libp2p_kad/

kbucket.rs

// Copyright 2018 Parity Technologies (UK) Ltd.
//
// Permission is hereby granted, free of charge, to any person obtaining a
// copy of this software and associated documentation files (the "Software"),
// to deal in the Software without restriction, including without limitation
// the rights to use, copy, modify, merge, publish, distribute, sublicense,
// and/or sell copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.

//! Implementation of a Kademlia routing table as used by a single peer
//! participating in a Kademlia DHT.
//!
//! The entry point for the API of this module is a [`KBucketsTable`].
//!
//! ## Pending Insertions
//!
//! When the bucket associated with the `Key` of an inserted entry is full
//! but contains disconnected nodes, it accepts a [`PendingEntry`].
//! Pending entries are inserted lazily when their timeout is found to be expired
//! upon querying the `KBucketsTable`. When that happens, the `KBucketsTable` records
//! an [`AppliedPending`] result which must be consumed by calling [`take_applied_pending`]
//! regularly and / or after performing lookup operations like [`entry`] and [`closest`].
//!
//! [`entry`]: KBucketsTable::entry
//! [`closest`]: KBucketsTable::closest
//! [`AppliedPending`]: bucket::AppliedPending
//! [`take_applied_pending`]: KBucketsTable::take_applied_pending
//! [`PendingEntry`]: entry::PendingEntry

// [Implementation Notes]
//
// 1. Routing Table Layout
//
// The routing table is currently implemented as a fixed-size "array" of
// buckets, ordered by increasing distance relative to a local key
// that identifies the local peer. This is an often-used, simplified
// implementation that approximates the properties of the b-tree (or prefix tree)
// implementation described in the full paper [0], whereby buckets are split on-demand.
// This should be treated as an implementation detail, however, so that the
// implementation may change in the future without breaking the API.
//
// 2. Replacement Cache
//
// In this implementation, the "replacement cache" for unresponsive peers
// consists of a single entry per bucket. Furthermore, this implementation is
// currently tailored to connection-oriented transports, meaning that the
// "LRU"-based ordering of entries in a bucket is actually based on the last reported
// connection status of the corresponding peers, from least-recently (dis)connected to
// most-recently (dis)connected, and controlled through the `Entry` API. As a result,
// the nodes in the buckets are not reordered as a result of RPC activity, but only as a
// result of nodes being marked as connected or disconnected. In particular,
// if a bucket is full and contains only entries for peers that are considered
// connected, no pending entry is accepted. See the `bucket` submodule for
// further details.
//
// [0]: https://pdos.csail.mit.edu/~petar/papers/maymounkov-kademlia-lncs.pdf

mod bucket;
mod entry;
#[allow(clippy::ptr_offset_with_cast)]
#[allow(clippy::assign_op_pattern)]
mod key;

pub use bucket::NodeStatus;
pub use entry::*;

use arrayvec::{self, ArrayVec};
use bucket::KBucket;
use std::collections::VecDeque;
use std::time::{Duration, Instant};

/// Maximum number of k-buckets.
const NUM_BUCKETS: usize = 256;

/// A `KBucketsTable` represents a Kademlia routing table.
#[derive(Debug, Clone)]
pub(crate) struct KBucketsTable<TKey, TVal> {
    /// The key identifying the local peer that owns the routing table.
    local_key: TKey,
    /// The buckets comprising the routing table.
    buckets: Vec<KBucket<TKey, TVal>>,
    /// The list of evicted entries that have been replaced with pending
    /// entries since the last call to [`KBucketsTable::take_applied_pending`].
    applied_pending: VecDeque<AppliedPending<TKey, TVal>>,
}

/// A (type-safe) index into a `KBucketsTable`, i.e. a non-negative integer in the
/// interval `[0, NUM_BUCKETS)`.
#[derive(Debug, Copy, Clone, PartialEq, Eq)]
struct BucketIndex(usize);

impl BucketIndex {
    /// Creates a new `BucketIndex` for a `Distance`.
    ///
    /// The given distance is interpreted as the distance from a `local_key` of
    /// a `KBucketsTable`. If the distance is zero, `None` is returned, in
    /// recognition of the fact that the only key with distance `0` to a
    /// `local_key` is the `local_key` itself, which does not belong in any
    /// bucket.
    fn new(d: &Distance) -> Option<BucketIndex> {
        d.ilog2().map(|i| BucketIndex(i as usize))
    }

    /// Gets the index value as an unsigned integer.
    fn get(&self) -> usize {
        self.0
    }

    /// Returns the minimum inclusive and maximum inclusive [`Distance`]
    /// included in the bucket for this index.
    fn range(&self) -> (Distance, Distance) {
        let min = Distance(U256::pow(U256::from(2), U256::from(self.0)));
        if self.0 == usize::from(u8::MAX) {
            (min, Distance(U256::MAX))
        } else {
            let max = Distance(U256::pow(U256::from(2), U256::from(self.0 + 1)) - 1);
            (min, max)
        }
    }

    /// Generates a random distance that falls into the bucket for this index.
    fn rand_distance(&self, rng: &mut impl rand::Rng) -> Distance {
        let mut bytes = [0u8; 32];
        let quot = self.0 / 8;
        for i in 0..quot {
            bytes[31 - i] = rng.gen();
        }
        let rem = (self.0 % 8) as u32;
        let lower = usize::pow(2, rem);
        let upper = usize::pow(2, rem + 1);
        bytes[31 - quot] = rng.gen_range(lower..upper) as u8;
        Distance(U256::from(bytes))
    }
}

impl<TKey, TVal> KBucketsTable<TKey, TVal>
where
    TKey: Clone + AsRef<KeyBytes>,
    TVal: Clone,
{
    /// Creates a new, empty Kademlia routing table with entries partitioned
    /// into buckets as per the Kademlia protocol.
    ///
    /// The given `pending_timeout` specifies the duration after creation of
    /// a [`PendingEntry`] after which it becomes eligible for insertion into
    /// a full bucket, replacing the least-recently (dis)connected node.
    pub(crate) fn new(local_key: TKey, pending_timeout: Duration) -> Self {
        KBucketsTable {
            local_key,
            buckets: (0..NUM_BUCKETS)
                .map(|_| KBucket::new(pending_timeout))
                .collect(),
            applied_pending: VecDeque::new(),
        }
    }

    /// Returns the local key.
    pub(crate) fn local_key(&self) -> &TKey {
        &self.local_key
    }

    /// Returns an `Entry` for the given key, representing the state of the entry
    /// in the routing table.
    pub(crate) fn entry<'a>(&'a mut self, key: &'a TKey) -> Entry<'a, TKey, TVal> {
        let index = BucketIndex::new(&self.local_key.as_ref().distance(key));
        if let Some(i) = index {
            let bucket = &mut self.buckets[i.get()];
            if let Some(applied) = bucket.apply_pending() {
                self.applied_pending.push_back(applied)
            }
            Entry::new(bucket, key)
        } else {
            Entry::SelfEntry
        }
    }

    /// Returns an iterator over all buckets.
    ///
    /// The buckets are ordered by proximity to the `local_key`, i.e. the first
    /// bucket is the closest bucket (containing at most one key).
    pub(crate) fn iter(&mut self) -> impl Iterator<Item = KBucketRef<'_, TKey, TVal>> + '_ {
        let applied_pending = &mut self.applied_pending;
        self.buckets.iter_mut().enumerate().map(move |(i, b)| {
            if let Some(applied) = b.apply_pending() {
                applied_pending.push_back(applied)
            }
            KBucketRef {
                index: BucketIndex(i),
                bucket: b,
            }
        })
    }

    /// Returns the bucket for the distance to the given key.
    ///
    /// Returns `None` if the given key refers to the local key.
    pub(crate) fn bucket<K>(&mut self, key: &K) -> Option<KBucketRef<'_, TKey, TVal>>
    where
        K: AsRef<KeyBytes>,
    {
        let d = self.local_key.as_ref().distance(key);
        if let Some(index) = BucketIndex::new(&d) {
            let bucket = &mut self.buckets[index.0];
            if let Some(applied) = bucket.apply_pending() {
                self.applied_pending.push_back(applied)
            }
            Some(KBucketRef { bucket, index })
        } else {
            None
        }
    }

    /// Consumes the next applied pending entry, if any.
    ///
    /// When an entry is attempted to be inserted and the respective bucket is full,
    /// it may be recorded as pending insertion after a timeout, see [`InsertResult::Pending`].
    ///
    /// If the oldest currently disconnected entry in the respective bucket does not change
    /// its status until the timeout of pending entry expires, it is evicted and
    /// the pending entry inserted instead. These insertions of pending entries
    /// happens lazily, whenever the `KBucketsTable` is accessed, and the corresponding
    /// buckets are updated accordingly. The fact that a pending entry was applied is
    /// recorded in the `KBucketsTable` in the form of `AppliedPending` results, which must be
    /// consumed by calling this function.
    pub(crate) fn take_applied_pending(&mut self) -> Option<AppliedPending<TKey, TVal>> {
        self.applied_pending.pop_front()
    }

    /// Returns an iterator over the keys closest to `target`, ordered by
    /// increasing distance.
    pub(crate) fn closest_keys<'a, T>(
        &'a mut self,
        target: &'a T,
    ) -> impl Iterator<Item = TKey> + 'a
    where
        T: AsRef<KeyBytes>,
    {
        let distance = self.local_key.as_ref().distance(target);
        ClosestIter {
            target,
            iter: None,
            table: self,
            buckets_iter: ClosestBucketsIter::new(distance),
            fmap: |b: &KBucket<TKey, _>| -> ArrayVec<_, { K_VALUE.get() }> {
                b.iter().map(|(n, _)| n.key.clone()).collect()
            },
        }
    }

    /// Returns an iterator over the nodes closest to the `target` key, ordered by
    /// increasing distance.
    pub(crate) fn closest<'a, T>(
        &'a mut self,
        target: &'a T,
    ) -> impl Iterator<Item = EntryView<TKey, TVal>> + 'a
    where
        T: Clone + AsRef<KeyBytes>,
        TVal: Clone,
    {
        let distance = self.local_key.as_ref().distance(target);
        ClosestIter {
            target,
            iter: None,
            table: self,
            buckets_iter: ClosestBucketsIter::new(distance),
            fmap: |b: &KBucket<_, TVal>| -> ArrayVec<_, { K_VALUE.get() }> {
                b.iter()
                    .map(|(n, status)| EntryView {
                        node: n.clone(),
                        status,
                    })
                    .collect()
            },
        }
    }

    /// Counts the number of nodes between the local node and the node
    /// closest to `target`.
    ///
    /// The number of nodes between the local node and the target are
    /// calculated by backtracking from the target towards the local key.
    pub(crate) fn count_nodes_between<T>(&mut self, target: &T) -> usize
    where
        T: AsRef<KeyBytes>,
    {
        let local_key = self.local_key.clone();
        let distance = target.as_ref().distance(&local_key);
        let mut iter = ClosestBucketsIter::new(distance).take_while(|i| i.get() != 0);
        if let Some(i) = iter.next() {
            let num_first = self.buckets[i.get()]
                .iter()
                .filter(|(n, _)| n.key.as_ref().distance(&local_key) <= distance)
                .count();
            let num_rest: usize = iter.map(|i| self.buckets[i.get()].num_entries()).sum();
            num_first + num_rest
        } else {
            0
        }
    }
}

/// An iterator over (some projection of) the closest entries in a
/// `KBucketsTable` w.r.t. some target `Key`.
struct ClosestIter<'a, TTarget, TKey, TVal, TMap, TOut> {
    /// A reference to the target key whose distance to the local key determines
    /// the order in which the buckets are traversed. The resulting
    /// array from projecting the entries of each bucket using `fmap` is
    /// sorted according to the distance to the target.
    target: &'a TTarget,
    /// A reference to all buckets of the `KBucketsTable`.
    table: &'a mut KBucketsTable<TKey, TVal>,
    /// The iterator over the bucket indices in the order determined by the
    /// distance of the local key to the target.
    buckets_iter: ClosestBucketsIter,
    /// The iterator over the entries in the currently traversed bucket.
    iter: Option<arrayvec::IntoIter<TOut, { K_VALUE.get() }>>,
    /// The projection function / mapping applied on each bucket as
    /// it is encountered, producing the next `iter`ator.
    fmap: TMap,
}

/// An iterator over the bucket indices, in the order determined by the `Distance` of
/// a target from the `local_key`, such that the entries in the buckets are incrementally
/// further away from the target, starting with the bucket covering the target.
struct ClosestBucketsIter {
    /// The distance to the `local_key`.
    distance: Distance,
    /// The current state of the iterator.
    state: ClosestBucketsIterState,
}

/// Operating states of a `ClosestBucketsIter`.
enum ClosestBucketsIterState {
    /// The starting state of the iterator yields the first bucket index and
    /// then transitions to `ZoomIn`.
    Start(BucketIndex),
    /// The iterator "zooms in" to to yield the next bucket cotaining nodes that
    /// are incrementally closer to the local node but further from the `target`.
    /// These buckets are identified by a `1` in the corresponding bit position
    /// of the distance bit string. When bucket `0` is reached, the iterator
    /// transitions to `ZoomOut`.
    ZoomIn(BucketIndex),
    /// Once bucket `0` has been reached, the iterator starts "zooming out"
    /// to buckets containing nodes that are incrementally further away from
    /// both the local key and the target. These are identified by a `0` in
    /// the corresponding bit position of the distance bit string. When bucket
    /// `255` is reached, the iterator transitions to state `Done`.
    ZoomOut(BucketIndex),
    /// The iterator is in this state once it has visited all buckets.
    Done,
}

impl ClosestBucketsIter {
    fn new(distance: Distance) -> Self {
        let state = match BucketIndex::new(&distance) {
            Some(i) => ClosestBucketsIterState::Start(i),
            None => ClosestBucketsIterState::Start(BucketIndex(0)),
        };
        Self { distance, state }
    }

    fn next_in(&self, i: BucketIndex) -> Option<BucketIndex> {
        (0..i.get()).rev().find_map(|i| {
            if self.distance.0.bit(i) {
                Some(BucketIndex(i))
            } else {
                None
            }
        })
    }

    fn next_out(&self, i: BucketIndex) -> Option<BucketIndex> {
        (i.get() + 1..NUM_BUCKETS).find_map(|i| {
            if !self.distance.0.bit(i) {
                Some(BucketIndex(i))
            } else {
                None
            }
        })
    }
}

impl Iterator for ClosestBucketsIter {
    type Item = BucketIndex;

    fn next(&mut self) -> Option<Self::Item> {
        match self.state {
            ClosestBucketsIterState::Start(i) => {
                self.state = ClosestBucketsIterState::ZoomIn(i);
                Some(i)
            }
            ClosestBucketsIterState::ZoomIn(i) => {
                if let Some(i) = self.next_in(i) {
                    self.state = ClosestBucketsIterState::ZoomIn(i);
                    Some(i)
                } else {
                    let i = BucketIndex(0);
                    self.state = ClosestBucketsIterState::ZoomOut(i);
                    Some(i)
                }
            }
            ClosestBucketsIterState::ZoomOut(i) => {
                if let Some(i) = self.next_out(i) {
                    self.state = ClosestBucketsIterState::ZoomOut(i);
                    Some(i)
                } else {
                    self.state = ClosestBucketsIterState::Done;
                    None
                }
            }
            ClosestBucketsIterState::Done => None,
        }
    }
}

impl<TTarget, TKey, TVal, TMap, TOut> Iterator for ClosestIter<'_, TTarget, TKey, TVal, TMap, TOut>
where
    TTarget: AsRef<KeyBytes>,
    TKey: Clone + AsRef<KeyBytes>,
    TVal: Clone,
    TMap: Fn(&KBucket<TKey, TVal>) -> ArrayVec<TOut, { K_VALUE.get() }>,
    TOut: AsRef<KeyBytes>,
{
    type Item = TOut;

    fn next(&mut self) -> Option<Self::Item> {
        loop {
            match &mut self.iter {
                Some(iter) => match iter.next() {
                    Some(k) => return Some(k),
                    None => self.iter = None,
                },
                None => {
                    if let Some(i) = self.buckets_iter.next() {
                        let bucket = &mut self.table.buckets[i.get()];
                        if let Some(applied) = bucket.apply_pending() {
                            self.table.applied_pending.push_back(applied)
                        }
                        let mut v = (self.fmap)(bucket);
                        v.sort_by(|a, b| {
                            self.target
                                .as_ref()
                                .distance(a.as_ref())
                                .cmp(&self.target.as_ref().distance(b.as_ref()))
                        });
                        self.iter = Some(v.into_iter());
                    } else {
                        return None;
                    }
                }
            }
        }
    }
}

/// A reference to a bucket.
pub struct KBucketRef<'a, TKey, TVal> {
    index: BucketIndex,
    bucket: &'a mut KBucket<TKey, TVal>,
}

impl<'a, TKey, TVal> KBucketRef<'a, TKey, TVal>
where
    TKey: Clone + AsRef<KeyBytes>,
    TVal: Clone,
{
    /// Returns the minimum inclusive and maximum inclusive distance for
    /// this bucket.
    pub fn range(&self) -> (Distance, Distance) {
        self.index.range()
    }

    /// Checks whether the bucket is empty.
    pub fn is_empty(&self) -> bool {
        self.num_entries() == 0
    }

    /// Returns the number of entries in the bucket.
    pub fn num_entries(&self) -> usize {
        self.bucket.num_entries()
    }

    /// Returns true if the bucket has a pending node.
    pub fn has_pending(&self) -> bool {
        self.bucket.pending().map_or(false, |n| !n.is_ready())
    }

    /// Tests whether the given distance falls into this bucket.
    pub fn contains(&self, d: &Distance) -> bool {
        BucketIndex::new(d).map_or(false, |i| i == self.index)
    }

    /// Generates a random distance that falls into this bucket.
    ///
    /// Together with a known key `a` (e.g. the local key), a random distance `d` for
    /// this bucket w.r.t `k` gives rise to the corresponding (random) key `b` s.t.
    /// the XOR distance between `a` and `b` is `d`. In other words, it gives
    /// rise to a random key falling into this bucket. See [`key::Key::for_distance`].
    pub fn rand_distance(&self, rng: &mut impl rand::Rng) -> Distance {
        self.index.rand_distance(rng)
    }

    /// Returns an iterator over the entries in the bucket.
    pub fn iter(&'a self) -> impl Iterator<Item = EntryRefView<'a, TKey, TVal>> {
        self.bucket.iter().map(move |(n, status)| EntryRefView {
            node: NodeRefView {
                key: &n.key,
                value: &n.value,
            },
            status,
        })
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use libp2p_identity::PeerId;
    use quickcheck::*;

    type TestTable = KBucketsTable<KeyBytes, ()>;

    impl Arbitrary for TestTable {
        fn arbitrary(g: &mut Gen) -> TestTable {
            let local_key = Key::from(PeerId::random());
            let timeout = Duration::from_secs(g.gen_range(1..360));
            let mut table = TestTable::new(local_key.clone().into(), timeout);
            let mut num_total = g.gen_range(0..100);
            for (i, b) in &mut table.buckets.iter_mut().enumerate().rev() {
                let ix = BucketIndex(i);
                let num = g.gen_range(0..usize::min(K_VALUE.get(), num_total) + 1);
                num_total -= num;
                for _ in 0..num {
                    let distance = ix.rand_distance(&mut rand::thread_rng());
                    let key = local_key.for_distance(distance);
                    let node = Node {
                        key: key.clone(),
                        value: (),
                    };
                    let status = NodeStatus::arbitrary(g);
                    match b.insert(node, status) {
                        InsertResult::Inserted => {}
                        _ => panic!(),
                    }
                }
            }
            table
        }
    }

    #[test]
    fn buckets_are_non_overlapping_and_exhaustive() {
        let local_key = Key::from(PeerId::random());
        let timeout = Duration::from_secs(0);
        let mut table = KBucketsTable::<KeyBytes, ()>::new(local_key.into(), timeout);

        let mut prev_max = U256::from(0);

        for bucket in table.iter() {
            let (min, max) = bucket.range();
            assert_eq!(Distance(prev_max + U256::from(1)), min);
            prev_max = max.0;
        }

        assert_eq!(U256::MAX, prev_max);
    }

    #[test]
    fn bucket_contains_range() {
        fn prop(ix: u8) {
            let index = BucketIndex(ix as usize);
            let mut bucket = KBucket::<Key<PeerId>, ()>::new(Duration::from_secs(0));
            let bucket_ref = KBucketRef {
                index,
                bucket: &mut bucket,
            };

            let (min, max) = bucket_ref.range();

            assert!(min <= max);

            assert!(bucket_ref.contains(&min));
            assert!(bucket_ref.contains(&max));

            if min != Distance(0.into()) {
                // ^ avoid underflow
                assert!(!bucket_ref.contains(&Distance(min.0 - 1)));
            }

            if max != Distance(U256::max_value()) {
                // ^ avoid overflow
                assert!(!bucket_ref.contains(&Distance(max.0 + 1)));
            }
        }

        quickcheck(prop as fn(_));
    }

    #[test]
    fn rand_distance() {
        fn prop(ix: u8) -> bool {
            let d = BucketIndex(ix as usize).rand_distance(&mut rand::thread_rng());
            let n = U256::from(<[u8; 32]>::from(d.0));
            let b = U256::from(2);
            let e = U256::from(ix);
            let lower = b.pow(e);
            let upper = b.checked_pow(e + U256::from(1)).unwrap_or(U256::MAX) - U256::from(1);
            lower <= n && n <= upper
        }
        quickcheck(prop as fn(_) -> _);
    }

    #[test]
    fn entry_inserted() {
        let local_key = Key::from(PeerId::random());
        let other_id = Key::from(PeerId::random());

        let mut table = KBucketsTable::<_, ()>::new(local_key, Duration::from_secs(5));
        if let Entry::Absent(entry) = table.entry(&other_id) {
            match entry.insert((), NodeStatus::Connected) {
                InsertResult::Inserted => (),
                _ => panic!(),
            }
        } else {
            panic!()
        }

        let res = table.closest_keys(&other_id).collect::<Vec<_>>();
        assert_eq!(res.len(), 1);
        assert_eq!(res[0], other_id);
    }

    #[test]
    fn entry_self() {
        let local_key = Key::from(PeerId::random());
        let mut table = KBucketsTable::<_, ()>::new(local_key.clone(), Duration::from_secs(5));
        match table.entry(&local_key) {
            Entry::SelfEntry => (),
            _ => panic!(),
        }
    }

    #[test]
    fn closest() {
        let local_key = Key::from(PeerId::random());
        let mut table = KBucketsTable::<_, ()>::new(local_key, Duration::from_secs(5));
        let mut count = 0;
        loop {
            if count == 100 {
                break;
            }
            let key = Key::from(PeerId::random());
            if let Entry::Absent(e) = table.entry(&key) {
                match e.insert((), NodeStatus::Connected) {
                    InsertResult::Inserted => count += 1,
                    _ => continue,
                }
            } else {
                panic!("entry exists")
            }
        }

        let mut expected_keys: Vec<_> = table
            .buckets
            .iter()
            .flat_map(|t| t.iter().map(|(n, _)| n.key.clone()))
            .collect();

        for _ in 0..10 {
            let target_key = Key::from(PeerId::random());
            let keys = table.closest_keys(&target_key).collect::<Vec<_>>();
            // The list of keys is expected to match the result of a full-table scan.
            expected_keys.sort_by_key(|k| k.distance(&target_key));
            assert_eq!(keys, expected_keys);
        }
    }

    #[test]
    fn applied_pending() {
        let local_key = Key::from(PeerId::random());
        let mut table = KBucketsTable::<_, ()>::new(local_key.clone(), Duration::from_millis(1));
        let expected_applied;
        let full_bucket_index;
        loop {
            let key = Key::from(PeerId::random());
            if let Entry::Absent(e) = table.entry(&key) {
                match e.insert((), NodeStatus::Disconnected) {
                    InsertResult::Full => {
                        if let Entry::Absent(e) = table.entry(&key) {
                            match e.insert((), NodeStatus::Connected) {
                                InsertResult::Pending { disconnected } => {
                                    expected_applied = AppliedPending {
                                        inserted: Node {
                                            key: key.clone(),
                                            value: (),
                                        },
                                        evicted: Some(Node {
                                            key: disconnected,
                                            value: (),
                                        }),
                                    };
                                    full_bucket_index = BucketIndex::new(&key.distance(&local_key));
                                    break;
                                }
                                _ => panic!(),
                            }
                        } else {
                            panic!()
                        }
                    }
                    _ => continue,
                }
            } else {
                panic!("entry exists")
            }
        }

        // Expire the timeout for the pending entry on the full bucket.`
        let full_bucket = &mut table.buckets[full_bucket_index.unwrap().get()];
        let elapsed = Instant::now().checked_sub(Duration::from_secs(1)).unwrap();
        full_bucket.pending_mut().unwrap().set_ready_at(elapsed);

        match table.entry(&expected_applied.inserted.key) {
            Entry::Present(_, NodeStatus::Connected) => {}
            x => panic!("Unexpected entry: {x:?}"),
        }

        match table.entry(&expected_applied.evicted.as_ref().unwrap().key) {
            Entry::Absent(_) => {}
            x => panic!("Unexpected entry: {x:?}"),
        }

        assert_eq!(Some(expected_applied), table.take_applied_pending());
        assert_eq!(None, table.take_applied_pending());
    }

    #[test]
    fn count_nodes_between() {
        fn prop(mut table: TestTable, target: Key<PeerId>) -> bool {
            let num_to_target = table.count_nodes_between(&target);
            let distance = table.local_key.distance(&target);
            let base2 = U256::from(2);
            let mut iter = ClosestBucketsIter::new(distance);
            iter.all(|i| {
                // Flip the distance bit related to the bucket.
                let d = Distance(distance.0 ^ (base2.pow(U256::from(i.get()))));
                let k = table.local_key.for_distance(d);
                if distance.0.bit(i.get()) {
                    // Bit flip `1` -> `0`, the key must be closer than `target`.
                    d < distance && table.count_nodes_between(&k) <= num_to_target
                } else {
                    // Bit flip `0` -> `1`, the key must be farther than `target`.
                    d > distance && table.count_nodes_between(&k) >= num_to_target
                }
            })
        }

        QuickCheck::new()
            .tests(10)
            .quickcheck(prop as fn(_, _) -> _)
    }
}