1 files changed, 140 insertions, 0 deletions
diff --git a/src/crypto/internal/boring/bcache/cache.go b/src/crypto/internal/boring/bcache/cache.go
new file mode 100644
index 0000000..7934d03
--- /dev/null
+++ b/src/crypto/internal/boring/bcache/cache.go
@@ -0,0 +1,140 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package bcache implements a GC-friendly cache (see [Cache]) for BoringCrypto.
+package bcache
+
+import (
+	"sync/atomic"
+	"unsafe"
+)
+
+// A Cache is a GC-friendly concurrent map from unsafe.Pointer to
+// unsafe.Pointer. It is meant to be used for maintaining shadow
+// BoringCrypto state associated with certain allocated structs, in
+// particular public and private RSA and ECDSA keys.
+//
+// The cache is GC-friendly in the sense that the keys do not
+// indefinitely prevent the garbage collector from collecting them.
+// Instead, at the start of each GC, the cache is cleared entirely. That
+// is, the cache is lossy, and the loss happens at the start of each GC.
+// This means that clients need to be able to cope with cache entries
+// disappearing, but it also means that clients don't need to worry about
+// cache entries keeping the keys from being collected.
+type Cache[K, V any] struct {
+	// The runtime atomically stores nil to ptable at the start of each GC.
+	ptable atomic.Pointer[cacheTable[K, V]]
+}
+
+type cacheTable[K, V any] [cacheSize]atomic.Pointer[cacheEntry[K, V]]
+
+// A cacheEntry is a single entry in the linked list for a given hash table entry.
+type cacheEntry[K, V any] struct {
+	k    *K                // immutable once created
+	v    atomic.Pointer[V] // read and written atomically to allow updates
+	next *cacheEntry[K, V] // immutable once linked into table
+}
+
+func registerCache(unsafe.Pointer) // provided by runtime
+
+// Register registers the cache with the runtime,
+// so that c.ptable can be cleared at the start of each GC.
+// Register must be called during package initialization.
+func (c *Cache[K, V]) Register() {
+	registerCache(unsafe.Pointer(&c.ptable))
+}
+
+// cacheSize is the number of entries in the hash table.
+// The hash is the pointer value mod cacheSize, a prime.
+// Collisions are resolved by maintaining a linked list in each hash slot.
+const cacheSize = 1021
+
+// table returns a pointer to the current cache hash table,
+// coping with the possibility of the GC clearing it out from under us.
+func (c *Cache[K, V]) table() *cacheTable[K, V] {
+	for {
+		p := c.ptable.Load()
+		if p == nil {
+			p = new(cacheTable[K, V])
+			if !c.ptable.CompareAndSwap(nil, p) {
+				continue
+			}
+		}
+		return p
+	}
+}
+
+// Clear clears the cache.
+// The runtime does this automatically at each garbage collection;
+// this method is exposed only for testing.
+func (c *Cache[K, V]) Clear() {
+	// The runtime does this at the start of every garbage collection
+	// (itself, not by calling this function).
+	c.ptable.Store(nil)
+}
+
+// Get returns the cached value associated with v,
+// which is either the value v corresponding to the most recent call to Put(k, v)
+// or nil if that cache entry has been dropped.
+func (c *Cache[K, V]) Get(k *K) *V {
+	head := &c.table()[uintptr(unsafe.Pointer(k))%cacheSize]
+	e := head.Load()
+	for ; e != nil; e = e.next {
+		if e.k == k {
+			return e.v.Load()
+		}
+	}
+	return nil
+}
+
+// Put sets the cached value associated with k to v.
+func (c *Cache[K, V]) Put(k *K, v *V) {
+	head := &c.table()[uintptr(unsafe.Pointer(k))%cacheSize]
+
+	// Strategy is to walk the linked list at head,
+	// same as in Get, to look for existing entry.
+	// If we find one, we update v atomically in place.
+	// If not, then we race to replace the start = *head
+	// we observed with a new k, v entry.
+	// If we win that race, we're done.
+	// Otherwise, we try the whole thing again,
+	// with two optimizations:
+	//
+	//  1. We track in noK the start of the section of
+	//     the list that we've confirmed has no entry for k.
+	//     The next time down the list, we can stop at noK,
+	//     because new entries are inserted at the front of the list.
+	//     This guarantees we never traverse an entry
+	//     multiple times.
+	//
+	//  2. We only allocate the entry to be added once,
+	//     saving it in add for the next attempt.
+	var add, noK *cacheEntry[K, V]
+	n := 0
+	for {
+		e := head.Load()
+		start := e
+		for ; e != nil && e != noK; e = e.next {
+			if e.k == k {
+				e.v.Store(v)
+				return
+			}
+			n++
+		}
+		if add == nil {
+			add = &cacheEntry[K, V]{k: k}
+			add.v.Store(v)
+		}
+		add.next = start
+		if n >= 1000 {
+			// If an individual list gets too long, which shouldn't happen,
+			// throw it away to avoid quadratic lookup behavior.
+			add.next = nil
+		}
+		if head.CompareAndSwap(start, add) {
+			return
+		}
+		noK = start
+	}
+}