128 lines
2.8 KiB
Go
128 lines
2.8 KiB
Go
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// Copyright (C) 2020 Storj Labs, Inc.
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// See LICENSE for copying information.
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package cache
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import (
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"container/list"
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"sync"
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"time"
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)
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// Options controls the details of the expiration policy.
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type Options struct {
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// Expiration is how long an entry will be valid. It is not
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// affected by LRU or anything: after this duration, the object
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// is invalidated. A non-positive value means no expiration.
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Expiration time.Duration
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// Capacity is how many objects to keep in memory.
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Capacity int
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}
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// cacheState contains all of the state for a cached entry.
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type cacheState struct {
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once sync.Once
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when time.Time
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order *list.Element
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value interface{}
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}
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// ExpiringLRU caches values for string keys with a time based expiration and
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// an LRU based eviciton policy.
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type ExpiringLRU struct {
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mu sync.Mutex
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opts Options
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data map[string]*cacheState
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order *list.List
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}
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// New constructs an ExpiringLRU with the given options.
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func New(opts Options) *ExpiringLRU {
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return &ExpiringLRU{
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opts: opts,
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data: make(map[string]*cacheState, opts.Capacity),
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order: list.New(),
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}
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}
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// Get returns the value for some key if it exists and is valid. If not
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// it will call the provided function. Concurrent calls will dedupe as
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// best as they are able. If the function returns an error, it is not
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// cached and further calls will try again.
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func (e *ExpiringLRU) Get(key string, fn func() (interface{}, error)) (
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value interface{}, err error) {
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if e.opts.Capacity <= 0 {
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return fn()
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}
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for {
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e.mu.Lock()
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state, ok := e.data[key]
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switch {
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case !ok:
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for len(e.data) >= e.opts.Capacity {
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back := e.order.Back()
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delete(e.data, back.Value.(string))
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e.order.Remove(back)
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}
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state = &cacheState{
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when: time.Now(),
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order: e.order.PushFront(key),
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}
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e.data[key] = state
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case e.opts.Expiration > 0 && time.Since(state.when) > e.opts.Expiration:
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delete(e.data, key)
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e.order.Remove(state.order)
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e.mu.Unlock()
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continue
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default:
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e.order.MoveToFront(state.order)
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}
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e.mu.Unlock()
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called := false
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state.once.Do(func() {
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called = true
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value, err = fn()
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if err == nil {
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// careful because we don't want a `(*T)(nil) != nil` situation
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// that's why we only assign to state.value if err == nil.
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state.value = value
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} else {
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// the once has been used. delete it so that any other waiters
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// will retry.
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e.mu.Lock()
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if e.data[key] == state {
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delete(e.data, key)
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e.order.Remove(state.order)
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}
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e.mu.Unlock()
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}
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})
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if called || state.value != nil {
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return state.value, err
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}
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}
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}
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// Delete explicitly removes a key from the cache if it exists.
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func (e *ExpiringLRU) Delete(key string) {
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e.mu.Lock()
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defer e.mu.Unlock()
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state, ok := e.data[key]
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if !ok {
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return
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}
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delete(e.data, key)
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e.order.Remove(state.order)
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}
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