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// Copyright (C) 2023 Luke Shumaker <lukeshu@lukeshu.com>
//
// SPDX-License-Identifier: GPL-2.0-or-later
package caching
import (
"context"
"fmt"
"sync"
)
// NewLRUCache returns a new thread-safe Cache with a simple
// Least-Recently-Used eviction policy.
//
// It is invalid (runtime-panic) to call NewLRUCache with a
// non-positive capacity or a nil source.
func NewLRUCache[K comparable, V any](cap int, src Source[K, V]) Cache[K, V] {
if cap <= 0 {
panic(fmt.Errorf("caching.NewLRUCache: invalid capacity: %v", cap))
}
if src == nil {
panic(fmt.Errorf("caching.NewLRUCache: nil source"))
}
ret := &lruCache[K, V]{
cap: cap,
src: src,
}
for i := 0; i < cap; i++ {
ret.unused.Store(new(LinkedListEntry[lruEntry[K, V]]))
}
return ret
}
type lruEntry[K comparable, V any] struct {
key K
val V
refs int
del chan struct{} // non-nil if a delete is waiting on .refs to drop to zero
}
type lruCache[K comparable, V any] struct {
cap int
src Source[K, V]
mu sync.Mutex
// Pinned entries are in .byName, but not in any LinkedList.
unused LinkedList[lruEntry[K, V]]
evictable LinkedList[lruEntry[K, V]] // only entries with .refs==0
byName map[K]*LinkedListEntry[lruEntry[K, V]]
waiters LinkedList[chan struct{}]
}
// Blocking primitives /////////////////////////////////////////////////////////
// Because of pinning, there might not actually be an available entry
// for us to use/evict. If we need an entry to use or evict, we'll
// call waitForAvail to block until there is en entry that is either
// unused or evictable. We'll give waiters FIFO priority.
func (c *lruCache[K, V]) waitForAvail() {
if !(c.unused.IsEmpty() && c.evictable.IsEmpty()) {
return
}
ch := make(chan struct{})
c.waiters.Store(&LinkedListEntry[chan struct{}]{Value: ch})
c.mu.Unlock()
<-ch
c.mu.Lock()
}
// notifyAvail is called when an entry becomes unused or evictable,
// and wakes up the highest-priority .waitForAvail() waiter (if there
// is one).
func (c *lruCache[K, V]) notifyAvail(entry *LinkedListEntry[lruEntry[K, V]]) {
waiter := c.waiters.Oldest
if waiter == nil {
return
}
c.waiters.Delete(waiter)
close(waiter.Value)
}
// Calling .Delete(k) on an entry that is pinned needs to block until
// the entry is no longer pinned.
func (c *lruCache[K, V]) unlockAndWaitForDel(entry *LinkedListEntry[lruEntry[K, V]]) {
if entry.Value.del == nil {
entry.Value.del = make(chan struct{})
}
ch := entry.Value.del
c.mu.Unlock()
<-ch
}
// notifyOfDel unblocks any calls to .Delete(k), notifying them that
// the entry has been deleted and they can now return.
func (c *lruCache[K, V]) notifyOfDel(entry *LinkedListEntry[arcLiveEntry[K, V]]) {
if entry.Value.del != nil {
close(entry.Value.del)
entry.Value.del = nil
}
}
// Main implementation /////////////////////////////////////////////////////////
// lruReplace is the LRU(c) replacement policy. It returns an entry
// that is not in any list.
func (c *lruCache[K, V]) lruReplace() *LinkedListEntry[lruEntry[K, V]] {
c.waitForAvail()
// If the cache isn't full, no need to do an eviction.
if entry := c.unused.Oldest; entry != nil {
c.unused.Delete(entry)
return entry
}
// Replace the oldest entry.
entry := c.evictable.Oldest
c.evictable.Delete(entry)
delete(c.byName, entry.Value.key)
return entry
}
// Acquire implements the 'Cache' interface.
func (c *lruCache[K, V]) Acquire(ctx context.Context, k K) *V {
c.mu.Lock()
defer c.mu.Unlock()
if c.byName == nil {
c.byName = make(map[K]*LinkedListEntry[lruEntry[K, V]], c.cap)
}
entry := c.byName[k]
if entry != nil {
if entry.Value.refs == 0 {
c.evictable.Delete(entry)
}
entry.Value.refs++
} else {
entry = c.lruReplace()
entry.Value.key = k
c.src.Load(ctx, k, &entry.Value.val)
entry.Value.refs = 1
c.byName[k] = entry
}
return &entry.Value.val
}
// Delete implements the 'Cache' interface.
func (c *lruCache[K, V]) Delete(k K) {
c.mu.Lock()
entry := c.byName[k]
if entry == nil {
return
}
if entry.Value.refs > 0 {
// Let .Release(k) do the deletion when the
// refcount drops to 0.
c.unlockAndWaitForDel(entry)
return
}
delete(c.byName, k)
c.evictable.Delete(entry)
c.unused.Store(entry)
// No need to call c.notifyAvail(); if we were able to delete
// it, it was already available.
c.mu.Unlock()
}
// Release implements the 'Cache' interface.
func (c *lruCache[K, V]) Release(k K) {
c.mu.Lock()
defer c.mu.Unlock()
entry := c.byName[k]
if entry == nil || entry.Value.refs <= 0 {
panic(fmt.Errorf("caching.lruCache.Release called on key that is not held: %v", k))
}
entry.Value.refs--
if entry.Value.refs == 0 {
if entry.Value.del != nil {
delete(c.byName, k)
c.unused.Store(entry)
} else {
c.evictable.Store(entry)
}
c.notifyAvail(entry)
}
}
// Flush implements the 'Cache' interface.
func (c *lruCache[K, V]) Flush(ctx context.Context) {
c.mu.Lock()
defer c.mu.Unlock()
for _, entry := range c.byName {
c.src.Flush(ctx, &entry.Value.val)
}
for entry := c.unused.Oldest; entry != nil; entry = entry.Newer {
c.src.Flush(ctx, &entry.Value.val)
}
}
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