82 lines
1.8 KiB
Go
82 lines
1.8 KiB
Go
// Copyright (c) Tailscale Inc & AUTHORS
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// SPDX-License-Identifier: BSD-3-Clause
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package cache
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import (
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"time"
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)
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// Single is a simple in-memory cache that stores a single value until a
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// defined time before it is re-fetched. It also supports returning a
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// previously-expired value if refreshing the value in the cache fails.
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//
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// Single is not safe for concurrent use.
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type Single[K comparable, V any] struct {
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key K
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val V
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goodUntil time.Time
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timeNow func() time.Time // for tests
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// ServeExpired indicates that if an error occurs when filling the
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// cache, an expired value can be returned instead of an error.
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//
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// This value should only be set when this struct is created.
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ServeExpired bool
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}
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var _ Cache[int, int] = (*Single[int, int])(nil)
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// Get will return the cached value, if any, or fill the cache by calling f and
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// return the corresponding value. If f returns an error and c.ServeExpired is
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// true, then a previous expired value can be returned with no error.
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func (c *Single[K, V]) Get(key K, f FillFunc[V]) (V, error) {
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var now time.Time
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if c.timeNow != nil {
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now = c.timeNow()
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} else {
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now = time.Now()
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}
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if c.key == key && now.Before(c.goodUntil) {
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return c.val, nil
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}
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// Re-fill cached entry
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val, until, err := f()
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if err == nil {
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c.key = key
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c.val = val
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c.goodUntil = until
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return val, nil
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}
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// Never serve an expired entry for the wrong key.
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if c.key == key && c.ServeExpired && !c.goodUntil.IsZero() {
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return c.val, nil
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}
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var zero V
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return zero, err
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}
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// Forget implements Cache.
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func (c *Single[K, V]) Forget(key K) {
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if c.key != key {
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return
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}
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c.Empty()
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}
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// Empty implements Cache.
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func (c *Single[K, V]) Empty() {
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c.goodUntil = time.Time{}
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var zeroKey K
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c.key = zeroKey
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var zeroVal V
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c.val = zeroVal
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}
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