154 lines
3.6 KiB
Go
154 lines
3.6 KiB
Go
package service
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import (
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"container/list"
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"context"
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"errors"
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"sync"
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"time"
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)
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var errReadCacheBusy = errors.New("热点读取正在回源，请稍后重试")
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var errReadCacheLoadAborted = errors.New("缓存回源异常中断")
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// 仅用于可短暂延迟的读投影。限制条目、正文预算和同时回源数；失败短暂冷却，
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// 不返回过期成功值，也不允许大量不同 key 将 singleflight 本身变成无界队列。
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type boundedReadCache[K comparable, V any] struct {
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mu sync.Mutex
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entries map[K]*readCacheEntry[K, V]
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lru list.List
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bytes, loads int
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maxEntries, maxBytes, maxLoads int
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ttl time.Duration
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}
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type readCacheEntry[K comparable, V any] struct {
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key K
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ready chan struct{}
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value V
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err error
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expires time.Time
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bytes int
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position *list.Element
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}
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func newBoundedReadCache[K comparable, V any](entries, bytes, loads int, ttl time.Duration) *boundedReadCache[K, V] {
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return &boundedReadCache[K, V]{entries: make(map[K]*readCacheEntry[K, V]), maxEntries: entries, maxBytes: bytes, maxLoads: loads, ttl: ttl}
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}
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func (c *boundedReadCache[K, V]) get(ctx context.Context, key K, load func(context.Context) (V, int, error)) (V, error) {
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var zero V
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if err := ctx.Err(); err != nil {
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return zero, err
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}
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c.mu.Lock()
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if e := c.entries[key]; e != nil {
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if ready := e.ready; ready != nil {
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c.mu.Unlock()
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select {
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case <-ctx.Done():
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return zero, ctx.Err()
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case <-ready:
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return e.value, e.err
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}
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}
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if time.Now().Before(e.expires) {
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c.lru.MoveToFront(e.position)
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value, err := e.value, e.err
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c.mu.Unlock()
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return value, err
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}
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c.remove(e)
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}
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if c.loads >= c.maxLoads {
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c.mu.Unlock()
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return zero, errReadCacheBusy
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}
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for len(c.entries) >= c.maxEntries {
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if !c.evict() {
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c.mu.Unlock()
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return zero, errReadCacheBusy
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}
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}
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e := &readCacheEntry[K, V]{key: key, ready: make(chan struct{})}
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c.entries[key] = e
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c.loads++
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c.mu.Unlock()
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// 让原始 panic 继续传播给调用方，同时释放等待者和容量，避免永久卡住热点 key。
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loaded := false
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defer func() {
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if loaded {
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return
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}
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c.mu.Lock()
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defer c.mu.Unlock()
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c.loads--
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e.err = errReadCacheLoadAborted
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if c.entries[key] == e {
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delete(c.entries, key)
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}
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close(e.ready)
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e.ready = nil
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}()
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value, bytes, err := load(ctx)
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loaded = true
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c.mu.Lock()
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defer c.mu.Unlock()
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c.loads--
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e.value, e.err = value, err
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ready := e.ready
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e.ready = nil
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if c.entries[key] == e {
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ttl := c.ttl
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if err != nil {
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ttl, bytes = time.Second, 256
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}
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bytes = max(bytes, 256)
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if bytes > c.maxBytes || errors.Is(err, context.Canceled) || errors.Is(err, context.DeadlineExceeded) {
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delete(c.entries, key)
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} else {
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for c.bytes+bytes > c.maxBytes {
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if !c.evict() {
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break
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}
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}
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e.bytes, e.expires = bytes, time.Now().Add(ttl)
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e.position = c.lru.PushFront(e)
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c.bytes += bytes
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}
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}
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close(ready)
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return value, err
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}
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func (c *boundedReadCache[K, V]) evict() bool {
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if oldest := c.lru.Back(); oldest != nil {
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c.remove(oldest.Value.(*readCacheEntry[K, V]))
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return true
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}
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return false
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}
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func (c *boundedReadCache[K, V]) remove(e *readCacheEntry[K, V]) {
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delete(c.entries, e.key)
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if e.position != nil {
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c.lru.Remove(e.position)
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c.bytes -= e.bytes
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}
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}
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func (c *boundedReadCache[K, V]) clear() {
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c.clearAndCount()
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}
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func (c *boundedReadCache[K, V]) clearAndCount() int {
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c.mu.Lock()
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defer c.mu.Unlock()
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count := len(c.entries)
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c.entries = make(map[K]*readCacheEntry[K, V])
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c.lru.Init()
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c.bytes = 0
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return count
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}
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