples"),
    ///     }
    /// }
    /// # Ok(()) }
    /// ```
    ///
    /// The `capacity` is the number of in-flight samples the buffer
    /// can hold; if a consumer falls behind by more than that, the
    /// next item is `Err(Missed(n))` with `n` = missed samples, and
    /// the consumer can decide whether to catch up or bail. This is
    /// explicit back-pressure, as opposed to [`latest`] which is
    /// lossy by design.
    ///
    /// The broadcast channel is created lazily on the first call —
    /// subscriptions with only `notify` / `read` / `latest` users
    /// pay nothing for it. Subsequent calls reuse the existing
    /// broadcast; the `capacity` argument is honoured only on the
    /// first call.
    ///
    /// [`latest`]: Self::latest
    pub fn stream<V>(&self, capacity: usize) -> impl Stream<Item = StreamResult<V>> + use<V>
    where
        V: GetParameterTuple + Send + 'static,
    {
        let sender = self.ensure_broadcast(capacity);
        let rx = sender.subscribe();
        let inner = Arc::clone(&self.inner);
        unfold(rx, move |mut rx| {
            let inner = Arc::clone(&inner);
            async move {
                loop {
                    match rx.recv().await {
                        Ok(buffer) => {
                            let decoded = {
                                let layout = inner.layout.read().unwrap();
                                decode_tuple::<V>(&layout, &buffer)
                            };
                            match decoded {
                                Some(decoded) => return Some((Ok(decoded), rx)),
                                // Unsupported protocol / malformed frame —
                                // skip silently and wait for the next one
                                // rather than ending the stream on transient
                                // garbage.
                                None => continue,
                            }
                        }
                        Err(broadcast::error::RecvError::Lagged(n)) => {
                            return Some((Err(Missed(n)), rx));
                        }
                        Err(broadcast::error::RecvError::Closed) => return None,
                    }
                }
            }
        })
    }

    /// Called by the subscribe receive thread when a new payload
    /// arrives. Updates every registered sink: watch, broadcast (if
    /// present), and the callback.
    pub(crate) fn update(&self, buffer: Vec<u8>) {
        // Watch: overwrite the current value. `send_replace` instead
        // of `send` because the latter errors when no receivers are
        // subscribed — which is the common case for subscriptions
        // that use only `notify` — and its failure silently drops
        // the value. `send_replace` stores unconditionally and still
        // notifies any receivers that exist.
        self.inner.buffer.send_replace(buffer.clone());
        // Broadcast: only present if someone's using `stream()`.
        if let Some(tx) = self.inner.broadcast.lock().unwrap().as_ref() {
            // Err means there are no active receivers — that's fine,
            // we drop the payload and keep going.
            let _ = tx.send(buffer);
        }
        // Callback: invoked with a reference to self so the user
        // can pull typed values via read / read_all.
        let cb = self.inner.callback.read().unwrap().clone();
        if let Some(cb) = cb {
            cb(self);
        }
    }

    fn ensure_broadcast(&self, capacity: usize) -> broadcast::Sender<Vec<u8>> {
        let mut guard = self.inner.broadcast.lock().unwrap();
        guard
            .get_or_insert_with(|| broadcast::channel(capacity).0)
            .clone()
    }
}

/// Convenience alias for the `Stream`'s item type.
pub type StreamResult<V> = std::result::Result<(TimeSpec, V), Missed>;

impl Clone for Subscription {
    fn clone(&self) -> Self {
        Self {
            inner: Arc::clone(&self.inner),
        }
    }
}

fn decode_tuple<V>(layout: &GroupLayout, buffer: &[u8]) -> Option<(TimeSpec, V)>
where
    V: GetParameterTuple,
{
    if buffer.is_empty() {
        return None;
    }
    const HEADER_LEN: usize = 4;
    let protocol_version = buffer[3];
    if protocol_version != 1 {
        return None;
    }
    let body = &buffer[HEADER_LEN..];
    let ts = TimeSpec::from_buffer(body)?;
    const TS_SIZE: usize = size_of::<TimeSpec>();
    let payload = &body[TS_SIZE..];
    let iter = layout
        .description
        .params
        .iter()
        .zip(layout.data_types.iter())
        .scan(0usize, |cursor, (param, dt)| {
            let size = param.size as usize;
            let slice = &payload[*cursor..*cursor + size];
            *cursor += size;
            Some((dt, slice))
        });
    V::get_parameters(iter).ok().map(|v| (ts, v))
}

fn decode_flat<V>(layout: &GroupLayout, buffer: &[u8]) -> Option<(TimeSpec, Vec<V>)>
where
    V: GetParameterValue + Default,
{
    if buffer.is_empty() {
        return None;
    }
    const HEADER_LEN: usize = 4;
    let protocol_version = buffer[3];
    if protocol_version != 1 {
        return None;
    }
    let body = &buffer[HEADER_LEN..];
    let ts = TimeSpec::from_buffer(body)?;
    const TS_SIZE: usize = size_of::<TimeSpec>();
    let payload = &body[TS_SIZE..];

    let mut values = Vec::new();
    let mut cursor = 0usize;
    for (param, &data_type) in layout.description.params.iter().zip(layout.data_types.iter()) {
        let size = param.size as usize;
        let data_size = param.info.data_size as usize;
        let n = param.info.number_of_elements as usize;
        let bytes = &payload[cursor..cursor + size];
        for i in 0..n {
            let start = i * data_size;
            let end = start + data_size;
            values.push(decode_parameter_value::<V>(data_type, &bytes[start..end]));
        }
        cursor += size;
    }
    Some((ts, values))
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::msg::{GroupParameterInfo, ParameterInfo, ParameterType, StatusCode};
    use std::sync::Mutex;

    fn param(path: &str, dtype: DataType, data_size: u32, n_elements: u32) -> GroupParameterInfo {
        GroupParameterInfo {
            index: 0,
            offset: 0,
            size: data_size * n_elements,
            info: ParameterInfo {
                id: 0,
                data_type: dtype as u32,
                data_size,
                number_of_elements: n_elements,
                flags: 0,
                permissions: 0,
                param_type: ParameterType::Parameter as i32,
                group_id: 0,
                unit: 0,
                path: path.to_string(),
            },
            status: StatusCode::Ok as i32,
        }
    }

    fn group(id: u32, alias: &str, params: Vec<GroupParameterInfo>) -> GroupStatusMsg {
        GroupStatusMsg {
            header: None,
            id,
            alias: alias.to_string(),
            params,
            status: StatusCode::Ok as i32,
        }
    }

    fn protocol1(body: &[u8]) -> Vec<u8> {
        let mut buf = vec![0u8, 0, 0, 1];
        buf.extend_from_slice(&[0u8; 16]); // zero TimeSpec
        buf.extend_from_slice(body);
        buf
    }

    #[test]
    fn id_and_name_reflect_group_msg() {
        let sub = Subscription::new(group(7, "grp", vec![]), 1);
        assert_eq!(sub.id(), 7);
        assert_eq!(sub.name(), "grp");
    }

    #[test]
    fn clone_is_shared() {
        let sub = Subscription::new(group(1, "g", vec![]), 1);
        let clone = sub.clone();
        assert!(Arc::ptr_eq(&sub.inner, &clone.inner));
    }

    #[test]
    fn read_returns_none_without_a_payload() {
        let sub = Subscription::new(group(1, "g", vec![param("x", DataType::Double, 8, 1)]), 1);
        assert!(sub.read::<f64>().is_none());
        assert!(sub.read_all::<f64>().is_none());
    }

    #[test]
    fn read_decodes_a_single_scalar_payload() {
        let sub = Subscription::new(group(1, "g", vec![param("x", DataType::Double, 8, 1)]), 1);
        sub.update(protocol1(&2.5_f64.to_le_bytes()));
        let (_ts, value) = sub.read::<f64>().expect("decode ok");
        assert_eq!(value, 2.5);
    }

    #[test]
    fn read_all_decodes_flattened_array() {
        let sub = Subscription::new(group(1, "g", vec![param("x", DataType::Double, 8, 3)]), 1);
        let mut body = Vec::new();
        body.extend_from_slice(&1.0f64.to_le_bytes());
        body.extend_from_slice(&2.0f64.to_le_bytes());
        body.extend_from_slice(&3.0f64.to_le_bytes());
        sub.update(protocol1(&body));
        let (_ts, values) = sub.read_all::<f64>().expect("decode");
        assert_eq!(values, vec![1.0, 2.0, 3.0]);
    }

    #[test]
    fn update_fires_the_callback() {
        let sub = Subscription::new(group(1, "g", vec![param("x", DataType::Double, 8, 1)]), 1);
        let hits = Arc::new(Mutex::new(0u32));
        let counter = Arc::clone(&hits);
        sub.notify(move |_| {
            *counter.lock().unwrap() += 1;
        });
        sub.update(protocol1(&0f64.to_le_bytes()));
        sub.update(protocol1(&0f64.to_le_bytes()));
        assert_eq!(*hits.lock().unwrap(), 2);
    }

    #[test]
    fn non_protocol_1_returns_none() {
        let sub = Subscription::new(group(1, "g", vec![param("x", DataType::Double, 8, 1)]), 1);
        let mut buf = vec![0u8, 0, 0, 0]; // protocol 0
        buf.extend_from_slice(&[0u8; 24]);
        sub.update(buf);
        assert!(sub.read::<f64>().is_none());
        assert!(sub.read_all::<f64>().is_none());
    }

    #[tokio::test]
    async fn latest_resolves_immediately_when_payload_already_present() {
        let sub = Subscription::new(group(1, "g", vec![param("x", DataType::Double, 8, 1)]), 1);
        sub.update(protocol1(&7.5f64.to_le_bytes()));
        let (_ts, v) = sub.latest::<f64>().await.expect("decode ok");
        assert_eq!(v, 7.5);
    }

    #[tokio::test]
    async fn latest_waits_for_the_first_payload() {
        let sub = Subscription::new(group(1, "g", vec![param("x", DataType::Double, 8, 1)]), 1);
        let sub_for_push = sub.clone();
        // Publish after a short delay — latest().await must pick it up.
        tokio::spawn(async move {
            tokio::time::sleep(std::time::Duration::from_millis(50)).await;
            sub_for_push.update(protocol1(&42.0f64.to_le_bytes()));
        });
        let (_ts, v) = sub.latest::<f64>().await.expect("decode ok");
        assert_eq!(v, 42.0);
    }

    #[tokio::test]
    async fn stream_delivers_consecutive_payloads() {
        use futures::StreamExt;

        let sub = Subscription::new(group(1, "g", vec![param("x", DataType::Double, 8, 1)]), 1);
        let mut stream = Box::pin(sub.stream::<f64>(16));

        // Publish three payloads; the stream must yield them in order.
        sub.update(protocol1(&1.0f64.to_le_bytes()));
        sub.update(protocol1(&2.0f64.to_le_bytes()));
        sub.update(protocol1(&3.0f64.to_le_bytes()));

        for expected in [1.0, 2.0, 3.0f64] {
            let item = tokio::time::timeout(std::time::Duration::from_millis(100), stream.next())
                .await
                .expect("stream must yield within 100 ms")
                .expect("stream is not exhausted");
            let (_ts, v) = item.expect("not lagged");
            assert_eq!(v, expected);
        }
    }

    #[tokio::test]
    async fn stream_surfaces_lag_as_err() {
        use futures::StreamExt;

        // Tiny capacity so we provoke lag trivially.
        let sub = Subscription::new(group(1, "g", vec![param("x", DataType::Double, 8, 1)]), 1);
        let mut stream = Box::pin(sub.stream::<f64>(2));

        // Push more than capacity before the stream consumes anything.
        for i in 0..8 {
            sub.update(protocol1(&(i as f64).to_le_bytes()));
        }

        // First pull should surface a Missed error. (Might also be the
        // first Ok if tokio happens to poll fast; assert on either
        // shape but confirm a Missed eventually appears.)
        let mut saw_miss = false;
        for _ in 0..8 {
            let item = tokio::time::timeout(std::time::Duration::from_millis(100), stream.next())
                .await
                .expect("stream yields")
                .expect("not exhausted");
            if let Err(Missed(n)) = item {
                assert!(n > 0, "Missed's inner count must be positive");
                saw_miss = true;
                break;
            }
        }
        assert!(saw_miss, "expected to observe at least one Missed item");
    }

    #[tokio::test]
    async fn stream_is_not_created_unless_requested() {
        // Confirm that not calling stream() keeps the broadcast Mutex
        // at None — no capacity paid for.
        let sub = Subscription::new(group(1, "g", vec![param("x", DataType::Double, 8, 1)]), 1);
        sub.update(protocol1(&1.0f64.to_le_bytes()));
        assert!(sub.inner.broadcast.lock().unwrap().is_none());
    }

    #[test]
    fn missed_formats_and_is_error() {
        let m = Missed(7);
        assert_eq!(m, Missed(7));
        assert_eq!(format!("{m}"), "stream consumer missed 7 samples");
        let _: &dyn std::error::Error = &m;
    }

    #[tokio::test]
    async fn latest_errors_on_unsupported_protocol_version() {
        let sub = Subscription::new(group(1, "g", vec![param("x", DataType::Double, 8, 1)]), 1);
        let mut buf = vec![0u8, 0, 0, 7]; // unknown protocol
        buf.extend_from_slice(&[0u8; 24]);
        sub.update(buf);
        let err = sub
            .latest::<f64>()
            .await
            .expect_err("unsupported protocol must error");
        assert!(matches!(err, MotorcortexError::Decode(_)));
    }

    #[test]
    fn fdiv_and_paths_reflect_the_constructor_args() {
        let params = vec![
            param("root/a", DataType::Double, 8, 1),
            param("root/b", DataType::Int32, 4, 1),
        ];
        let sub = Subscription::new(group(1, "g", params), 7);
        assert_eq!(sub.fdiv(), 7);
        assert_eq!(sub.paths(), vec!["root/a".to_string(), "root/b".to_string()]);
    }

    #[test]
    fn rebind_swaps_id_and_layout() {
        let sub = Subscription::new(
            group(11, "grp", vec![param("root/a", DataType::Double, 8, 1)]),
            1,
        );
        assert_eq!(sub.id(), 11);
        assert_eq!(sub.paths(), vec!["root/a".to_string()]);

        let new_group = group(
            42,
            "grp",
            vec![
                param("root/x", DataType::Double, 8, 1),
                param("root/y", DataType::Int32, 4, 1),
            ],
        );
        sub.rebind(new_group);
        assert_eq!(sub.id(), 42);
        assert_eq!(sub.paths(), vec!["root/x".to_string(), "root/y".to_string()]);
        // Alias + fdiv are fixed across rebinds.
        assert_eq!(sub.name(), "grp");
        assert_eq!(sub.fdiv(), 1);
    }

    #[test]
    fn rebind_is_visible_to_outstanding_clones() {
        let sub = Subscription::new(
            group(1, "g", vec![param("root/a", DataType::Double, 8, 1)]),
            1,
        );
        let clone = sub.clone();
        let new_group = group(99, "g", vec![param("root/b", DataType::Double, 8, 1)]);
        sub.rebind(new_group);
        // Both handles share the same inner Arc, so the clone observes
        // the rebind without any additional plumbing.
        assert_eq!(clone.id(), 99);
        assert_eq!(clone.paths(), vec!["root/b".to_string()]);
    }
}