orbit_core/fleet/

mod.rs

//! `Fleet` — the per-process handle for one node address in a fleet's
//! shared physical geometry.

use std::sync::Arc;
use std::sync::atomic::{AtomicU64, Ordering};

use bytes::Bytes;
use dashmap::DashMap;

use crate::OrbitTyped;
use crate::error::{Error, Result};
use crate::id::NetId64;
#[cfg(unix)]
use crate::ring::shm::{ShmRing, ShmRingRegistry};
use crate::ring::{Frame, Ring, RingRegistry, RingTopology};
#[cfg(any(target_os = "linux", target_os = "freebsd", target_os = "macos"))]
use crate::ring::{ParkedRingEventFd, RingEventFd};

mod cursor;
pub use cursor::{FleetLaneCursor, FleetLanePoll};

/// Read-only namespace handle for inspecting an existing SHM fleet.
///
/// Unlike [`Fleet`], this handle has no node id, creates no rings, and can
/// never publish, reset, or unlink. Opening a ring maps only an object that
/// already exists; dropping the observer or a ring view only unmaps local
/// memory.
#[cfg(unix)]
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct FleetObserver {
    name: String,
    uid: u32
}

#[cfg(unix)]
impl FleetObserver {
    /// Create an observer for the effective user's existing fleet namespace.
    ///
    /// This constructor itself performs no SHM operation. Each [`Self::ring`]
    /// call attaches to one exact existing kind and returns `NotFound` when it
    /// is absent.
    pub fn attach_existing(name: impl Into<String>) -> std::io::Result<Self> {
        // SAFETY: `geteuid` has no error path.
        let uid = unsafe { libc::geteuid() };
        Self::attach_existing_for_uid(name, uid)
    }

    /// Address an explicit uid-scoped fleet namespace.
    ///
    /// POSIX permissions still decide whether the caller may read another
    /// user's SHM objects.
    pub fn attach_existing_for_uid(
        name: impl Into<String>,
        uid: u32
    ) -> std::io::Result<Self> {
        let name = name.into();
        if name.is_empty() {
            return Err(std::io::Error::new(
                std::io::ErrorKind::InvalidInput,
                "fleet name must not be empty"
            ));
        }
        if name.contains('/') || name.contains('\0') {
            return Err(std::io::Error::new(
                std::io::ErrorKind::InvalidInput,
                "fleet name must not contain '/' or a NUL byte"
            ));
        }
        Ok(Self { name, uid })
    }

    pub fn name(&self) -> &str {
        &self.name
    }

    pub fn uid(&self) -> u32 {
        self.uid
    }

    /// Attach read-only to one exact existing ring kind.
    pub fn ring(
        &self,
        kind: u8
    ) -> std::io::Result<crate::ring::shm::ShmRingView> {
        crate::ring::shm::ShmRingView::attach_existing_for_uid(&self.name, kind, self.uid)
    }

    /// Attach read-only using an explicit per-ring maximum access policy.
    pub fn ring_with_policy(
        &self,
        kind: u8,
        policy: crate::shm::ShmAccessPolicy
    ) -> std::io::Result<crate::ring::shm::ShmRingView> {
        crate::ring::shm::ShmRingView::attach_existing_for_uid_with_policy(
            &self.name, kind, self.uid, policy
        )
    }

    /// Attach read-only and verify a linked [`OrbitTyped`] contract.
    pub fn typed_ring<T: OrbitTyped>(&self) -> std::io::Result<crate::ring::shm::ShmRingView> {
        self.typed_ring_with_policy::<T>(crate::shm::ShmAccessPolicy::default())
    }

    /// Attach with a policy and additionally verify the typed ring layout.
    pub fn typed_ring_with_policy<T: OrbitTyped>(
        &self,
        policy: crate::shm::ShmAccessPolicy
    ) -> std::io::Result<crate::ring::shm::ShmRingView> {
        let view = self.ring_with_policy(T::KIND, policy)?;
        if view.metadata().spec != T::RING_SPEC {
            return Err(std::io::Error::new(
                std::io::ErrorKind::InvalidData,
                format!(
                    "OrbitTyped KIND {} declares {:?}; existing spec is {:?}",
                    T::KIND,
                    T::RING_SPEC,
                    view.metadata().spec
                )
            ));
        }
        Ok(view)
    }
}

/// A node's physical writer slot inside the fleet.
///
/// Orbit validates the address against fleet capacity but does not allocate
/// it. The embedding runtime must ensure that simultaneously active writers
/// receive distinct ids. Read-only inspectors can examine SHM without joining
/// as a writer.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[repr(transparent)]
pub struct NodeId(pub u16);

impl NodeId {
    pub const ZERO: Self = Self(0);

    pub const fn new(value: u16) -> Self {
        Self(value)
    }

    pub const fn get(self) -> u16 {
        self.0
    }
}

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

/// Per-process handle into the fleet. Cheap to clone — the inner
/// state is `Arc`-shared.
#[derive(Clone)]
pub struct Fleet {
    inner: Arc<FleetInner>
}

struct FleetInner {
    name: Arc<str>,
    fleet_capacity: u16,
    node_id: NodeId,
    /// Per-KIND counter for `next_id` calls that don't go through a
    /// ring (i.e. when the caller wants a fleet-unique id without
    /// allocating a ring slot). V0: process-local atomic. V1: still
    /// here, parallel to the ring's own write-position.
    id_counters: DashMap<u8, Arc<AtomicU64>>,
    /// Per-KIND ring buffers — orbit's runtime substrate. Either
    /// in-process for unit-test / single-process use, or POSIX SHM
    /// for real cross-process visibility (V1, master+worker fleet).
    backing: RingBacking,
    /// Held for as long as this process is in a shared-memory fleet, so
    /// a lifecycle tool can tell a live fleet from a stopped one and refuse
    /// to remove what is in use. Read-only attachments hold none.
    #[cfg(unix)]
    #[allow(dead_code)]
    membership: Option<crate::shm::FleetMembership>
}

/// Backing storage for the fleet's ring buffers — chosen at
/// `Fleet::join` / `Fleet::join_shm` time and frozen for the
/// fleet's lifetime.
enum RingBacking {
    /// Process-local DashMap of `Ring` instances. No cross-process
    /// visibility — peers running other processes do not see this
    /// fleet's writes. Useful for unit tests and embedded scenarios.
    InMemory(RingRegistry),
    /// POSIX-SHM-backed `ShmRing` instances. Multiple processes
    /// joining the same fleet name share the same kernel-level
    /// memory; writes from one are visible to all immediately.
    #[cfg(unix)]
    Shm(ShmRingRegistry)
}

impl Fleet {
    /// Join (or create) a fleet under `name` with `fleet_capacity` physical
    /// node lanes. In-memory backings remain process-local.
    pub fn join(
        name: &str,
        fleet_capacity: u16
    ) -> Result<Self> {
        Self::join_as(name, fleet_capacity, NodeId::ZERO)
    }

    /// Join (or create) a process-local fleet with an explicit node id.
    pub fn join_as(
        name: &str,
        fleet_capacity: u16,
        node_id: NodeId
    ) -> Result<Self> {
        if fleet_capacity == 0 {
            return Err(Error::EmptyFleet);
        }
        if node_id.get() >= fleet_capacity {
            return Err(Error::NodeOutsideFleet { node_id: node_id.get(), fleet_capacity });
        }
        Ok(Self {
            inner: Arc::new(FleetInner {
                name: Arc::from(name),
                fleet_capacity,
                node_id,
                id_counters: DashMap::new(),
                backing: RingBacking::InMemory(RingRegistry::new(fleet_capacity)),
                #[cfg(unix)]
                membership: None
            })
        })
    }

    /// Join (or create) a fleet whose ring storage is backed by
    /// POSIX shared memory. Multiple processes calling this with
    /// the same `name` share the same kernel-level
    /// segments — the fleet sees each other's writes.
    ///
    /// Each `OrbitTyped` kind gets its own SHM segment whose layout is
    /// declared by `OrbitTyped::RING_SPEC`.
    ///
    /// Cross-process naming: segments are `/orbit-{name}-{kind}-{uid}`.
    /// macOS limits POSIX SHM names to 31 chars (PSHMNAMLEN); a
    /// short fleet name is required there.
    #[cfg(unix)]
    pub fn join_shm(
        name: &str,
        fleet_capacity: u16
    ) -> Result<Self> {
        Self::join_shm_as(name, fleet_capacity, NodeId::ZERO)
    }

    /// Join (or create) a SHM-backed fleet with an explicit node id.
    ///
    /// Per-node rings require one active process membership per node id.
    /// Orbit validates the id range but does not own process lifecycle and
    /// therefore cannot prevent duplicate live memberships.
    #[cfg(unix)]
    pub fn join_shm_as(
        name: &str,
        fleet_capacity: u16,
        node_id: NodeId
    ) -> Result<Self> {
        Self::join_shm_as_with_policies(name, fleet_capacity, node_id, [])
    }

    /// Join with immutable per-kind SHM policies. Unspecified kinds retain
    /// `OwnerOnly`. This configures typed rings, not separate semantic tables,
    /// membership locks or process credentials. All writers must agree on policy.
    #[cfg(unix)]
    pub fn join_shm_as_with_policies(
        name: &str,
        fleet_capacity: u16,
        node_id: NodeId,
        policies: impl IntoIterator<Item = (u8, crate::shm::ShmAccessPolicy)>
    ) -> Result<Self> {
        if fleet_capacity == 0 {
            return Err(Error::EmptyFleet);
        }
        if node_id.get() >= fleet_capacity {
            return Err(Error::NodeOutsideFleet { node_id: node_id.get(), fleet_capacity });
        }
        let name: Arc<str> = Arc::from(name);
        let membership = crate::shm::join_fleet_membership(&name).map_err(Error::Io)?;
        Ok(Self {
            inner: Arc::new(FleetInner {
                name: Arc::clone(&name),
                fleet_capacity,
                node_id,
                id_counters: DashMap::new(),
                backing: RingBacking::Shm(ShmRingRegistry::with_policies(
                    name.as_ref(),
                    fleet_capacity,
                    policies
                )),
                membership: Some(membership)
            })
        })
    }

    pub fn name(&self) -> &str {
        &self.inner.name
    }

    /// Number of physical node lanes reserved for this fleet.
    pub fn fleet_capacity(&self) -> u16 {
        self.inner.fleet_capacity
    }

    pub fn node_id(&self) -> NodeId {
        self.inner.node_id
    }

    /// Mint a fresh fleet-unique [`NetId64`] for type `T` *without*
    /// publishing anything. Use this when the caller only needs the
    /// id (e.g. minting an id to attach to data being persisted to
    /// DB before going through the ring).
    ///
    /// For most use cases prefer [`Fleet::publish`] — it mints AND
    /// stores in a single atomic step.
    pub fn next_id<T: OrbitTyped>(&self) -> NetId64 {
        let counter_arc = self
            .inner
            .id_counters
            .entry(T::KIND)
            .or_insert_with(|| Arc::new(AtomicU64::new(0)))
            .clone();
        let counter = counter_arc.fetch_add(1, Ordering::Relaxed);
        NetId64::make(T::KIND, self.node_id().get(), counter)
    }

    /// True when this fleet's ring storage is backed by POSIX SHM
    /// (visible across processes). False for in-memory fleets.
    pub fn is_shm(&self) -> bool {
        #[cfg(unix)]
        {
            matches!(self.inner.backing, RingBacking::Shm(_))
        }
        #[cfg(not(unix))]
        {
            false
        }
    }

    /// Get-or-create the in-memory ring for type `T`. Only valid
    /// for fleets created via [`Fleet::join`]; SHM-backed fleets
    /// should use [`Fleet::shm_ring`] instead.
    ///
    /// # Panics
    ///
    /// Panics if called on a SHM-backed fleet.
    pub fn ring<T: OrbitTyped>(&self) -> Arc<Ring> {
        match &self.inner.backing {
            RingBacking::InMemory(r) => r.get_or_create::<T>(),
            #[cfg(unix)]
            RingBacking::Shm(_) => {
                panic!("Fleet::ring called on SHM-backed fleet — use Fleet::shm_ring instead");
            }
        }
    }

    /// Get-or-create the SHM ring for type `T`. Only valid on fleets
    /// created via [`Fleet::join_shm`].
    ///
    /// # Errors
    ///
    /// Returns an `io::Error` if the SHM segment cannot be opened
    /// (permissions, name too long, etc.).
    ///
    /// # Panics
    ///
    /// Panics if called on an in-memory fleet.
    #[cfg(unix)]
    pub fn shm_ring<T: OrbitTyped>(&self) -> std::io::Result<Arc<ShmRing>> {
        match &self.inner.backing {
            RingBacking::Shm(r) => r.get_or_create_for::<T>(),
            RingBacking::InMemory(_) => {
                panic!("Fleet::shm_ring called on in-memory fleet — use Fleet::ring instead");
            }
        }
    }

    /// Publish a payload to the ring for type `T`. Mints a [`NetId64`],
    /// writes the [`Frame`] to the appropriate shared or node-owned lane,
    /// and returns the id.
    ///
    /// # Panics
    ///
    /// Panics if the ring cannot be opened or the payload exceeds
    /// `T::RING_SPEC.payload_capacity`. Ring failures are
    /// operator-visible, not silently ignored.
    pub fn publish<T: OrbitTyped>(
        &self,
        frame_kind: u8,
        ver: u64,
        payload: Bytes
    ) -> NetId64 {
        match &self.inner.backing {
            RingBacking::InMemory(r) => {
                let ring = r.get_or_create::<T>();
                ring.write(self.node_id(), frame_kind, ver, payload)
            }
            #[cfg(unix)]
            RingBacking::Shm(r) => {
                let ring =
                    r.get_or_create_for::<T>().expect("SHM ring open failed — fleet unusable");
                ring.write(self.node_id(), frame_kind, ver, payload).expect("SHM ring write failed")
            }
        }
    }

    /// Publish a contiguous batch into one ring lane.
    ///
    /// The returned ids are ordered and consecutive. For per-node rings the
    /// lane head becomes visible only after every frame in the batch has been
    /// committed. Semantic layers can use this to publish a multi-slot blob,
    /// then publish a separate descriptor that references the first id and
    /// frame count.
    ///
    /// # Panics
    ///
    /// Panics if the ring cannot be opened, a payload exceeds the declared
    /// slot capacity, or the batch itself is larger than the ring.
    pub fn publish_batch<T: OrbitTyped>(
        &self,
        frame_kind: u8,
        ver: u64,
        payloads: Vec<Bytes>
    ) -> Vec<NetId64> {
        match &self.inner.backing {
            RingBacking::InMemory(r) => {
                let ring = r.get_or_create::<T>();
                ring.write_batch(self.node_id(), frame_kind, ver, payloads)
            }
            #[cfg(unix)]
            RingBacking::Shm(r) => {
                let ring =
                    r.get_or_create_for::<T>().expect("SHM ring open failed — fleet unusable");
                ring.write_batch(self.node_id(), frame_kind, ver, payloads)
                    .expect("SHM ring batch write failed")
            }
        }
    }

    /// Look up a previously-published frame by its id. Returns the
    /// frame if its slot still holds the same id (i.e. the ring has
    /// not wrapped past it).
    pub fn read(
        &self,
        id: NetId64
    ) -> Option<Frame> {
        match &self.inner.backing {
            RingBacking::InMemory(r) => r.lookup(id.kind())?.read(id),
            #[cfg(unix)]
            RingBacking::Shm(r) => r.lookup(id.kind())?.read(id)
        }
    }

    /// Read the most recent frame for type `T`. Per-node rings read this
    /// fleet handle's local lane; shared rings read their sole lane.
    pub fn read_head<T: OrbitTyped>(&self) -> Option<Frame> {
        if T::RING_SPEC.topology == RingTopology::PerNode {
            let head = self.lane_head::<T>(self.node_id());
            return (head > 0).then(|| self.read_lane_at::<T>(self.node_id(), head - 1)).flatten();
        }
        match &self.inner.backing {
            RingBacking::InMemory(r) => {
                let ring = r.get_or_create::<T>();
                ring.read_head()
            }
            #[cfg(unix)]
            RingBacking::Shm(r) => {
                let ring = r.get_or_create_for::<T>().ok()?;
                ring.read_head()
            }
        }
    }

    /// Current head for type `T`'s ring. Per-node rings report this fleet
    /// handle's local committed head; shared rings report their sole lane's
    /// visible head.
    /// Lazily
    /// creates / attaches the ring on first access — important for
    /// cross-process readers, where a child process may need to
    /// attach to a SHM segment a peer already populated. Returns 0
    /// when the ring is fresh / no counters have been claimed.
    pub fn head<T: OrbitTyped>(&self) -> u64 {
        if T::RING_SPEC.topology == RingTopology::PerNode {
            return self.lane_head::<T>(self.node_id());
        }
        match &self.inner.backing {
            RingBacking::InMemory(r) => r.get_or_create::<T>().head(),
            #[cfg(unix)]
            RingBacking::Shm(r) => r.get_or_create_for::<T>().map(|ring| ring.head()).unwrap_or(0)
        }
    }

    /// Read whatever frame currently occupies `counter % capacity`.
    /// Per-node rings read this fleet handle's local lane. Lazily attaches
    /// the ring on first access (same rationale as [`Fleet::head`]).
    /// Returns `None` if the slot is empty/torn or attach fails.
    ///
    /// Used by walking readers; for typed handle-based reads,
    /// prefer [`Fleet::read`].
    pub fn read_at<T: OrbitTyped>(
        &self,
        counter: u64
    ) -> Option<Frame> {
        if T::RING_SPEC.topology == RingTopology::PerNode {
            return self.read_lane_at::<T>(self.node_id(), counter);
        }
        match &self.inner.backing {
            RingBacking::InMemory(r) => r.get_or_create::<T>().read_at(counter),
            #[cfg(unix)]
            RingBacking::Shm(r) => r.get_or_create_for::<T>().ok()?.read_at(counter)
        }
    }

    pub(crate) fn read_state_at<T: OrbitTyped>(
        &self,
        counter: u64
    ) -> crate::ring::cursor::RingRead {
        if T::RING_SPEC.topology == RingTopology::PerNode {
            return self.read_lane_state_at::<T>(self.node_id(), counter);
        }
        match &self.inner.backing {
            RingBacking::InMemory(r) => r.get_or_create::<T>().read_state_at(counter),
            #[cfg(unix)]
            RingBacking::Shm(r) => r
                .get_or_create_for::<T>()
                .map(|ring| ring.read_state_at(counter))
                .unwrap_or(crate::ring::cursor::RingRead::Unavailable)
        }
    }

    /// Current head for one physical node lane.
    ///
    /// On a shared ring every node id addresses the sole shared lane.
    pub fn lane_head<T: OrbitTyped>(
        &self,
        node_id: NodeId
    ) -> u64 {
        match &self.inner.backing {
            RingBacking::InMemory(r) => r.get_or_create::<T>().lane_head(node_id),
            #[cfg(unix)]
            RingBacking::Shm(r) => {
                r.get_or_create_for::<T>().map(|ring| ring.lane_head(node_id)).unwrap_or(0)
            }
        }
    }

    /// Read the frame currently occupying one node lane's counter slot.
    pub fn read_lane_at<T: OrbitTyped>(
        &self,
        node_id: NodeId,
        counter: u64
    ) -> Option<Frame> {
        match &self.inner.backing {
            RingBacking::InMemory(r) => r.get_or_create::<T>().read_lane_at(node_id, counter),
            #[cfg(unix)]
            RingBacking::Shm(r) => r.get_or_create_for::<T>().ok()?.read_lane_at(node_id, counter)
        }
    }

    pub(crate) fn read_lane_state_at<T: OrbitTyped>(
        &self,
        node_id: NodeId,
        counter: u64
    ) -> crate::ring::cursor::RingRead {
        match &self.inner.backing {
            RingBacking::InMemory(r) => r.get_or_create::<T>().read_lane_state_at(node_id, counter),
            #[cfg(unix)]
            RingBacking::Shm(r) => r
                .get_or_create_for::<T>()
                .map(|ring| ring.read_lane_state_at(node_id, counter))
                .unwrap_or(crate::ring::cursor::RingRead::Unavailable)
        }
    }

    /// Capacity of the ring for type `T`. Lazily attaches the ring
    /// on first access. Falls back to `T::RING_SPEC.capacity` when
    /// SHM attach fails.
    pub fn ring_capacity<T: OrbitTyped>(&self) -> usize {
        match &self.inner.backing {
            RingBacking::InMemory(r) => r.get_or_create::<T>().capacity(),
            #[cfg(unix)]
            RingBacking::Shm(r) => r
                .get_or_create_for::<T>()
                .map(|ring| ring.capacity())
                .unwrap_or(T::RING_SPEC.capacity)
        }
    }

    /// Allocate one semantic version shared by every writer lane of `T`.
    ///
    /// This is separate from each lane's physical frame counter. It is useful
    /// for semantic layers that retain per-node write scalability but require
    /// a deterministic fleet-wide last-write-wins order.
    pub fn next_ring_version<T: OrbitTyped>(&self) -> u64 {
        match &self.inner.backing {
            RingBacking::InMemory(r) => r.get_or_create::<T>().next_version(),
            #[cfg(unix)]
            RingBacking::Shm(r) => r
                .get_or_create_for::<T>()
                .expect("SHM ring open failed — fleet unusable")
                .next_version()
        }
    }

    /// Return the last semantic version allocated for `T` without advancing it.
    pub fn current_ring_version<T: OrbitTyped>(&self) -> u64 {
        match &self.inner.backing {
            RingBacking::InMemory(r) => r.get_or_create::<T>().current_version(),
            #[cfg(unix)]
            RingBacking::Shm(r) => r
                .get_or_create_for::<T>()
                .expect("SHM ring open failed — fleet unusable")
                .current_version()
        }
    }

    /// Clear every lane for `T` and reset all heads to zero.
    ///
    /// This is an owner-side boot cleanup primitive. It is safe for
    /// runtime state such as events and periodic metrics when the
    /// embedding application calls it before peer processes begin
    /// publishing. It is not a coordination protocol; callers must not
    /// reset a ring while other fleet members are actively writing it.
    pub fn reset_ring<T: OrbitTyped>(&self) -> std::io::Result<()> {
        match &self.inner.backing {
            RingBacking::InMemory(r) => {
                r.get_or_create::<T>().reset();
                Ok(())
            }
            #[cfg(unix)]
            RingBacking::Shm(r) => {
                r.get_or_create_for::<T>()?.reset();
                Ok(())
            }
        }
    }

    #[cfg(any(target_os = "linux", target_os = "freebsd", target_os = "macos"))]
    /// Create a process-local readiness fd for one notified SHM ring.
    ///
    /// The fd only signals that the ring generation changed. After draining
    /// it, callers must poll the ring with their own cursor. Multiple writes
    /// may coalesce into one readiness notification.
    pub fn ring_event_fd<T: OrbitTyped>(&self) -> std::io::Result<RingEventFd> {
        match &self.inner.backing {
            RingBacking::Shm(rings) => RingEventFd::new(rings.get_or_create_for::<T>()?),
            RingBacking::InMemory(_) => Err(std::io::Error::new(
                std::io::ErrorKind::Unsupported,
                "Orbit eventfd requires a shared-memory fleet"
            ))
        }
    }

    #[cfg(any(target_os = "linux", target_os = "freebsd", target_os = "macos"))]
    /// Publish one frame and notify native waiters after it commits.
    pub fn publish_notified<T: OrbitTyped>(
        &self,
        frame_kind: u8,
        ver: u64,
        payload: Bytes
    ) -> std::io::Result<NetId64> {
        match &self.inner.backing {
            RingBacking::Shm(rings) => {
                let ring = rings.get_or_create_for::<T>()?;
                let id = ring.write(self.node_id(), frame_kind, ver, payload)?;
                RingEventFd::notify(&ring)?;
                Ok(id)
            }
            // Process-local fleets do not need a kernel wake bridge.
            RingBacking::InMemory(rings) => {
                Ok(rings.get_or_create::<T>().write(self.node_id(), frame_kind, ver, payload))
            }
        }
    }

    #[cfg(any(target_os = "linux", target_os = "freebsd", target_os = "macos"))]
    /// Readiness for a ring published with
    /// [`publish_notified_parked`](Self::publish_notified_parked). See
    /// [`ParkedRingEventFd`]: every reader of such a ring uses this, not
    /// [`ring_event_fd`](Self::ring_event_fd).
    pub fn ring_event_fd_parked<T: OrbitTyped>(&self) -> std::io::Result<ParkedRingEventFd> {
        match &self.inner.backing {
            RingBacking::Shm(rings) => ParkedRingEventFd::new(rings.get_or_create_for::<T>()?),
            RingBacking::InMemory(_) => Err(std::io::Error::new(
                std::io::ErrorKind::Unsupported,
                "Orbit parked readiness requires a shared-memory fleet"
            ))
        }
    }

    #[cfg(any(target_os = "linux", target_os = "freebsd", target_os = "macos"))]
    /// Publish one frame and wake the ring's readers only if one is parked.
    ///
    /// For rings read through [`ring_event_fd_parked`](Self::ring_event_fd_parked):
    /// while every reader is busy draining, a publish is a write and an
    /// atomic, with no syscall. Rings published with
    /// [`publish_notified`](Self::publish_notified) are not affected.
    pub fn publish_notified_parked<T: OrbitTyped>(
        &self,
        frame_kind: u8,
        ver: u64,
        payload: Bytes
    ) -> std::io::Result<NetId64> {
        match &self.inner.backing {
            RingBacking::Shm(rings) => {
                let ring = rings.get_or_create_for::<T>()?;
                let id = ring.write(self.node_id(), frame_kind, ver, payload)?;
                ParkedRingEventFd::notify(&ring)?;
                Ok(id)
            }
            RingBacking::InMemory(rings) => {
                Ok(rings.get_or_create::<T>().write(self.node_id(), frame_kind, ver, payload))
            }
        }
    }

    #[cfg(any(target_os = "linux", target_os = "freebsd", target_os = "macos"))]
    /// Publish one contiguous batch and wake the ring's readers only if one
    /// is parked — [`publish_notified_parked`](Self::publish_notified_parked)
    /// for a batch. A per-node lane commits the whole batch before a reader
    /// can see any of it.
    pub fn publish_batch_notified_parked<T: OrbitTyped>(
        &self,
        frame_kind: u8,
        ver: u64,
        payloads: Vec<Bytes>
    ) -> std::io::Result<Vec<NetId64>> {
        match &self.inner.backing {
            RingBacking::Shm(rings) => {
                let ring = rings.get_or_create_for::<T>()?;
                let ids = ring.write_batch(self.node_id(), frame_kind, ver, payloads)?;
                if !ids.is_empty() {
                    ParkedRingEventFd::notify(&ring)?;
                }
                Ok(ids)
            }
            RingBacking::InMemory(rings) => Ok(rings.get_or_create::<T>().write_batch(
                self.node_id(),
                frame_kind,
                ver,
                payloads
            ))
        }
    }

    #[cfg(any(target_os = "linux", target_os = "freebsd", target_os = "macos"))]
    /// Publish one contiguous batch and notify native waiters once after the
    /// complete batch commits.
    pub fn publish_batch_notified<T: OrbitTyped>(
        &self,
        frame_kind: u8,
        ver: u64,
        payloads: Vec<Bytes>
    ) -> std::io::Result<Vec<NetId64>> {
        match &self.inner.backing {
            RingBacking::Shm(rings) => {
                let ring = rings.get_or_create_for::<T>()?;
                let ids = ring.write_batch(self.node_id(), frame_kind, ver, payloads)?;
                if !ids.is_empty() {
                    RingEventFd::notify(&ring)?;
                }
                Ok(ids)
            }
            RingBacking::InMemory(rings) => Ok(rings.get_or_create::<T>().write_batch(
                self.node_id(),
                frame_kind,
                ver,
                payloads
            ))
        }
    }
}

impl std::fmt::Debug for Fleet {
    fn fmt(
        &self,
        f: &mut std::fmt::Formatter<'_>
    ) -> std::fmt::Result {
        f.debug_struct("Fleet")
            .field("name", &self.inner.name)
            .field("fleet_capacity", &self.inner.fleet_capacity)
            .field("node_id", &self.inner.node_id)
            .field("id_counters", &self.inner.id_counters.len())
            .finish()
    }
}