subetha_cxc/

peer_directory.rs

//! `PeerDirectory` - the shared topology substrate behind the
//! automatic [`AdaptiveRing`](crate::AdaptiveRing).
//!
//! One small MMF region (file / named-shm / anonymous, mirroring the
//! ring backings' three locales) that every process attached to the
//! same ring maps. It carries the facts the automatic behavior needs
//! to be CROSS-PROCESS instead of per-process:
//!
//! - **Peer slot claims.** Producers and consumers claim / release
//!   slot ids in shared bitmaps, so ids are unique across processes
//!   and slots recycle on release. Registration in any process is
//!   visible to every process.
//! - **Ring publication.** `published()` is the count of per-producer
//!   ring backings whose files exist and are fully initialised.
//!   A grower creates the backing files FIRST, then advances the
//!   count (Release); openers that observe the count (Acquire) can
//!   open the files without racing initialisation.
//! - **Topology epoch.** One shared counter bumped on every peer /
//!   publication change. Hot paths compare it against a process-local
//!   cache - one relaxed load per op while the topology is stable -
//!   and run the sync slow path only on change.
//! - **MPMC ring ownership.** A per-ring owner table replaces the
//!   modulus partition so the consumer set can grow and shrink at
//!   runtime. Each per-producer ring is drained by exactly ONE
//!   consumer slot (the Lamport cores are single-reader); ownership
//!   moves only by (a) CAS-claim of an unowned ring, (b) the current
//!   owner handing off after a rebalance request, or (c) takeover of
//!   a slot whose process is gone (pid liveness probe) or whose slot
//!   was released. (a)-(c) all keep the single-reader invariant: no
//!   two live consumers ever pop the same ring concurrently.
//!
//! The region is fixed-size: [`PRODUCER_SLOT_CEILING`] and
//! [`CONSUMER_SLOT_CEILING`] are substrate-wide architectural
//! ceilings (like the Vyukov 56-byte slot), NOT per-ring tunables -
//! per-ring limits come from the caller's declared
//! [`RingContract`](crate::ring_contract::RingContract), and only a declared
//! contract makes registration fallible. Slots recycle on release, so
//! the ceilings bound CONCURRENT peers, not lifetime attachments.

use std::fs::{File, OpenOptions};
use std::path::Path;
use std::sync::atomic::{AtomicU32, AtomicU64, Ordering as AtomOrd};

use memmap2::{MmapMut, MmapOptions};

use crate::shared_ring::RingError;

/// Substrate-wide ceiling on CONCURRENTLY-claimed producer slots per
/// ring. Backing files are created on demand, so the ceiling costs
/// only directory bytes (one owner word + one bitmap bit per slot),
/// not ring storage.
pub const PRODUCER_SLOT_CEILING: usize = 4096;

/// Substrate-wide ceiling on concurrently-claimed consumer slots per
/// ring. Sized past the ring contract's expressible `u8` bound.
pub const CONSUMER_SLOT_CEILING: usize = 256;

/// "No consumer" marker in the ring owner table.
pub const OWNER_NONE: u16 = u16::MAX;

const DIR_MAGIC: u32 = 0x5045_4552; // "PEER"

const P_WORDS: usize = PRODUCER_SLOT_CEILING / 64;
const C_WORDS: usize = CONSUMER_SLOT_CEILING / 64;

#[repr(C)]
struct DirHeader {
    magic: AtomicU32,
    _rsvd: u32,
    /// Topology epoch: bumped on every claim / release / publish.
    epoch: AtomicU64,
    /// Per-producer ring backings that exist and are initialised.
    published: AtomicU32,
    active_producers: AtomicU32,
    active_consumers: AtomicU32,
    _pad: u32,
}

const OFF_P_BITMAP: usize = std::mem::size_of::<DirHeader>();
const OFF_C_BITMAP: usize = OFF_P_BITMAP + P_WORDS * 8;
const OFF_C_PIDS: usize = OFF_C_BITMAP + C_WORDS * 8;
const OFF_P_PIDS: usize = OFF_C_PIDS + CONSUMER_SLOT_CEILING * 8;
const OFF_OWNERS: usize = OFF_P_PIDS + PRODUCER_SLOT_CEILING * 8;

/// Total mapped size of a peer directory region.
pub const fn peer_directory_size() -> usize {
    OFF_OWNERS + PRODUCER_SLOT_CEILING * 8
}

/// Backing-store owner; mirrors the ordering region's pattern.
#[allow(dead_code)]
enum DirBacking {
    Anon(MmapMut),
    File(File, MmapMut),
    Shm(crate::shm_file::ShmFile),
}

/// The mapped peer directory in any of the three locales.
pub struct PeerDirectory {
    _backing: DirBacking,
    raw_ptr: *mut u8,
}

unsafe impl Send for PeerDirectory {}
unsafe impl Sync for PeerDirectory {}

/// Zero the claims / counters and stamp the magic. Called exactly
/// once per region lifetime (creator side); attachers never re-init.
unsafe fn init_dir_layout(ptr: *mut u8) {
    unsafe {
        std::ptr::write_bytes(ptr, 0, peer_directory_size());
        // Owner table starts all-OWNER_NONE, not zero.
        let owners = ptr.add(OFF_OWNERS) as *mut u64;
        let none = pack_owner(OWNER_NONE, OWNER_NONE);
        for i in 0..PRODUCER_SLOT_CEILING {
            std::ptr::write(owners.add(i), none);
        }
        let header = &*(ptr as *const DirHeader);
        header.magic.store(DIR_MAGIC, AtomOrd::Release);
    }
}

#[inline]
const fn pack_owner(owner: u16, pending: u16) -> u64 {
    (owner as u64) | ((pending as u64) << 16)
}

#[inline]
const fn unpack_owner(word: u64) -> (u16, u16) {
    (word as u16, (word >> 16) as u16)
}

impl PeerDirectory {
    /// Anonymous in-process directory (the Anon ring locale).
    pub fn create_anon() -> Result<Self, RingError> {
        let mut mmap = MmapOptions::new().len(peer_directory_size()).map_anon()?;
        unsafe { init_dir_layout(mmap.as_mut_ptr()) };
        let raw_ptr = mmap.as_mut_ptr();
        Ok(Self { _backing: DirBacking::Anon(mmap), raw_ptr })
    }

    /// File-backed directory at `path`, initialised by the creator.
    pub fn create(path: impl AsRef<Path>) -> Result<Self, RingError> {
        let total = peer_directory_size();
        let file = OpenOptions::new()
            .read(true).write(true).create(true).truncate(true)
            .open(path.as_ref())?;
        file.set_len(total as u64)?;
        let mut mmap = unsafe { MmapOptions::new().len(total).map_mut(&file)? };
        unsafe { init_dir_layout(mmap.as_mut_ptr()) };
        let raw_ptr = mmap.as_mut_ptr();
        Ok(Self { _backing: DirBacking::File(file, mmap), raw_ptr })
    }

    /// Open an existing file-backed directory; validates the magic
    /// and never re-initialises (live claims survive the attach).
    pub fn open(path: impl AsRef<Path>) -> Result<Self, RingError> {
        let total = peer_directory_size();
        let file = OpenOptions::new().read(true).write(true).open(path.as_ref())?;
        if (file.metadata()?.len() as usize) < total {
            return Err(RingError::LayoutMismatch);
        }
        let mmap = unsafe { MmapOptions::new().len(total).map_mut(&file)? };
        let header = unsafe { &*(mmap.as_ptr() as *const DirHeader) };
        if header.magic.load(AtomOrd::Acquire) != DIR_MAGIC {
            return Err(RingError::LayoutMismatch);
        }
        let raw_ptr = mmap.as_ptr() as *mut u8;
        Ok(Self { _backing: DirBacking::File(file, mmap), raw_ptr })
    }

    /// Named-shm directory. `create_or_open` semantics: the region is
    /// initialised only when its magic is absent, so racing attachers
    /// never wipe live claims.
    pub fn create_or_open_shm(name: &str) -> Result<Self, RingError> {
        let mut shm = crate::shm_file::ShmFile::create_or_open_named(
            name, peer_directory_size(),
        ).map_err(|e| RingError::IoError(e.kind()))?;
        if shm.len() < peer_directory_size() {
            return Err(RingError::LayoutMismatch);
        }
        let raw_ptr = shm.as_mut_slice().as_mut_ptr();
        let dir = Self { _backing: DirBacking::Shm(shm), raw_ptr };
        let header = dir.header();
        if header.magic
            .compare_exchange(0, 1, AtomOrd::AcqRel, AtomOrd::Acquire)
            .is_ok()
        {
            // We won the init race: full layout write, then publish
            // the real magic (attachers spin below until it lands).
            unsafe { init_dir_layout(raw_ptr) };
        } else {
            let mut spins = 0u32;
            while dir.header().magic.load(AtomOrd::Acquire) != DIR_MAGIC {
                std::hint::spin_loop();
                spins += 1;
                if spins > 100_000_000 {
                    return Err(RingError::LayoutMismatch);
                }
            }
        }
        Ok(dir)
    }

    #[inline]
    fn header(&self) -> &DirHeader {
        unsafe { &*(self.raw_ptr as *const DirHeader) }
    }

    #[inline]
    fn bitmap_word(&self, base: usize, i: usize) -> &AtomicU64 {
        unsafe { &*(self.raw_ptr.add(base + i * 8) as *const AtomicU64) }
    }

    #[inline]
    fn owner_word(&self, ring: usize) -> &AtomicU64 {
        debug_assert!(ring < PRODUCER_SLOT_CEILING);
        unsafe { &*(self.raw_ptr.add(OFF_OWNERS + ring * 8) as *const AtomicU64) }
    }

    #[inline]
    fn pid_word(&self, slot: usize) -> &AtomicU64 {
        debug_assert!(slot < CONSUMER_SLOT_CEILING);
        unsafe { &*(self.raw_ptr.add(OFF_C_PIDS + slot * 8) as *const AtomicU64) }
    }

    #[inline]
    fn producer_pid_word(&self, slot: usize) -> &AtomicU64 {
        debug_assert!(slot < PRODUCER_SLOT_CEILING);
        unsafe { &*(self.raw_ptr.add(OFF_P_PIDS + slot * 8) as *const AtomicU64) }
    }

    /// Current topology epoch. Hot paths compare this against a
    /// process-local cache; equality means nothing changed.
    #[inline]
    pub fn epoch(&self) -> u64 {
        self.header().epoch.load(AtomOrd::Acquire)
    }

    /// Bump the topology epoch (any peer / publication change).
    pub fn bump_epoch(&self) -> u64 {
        self.header().epoch.fetch_add(1, AtomOrd::AcqRel) + 1
    }

    /// Ring backings published (files exist + initialised).
    #[inline]
    pub fn published(&self) -> usize {
        self.header().published.load(AtomOrd::Acquire) as usize
    }

    /// Advance the published-ring count to `to` after creating the
    /// backing files for every slot below it. Monotone max, so
    /// concurrent growers publishing different highs converge.
    pub fn publish_rings(&self, to: usize) {
        self.header().published.fetch_max(to as u32, AtomOrd::AcqRel);
        self.bump_epoch();
    }

    /// Live producer count across all attached processes.
    #[inline]
    pub fn active_producers(&self) -> usize {
        self.header().active_producers.load(AtomOrd::Acquire) as usize
    }

    /// Live consumer count across all attached processes.
    #[inline]
    pub fn active_consumers(&self) -> usize {
        self.header().active_consumers.load(AtomOrd::Acquire) as usize
    }

    /// Claim the lowest free producer slot. `None` only at the
    /// substrate ceiling ([`PRODUCER_SLOT_CEILING`] CONCURRENT
    /// producers).
    pub fn claim_producer_slot(&self) -> Option<usize> {
        let slot = self.claim_bit(OFF_P_BITMAP, P_WORDS)?;
        self.producer_pid_word(slot).store(std::process::id() as u64, AtomOrd::Release);
        self.header().active_producers.fetch_add(1, AtomOrd::AcqRel);
        self.bump_epoch();
        Some(slot)
    }

    /// Release a producer slot claimed by
    /// [`claim_producer_slot`](Self::claim_producer_slot).
    pub fn release_producer_slot(&self, slot: usize) {
        if slot < PRODUCER_SLOT_CEILING {
            self.producer_pid_word(slot).store(0, AtomOrd::Release);
        }
        if self.release_bit(OFF_P_BITMAP, P_WORDS, slot) {
            self.header().active_producers.fetch_sub(1, AtomOrd::AcqRel);
            self.bump_epoch();
        }
    }

    /// Release every peer slot whose recorded process is gone
    /// (crashed / exited without unregistering). Called from the
    /// topology sync SLOW path only - it probes at most one pid per
    /// claimed slot. Rings owned by reaped consumer slots become
    /// claimable via the normal takeover / claim paths.
    pub fn reap_dead_peers(&self) {
        for slot in self.claimed_slots(OFF_P_BITMAP, P_WORDS) {
            let pid = self.producer_pid_word(slot).load(AtomOrd::Acquire);
            if pid != 0 && pid != std::process::id() as u64
                && !process_alive(pid as u32)
            {
                self.release_producer_slot(slot);
            }
        }
        for slot in self.claimed_slots(OFF_C_BITMAP, C_WORDS) {
            let pid = self.pid_word(slot).load(AtomOrd::Acquire);
            if pid != 0 && pid != std::process::id() as u64
                && !process_alive(pid as u32)
            {
                self.release_consumer_slot(slot);
            }
        }
    }

    fn claimed_slots(&self, base: usize, words: usize) -> Vec<usize> {
        let mut slots = Vec::new();
        for w in 0..words {
            let mut word = self.bitmap_word(base, w).load(AtomOrd::Acquire);
            while word != 0 {
                let bit = word.trailing_zeros() as usize;
                slots.push(w * 64 + bit);
                word &= word - 1;
            }
        }
        slots
    }

    /// Claim the lowest free consumer slot, recording the claiming
    /// process id for the crash-takeover liveness probe.
    pub fn claim_consumer_slot(&self) -> Option<usize> {
        let slot = self.claim_bit(OFF_C_BITMAP, C_WORDS)?;
        self.pid_word(slot).store(std::process::id() as u64, AtomOrd::Release);
        self.header().active_consumers.fetch_add(1, AtomOrd::AcqRel);
        self.bump_epoch();
        Some(slot)
    }

    /// Release a consumer slot. The caller transfers its ring
    /// ownership out FIRST (it is the single owner, so direct owner
    /// writes are safe), then releases.
    pub fn release_consumer_slot(&self, slot: usize) {
        if slot >= CONSUMER_SLOT_CEILING {
            return;
        }
        self.pid_word(slot).store(0, AtomOrd::Release);
        if self.release_bit(OFF_C_BITMAP, C_WORDS, slot) {
            self.header().active_consumers.fetch_sub(1, AtomOrd::AcqRel);
            self.bump_epoch();
        }
    }

    /// Whether `slot` currently holds a consumer claim.
    pub fn consumer_slot_claimed(&self, slot: usize) -> bool {
        if slot >= CONSUMER_SLOT_CEILING {
            return false;
        }
        let word = self.bitmap_word(OFF_C_BITMAP, slot / 64).load(AtomOrd::Acquire);
        word & (1u64 << (slot % 64)) != 0
    }

    /// Dense list of currently-claimed consumer slots (rebalance
    /// input). Snapshot semantics: claims racing the scan are picked
    /// up by the next epoch-triggered rebalance.
    pub fn claimed_consumer_slots(&self) -> Vec<u16> {
        let mut slots = Vec::new();
        for w in 0..C_WORDS {
            let mut word = self.bitmap_word(OFF_C_BITMAP, w).load(AtomOrd::Acquire);
            while word != 0 {
                let bit = word.trailing_zeros() as usize;
                slots.push((w * 64 + bit) as u16);
                word &= word - 1;
            }
        }
        slots
    }

    /// `(owner, pending)` for one ring's owner-table entry.
    #[inline]
    pub fn ring_owner(&self, ring: usize) -> (u16, u16) {
        unpack_owner(self.owner_word(ring).load(AtomOrd::Acquire))
    }

    /// CAS-claim an unowned ring for `me`. The only ownership entry
    /// point that does not go through the current owner, and it
    /// requires owner == OWNER_NONE, so the single-reader invariant
    /// holds by construction.
    pub fn try_claim_ring(&self, ring: usize, me: u16) -> bool {
        let cur = pack_owner(OWNER_NONE, OWNER_NONE);
        self.owner_word(ring)
            .compare_exchange(cur, pack_owner(me, OWNER_NONE),
                              AtomOrd::AcqRel, AtomOrd::Acquire)
            .is_ok()
    }

    /// Request that `ring` move to `target`. The CURRENT owner
    /// applies the handoff on its next scan ([`Self::apply_handoff`]);
    /// until then it keeps draining, so no items strand.
    pub fn request_handoff(&self, ring: usize, target: u16) {
        let word = self.owner_word(ring);
        let mut cur = word.load(AtomOrd::Acquire);
        loop {
            let (owner, _) = unpack_owner(cur);
            if owner == target || owner == OWNER_NONE {
                return; // already there / claimable directly
            }
            match word.compare_exchange(cur, pack_owner(owner, target),
                                        AtomOrd::AcqRel, AtomOrd::Acquire) {
                Ok(_) => return,
                Err(actual) => cur = actual,
            }
        }
    }

    /// Owner-side handoff: if `me` owns `ring` and a handoff is
    /// pending, transfer ownership and return the new owner. Called
    /// from the owner's own pop scan - the single-writer transfer.
    pub fn apply_handoff(&self, ring: usize, me: u16) -> Option<u16> {
        let word = self.owner_word(ring);
        let cur = word.load(AtomOrd::Acquire);
        let (owner, pending) = unpack_owner(cur);
        if owner != me || pending == OWNER_NONE {
            return None;
        }
        match word.compare_exchange(cur, pack_owner(pending, OWNER_NONE),
                                    AtomOrd::AcqRel, AtomOrd::Acquire) {
            Ok(_) => {
                self.bump_epoch();
                Some(pending)
            }
            Err(_) => None,
        }
    }

    /// Direct ownership transfer by the CURRENT owner (unregister
    /// path: the leaving consumer parcels its rings out itself).
    pub fn transfer_ring(&self, ring: usize, me: u16, to: u16) {
        let word = self.owner_word(ring);
        let mut cur = word.load(AtomOrd::Acquire);
        loop {
            let (owner, _) = unpack_owner(cur);
            if owner != me {
                return;
            }
            match word.compare_exchange(cur, pack_owner(to, OWNER_NONE),
                                        AtomOrd::AcqRel, AtomOrd::Acquire) {
                Ok(_) => return,
                Err(actual) => cur = actual,
            }
        }
    }

    /// Crash takeover: steal `ring` from `dead_owner` only when that
    /// slot is unclaimed OR its recorded process is gone. Both cases
    /// preclude a concurrent pop by the old owner, preserving the
    /// single-reader invariant. An alive-but-idle owner is never
    /// stolen from.
    pub fn try_takeover(&self, ring: usize, dead_owner: u16, me: u16) -> bool {
        let slot = dead_owner as usize;
        if self.consumer_slot_claimed(slot) {
            let pid = self.pid_word(slot).load(AtomOrd::Acquire);
            if pid == std::process::id() as u64 || pid == 0 || process_alive(pid as u32) {
                return false;
            }
        }
        let word = self.owner_word(ring);
        let cur = word.load(AtomOrd::Acquire);
        let (owner, _) = unpack_owner(cur);
        if owner != dead_owner {
            return false;
        }
        let swapped = word.compare_exchange(
            cur, pack_owner(me, OWNER_NONE), AtomOrd::AcqRel, AtomOrd::Acquire,
        ).is_ok();
        if swapped {
            self.bump_epoch();
        }
        swapped
    }

    fn claim_bit(&self, base: usize, words: usize) -> Option<usize> {
        for w in 0..words {
            let word = self.bitmap_word(base, w);
            let mut cur = word.load(AtomOrd::Acquire);
            loop {
                if cur == u64::MAX {
                    break; // word full; next word
                }
                let bit = (!cur).trailing_zeros() as usize;
                match word.compare_exchange(cur, cur | (1u64 << bit),
                                            AtomOrd::AcqRel, AtomOrd::Acquire) {
                    Ok(_) => return Some(w * 64 + bit),
                    Err(actual) => cur = actual,
                }
            }
        }
        None
    }

    fn release_bit(&self, base: usize, words: usize, slot: usize) -> bool {
        if slot >= words * 64 {
            return false;
        }
        let word = self.bitmap_word(base, slot / 64);
        let mask = 1u64 << (slot % 64);
        word.fetch_and(!mask, AtomOrd::AcqRel) & mask != 0
    }
}

/// Whether the OS process `pid` is alive. Used only by the crash
/// takeover; a false ALIVE (pid reuse) is the benign direction (the
/// ring stays with the stale owner until an explicit release).
#[cfg(unix)]
fn process_alive(pid: u32) -> bool {
    let rc = unsafe { libc::kill(pid as libc::pid_t, 0) };
    rc == 0
        || std::io::Error::last_os_error().raw_os_error() == Some(libc::EPERM)
}

#[cfg(windows)]
fn process_alive(pid: u32) -> bool {
    use windows_sys::Win32::Foundation::{CloseHandle, STILL_ACTIVE};
    use windows_sys::Win32::System::Threading::{
        GetExitCodeProcess, OpenProcess, PROCESS_QUERY_LIMITED_INFORMATION,
    };
    unsafe {
        let handle = OpenProcess(PROCESS_QUERY_LIMITED_INFORMATION, 0, pid);
        if handle.is_null() {
            // Access denied still proves existence; a missing pid
            // yields ERROR_INVALID_PARAMETER instead.
            return std::io::Error::last_os_error().raw_os_error()
                == Some(5 /* ERROR_ACCESS_DENIED */);
        }
        let mut code: u32 = 0;
        let ok = GetExitCodeProcess(handle, &mut code);
        CloseHandle(handle);
        ok != 0 && code == STILL_ACTIVE as u32
    }
}

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

    #[test]
    fn producer_slots_claim_release_recycle() {
        let dir = PeerDirectory::create_anon().unwrap();
        assert_eq!(dir.claim_producer_slot(), Some(0));
        assert_eq!(dir.claim_producer_slot(), Some(1));
        assert_eq!(dir.claim_producer_slot(), Some(2));
        assert_eq!(dir.active_producers(), 3);
        dir.release_producer_slot(1);
        assert_eq!(dir.active_producers(), 2);
        // Lowest free slot recycles.
        assert_eq!(dir.claim_producer_slot(), Some(1));
        assert_eq!(dir.active_producers(), 3);
    }

    #[test]
    fn epoch_bumps_on_every_topology_change() {
        let dir = PeerDirectory::create_anon().unwrap();
        let e0 = dir.epoch();
        let p = dir.claim_producer_slot().unwrap();
        assert!(dir.epoch() > e0);
        let e1 = dir.epoch();
        dir.release_producer_slot(p);
        assert!(dir.epoch() > e1);
        let e2 = dir.epoch();
        dir.publish_rings(4);
        assert!(dir.epoch() > e2);
        assert_eq!(dir.published(), 4);
        // publish is monotone max.
        dir.publish_rings(2);
        assert_eq!(dir.published(), 4);
    }

    #[test]
    fn ring_ownership_claim_handoff_transfer() {
        let dir = PeerDirectory::create_anon().unwrap();
        assert_eq!(dir.ring_owner(0), (OWNER_NONE, OWNER_NONE));
        assert!(dir.try_claim_ring(0, 3));
        assert!(!dir.try_claim_ring(0, 5), "claimed ring must reject CAS");
        assert_eq!(dir.ring_owner(0), (3, OWNER_NONE));

        // Rebalance request parks as pending until the owner applies.
        dir.request_handoff(0, 7);
        assert_eq!(dir.ring_owner(0), (3, 7));
        assert_eq!(dir.apply_handoff(0, 5), None, "non-owner cannot apply");
        assert_eq!(dir.apply_handoff(0, 3), Some(7));
        assert_eq!(dir.ring_owner(0), (7, OWNER_NONE));

        // Unregister-path direct transfer by the owner.
        dir.transfer_ring(0, 7, 2);
        assert_eq!(dir.ring_owner(0), (2, OWNER_NONE));
    }

    #[test]
    fn takeover_requires_dead_or_released_owner() {
        let dir = PeerDirectory::create_anon().unwrap();
        let slot = dir.claim_consumer_slot().unwrap() as u16;
        assert!(dir.try_claim_ring(0, slot));
        // The claiming slot belongs to THIS live process: never stolen.
        assert!(!dir.try_takeover(0, slot, 9));
        // Released slot: takeover permitted.
        dir.release_consumer_slot(slot as usize);
        assert!(dir.try_takeover(0, slot, 9));
        assert_eq!(dir.ring_owner(0), (9, OWNER_NONE));
    }

    #[test]
    fn consumer_slots_record_pid_and_enumerate_densely() {
        let dir = PeerDirectory::create_anon().unwrap();
        let a = dir.claim_consumer_slot().unwrap();
        let b = dir.claim_consumer_slot().unwrap();
        let c = dir.claim_consumer_slot().unwrap();
        assert_eq!((a, b, c), (0, 1, 2));
        dir.release_consumer_slot(b);
        assert_eq!(dir.claimed_consumer_slots(), vec![0u16, 2]);
        assert_eq!(dir.active_consumers(), 2);
    }

    #[test]
    fn file_backed_directory_shares_claims_across_handles() {
        let base = std::env::temp_dir()
            .join(format!("subetha_peerdir_{}", std::process::id()));
        let path = base.with_extension("peers.bin");
        let dir_a = PeerDirectory::create(&path).unwrap();
        let p = dir_a.claim_producer_slot().unwrap();
        dir_a.publish_rings(p + 1);

        let dir_b = PeerDirectory::open(&path).unwrap();
        assert_eq!(dir_b.active_producers(), 1);
        assert_eq!(dir_b.published(), p + 1);
        assert_eq!(dir_b.claim_producer_slot(), Some(p + 1));
        assert_eq!(dir_a.active_producers(), 2, "claim visible both ways");

        drop(dir_a);
        drop(dir_b);
        std::fs::remove_file(&path).ok();
    }
}