Use a new fixed-size map for path secret storage

This tightly bounds the maximum memory usage of the path secret storage.

dc/s2n-quic-dc/Cargo.toml

@@ -41,6 +41,7 @@ tokio = { version = "1", default-features = false, features = ["sync"] }
 tracing = "0.1"
 zerocopy = { version = "0.7", features = ["derive"] }
 zeroize = "1"
+parking_lot = "0.12"
 
 [dev-dependencies]
 bolero = "0.11"

dc/s2n-quic-dc/src/fixed_map.rs

@@ -0,0 +1,170 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+//! A fixed-allocation concurrent HashMap.
+//!
+//! This implements a concurrent map backed by a fixed-size allocation created at construction
+//! time, with a fixed memory footprint. The expectation is that all storage is inline (to the
+//! extent possible) reducing the likelihood.
+
+use core::{
+    hash::Hash,
+    sync::atomic::{AtomicU8, Ordering},
+};
+use parking_lot::{MappedRwLockReadGuard, RwLock, RwLockReadGuard, RwLockUpgradableReadGuard};
+use std::{collections::hash_map::RandomState, hash::BuildHasher};
+
+pub struct Map<K, V, S = RandomState> {
+    slots: Box<[Slot<K, V>]>,
+    hash_builder: S,
+}
+
+impl<K, V, S> Map<K, V, S>
+where
+    K: Hash + Eq,
+    S: BuildHasher,
+{
+    pub fn with_capacity(entries: usize, hasher: S) -> Self {
+        let map = Map {
+            slots: (0..std::cmp::min(1, (entries + SLOT_CAPACITY) / SLOT_CAPACITY))
+                .map(|_| Slot::new())
+                .collect::<Vec<_>>()
+                .into_boxed_slice(),
+            hash_builder: hasher,
+        };
+        assert!(map.slots.len().is_power_of_two());
+        assert!(u32::try_from(map.slots.len()).is_ok());
+        map
+    }
+
+    pub fn clear(&self) {
+        for slot in self.slots.iter() {
+            slot.clear();
+        }
+    }
+
+    pub fn len(&self) -> usize {
+        self.slots.iter().map(|s| s.len()).sum()
+    }
+
+    // can't lend references to values outside of a lock, so Iterator interface doesn't work
+    #[allow(unused)]
+    pub fn iter(&self, mut f: impl FnMut(&K, &V)) {
+        for slot in self.slots.iter() {
+            // this feels more readable than flatten
+            #[allow(clippy::manual_flatten)]
+            for entry in slot.values.read().iter() {
+                if let Some(v) = entry {
+                    f(&v.0, &v.1);
+                }
+            }
+        }
+    }
+
+    pub fn retain(&self, mut f: impl FnMut(&K, &V) -> bool) {
+        for slot in self.slots.iter() {
+            // this feels more readable than flatten
+            #[allow(clippy::manual_flatten)]
+            for entry in slot.values.write().iter_mut() {
+                if let Some(v) = entry {
+                    if !f(&v.0, &v.1) {
+                        *entry = None;
+                    }
+                }
+            }
+        }
+    }
+
+    fn slot_by_hash(&self, key: &K) -> &Slot<K, V> {
+        let hash = self.hash_builder.hash_one(key);
+        // needed for bit-and modulus, checked in new as a non-debug assert!.
+        debug_assert!(self.slots.len().is_power_of_two());
+        let slot_idx = hash as usize & (self.slots.len() - 1);
+        &self.slots[slot_idx]
+    }
+
+    /// Returns Some(v) if overwriting a previous value for the same key.
+    pub fn insert(&self, key: K, value: V) -> Option<V> {
+        self.slot_by_hash(&key).put(key, value)
+    }
+
+    pub fn contains_key(&self, key: &K) -> bool {
+        self.get_by_key(key).is_some()
+    }
+
+    pub fn get_by_key(&self, key: &K) -> Option<MappedRwLockReadGuard<'_, V>> {
+        self.slot_by_hash(key).get_by_key(key)
+    }
+}
+
+// Balance of speed of access (put or get) and likelihood of false positive eviction.
+const SLOT_CAPACITY: usize = 32;
+
+struct Slot<K, V> {
+    next_write: AtomicU8,
+    values: RwLock<[Option<(K, V)>; SLOT_CAPACITY]>,
+}
+
+impl<K, V> Slot<K, V>
+where
+    K: Hash + Eq,
+{
+    fn new() -> Self {
+        Slot {
+            next_write: AtomicU8::new(0),
+            values: RwLock::new(std::array::from_fn(|_| None)),
+        }
+    }
+
+    fn clear(&self) {
+        *self.values.write() = std::array::from_fn(|_| None);
+    }
+
+    /// Returns Some(v) if overwriting a previous value for the same key.
+    fn put(&self, new_key: K, new_value: V) -> Option<V> {
+        let values = self.values.upgradable_read();
+        for (value_idx, value) in values.iter().enumerate() {
+            // overwrite if same key or if no key/value pair yet
+            if value.as_ref().map_or(true, |(k, _)| *k == new_key) {
+                let mut values = RwLockUpgradableReadGuard::upgrade(values);
+                let old = values[value_idx].take().map(|v| v.1);
+                values[value_idx] = Some((new_key, new_value));
+                return old;
+            }
+        }
+
+        let mut values = RwLockUpgradableReadGuard::upgrade(values);
+
+        // If `new_key` isn't already in this slot, replace one of the existing entries with the
+        // new key. For now we rotate through based on `next_write`.
+        let replacement = self.next_write.fetch_add(1, Ordering::Relaxed) as usize % SLOT_CAPACITY;
+        values[replacement] = Some((new_key, new_value));
+        None
+    }
+
+    fn get_by_key(&self, needle: &K) -> Option<MappedRwLockReadGuard<'_, V>> {
+        // Scan each value and check if our requested needle is present.
+        let values = self.values.read();
+        for (value_idx, value) in values.iter().enumerate() {
+            if value.as_ref().map_or(false, |(k, _)| *k == *needle) {
+                return Some(RwLockReadGuard::map(values, |values| {
+                    &values[value_idx].as_ref().unwrap().1
+                }));
+            }
+        }
+
+        None
+    }
+
+    fn len(&self) -> usize {
+        let values = self.values.read();
+        let mut len = 0;
+        for value in values.iter().enumerate() {
+            len += value.1.is_some() as usize;
+        }
+        len
+    }
+}
+
+#[cfg(test)]
+mod test;

dc/s2n-quic-dc/src/fixed_map/test.rs

@@ -0,0 +1,33 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+use super::*;
+
+#[test]
+fn slot_insert_and_get() {
+    let slot = Slot::new();
+    assert!(slot.get_by_key(&3).is_none());
+    assert_eq!(slot.put(3, "key 1"), None);
+    // still same slot, but new generation
+    assert_eq!(slot.put(3, "key 2"), Some("key 1"));
+    // still same slot, but new generation
+    assert_eq!(slot.put(3, "key 3"), Some("key 2"));
+
+    // new slot
+    assert_eq!(slot.put(5, "key 4"), None);
+    assert_eq!(slot.put(6, "key 4"), None);
+}
+
+#[test]
+fn slot_clear() {
+    let slot = Slot::new();
+    assert_eq!(slot.put(3, "key 1"), None);
+    // still same slot, but new generation
+    assert_eq!(slot.put(3, "key 2"), Some("key 1"));
+    // still same slot, but new generation
+    assert_eq!(slot.put(3, "key 3"), Some("key 2"));
+
+    slot.clear();
+
+    assert_eq!(slot.len(), 0);
+}

dc/s2n-quic-dc/src/lib.rs

@@ -10,6 +10,7 @@ pub mod control;
 pub mod credentials;
 pub mod crypto;
 pub mod datagram;
+mod fixed_map;
 pub mod msg;
 pub mod packet;
 pub mod path;