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120 lines
3.1 KiB
Go
120 lines
3.1 KiB
Go
// Copyright (c) 2015-2024 MinIO, Inc.
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//
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// This file is part of MinIO Object Storage stack
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//
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// This program is free software: you can redistribute it and/or modify
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// it under the terms of the GNU Affero General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// This program is distributed in the hope that it will be useful
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU Affero General Public License for more details.
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//
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// You should have received a copy of the GNU Affero General Public License
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// along with this program. If not, see <http://www.gnu.org/licenses/>.
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package cachevalue
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import (
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"sync"
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"time"
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)
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// Cache contains a synchronized value that is considered valid
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// for a specific amount of time.
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// An Update function must be set to provide an updated value when needed.
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type Cache[I any] struct {
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// Update must return an updated value.
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// If an error is returned the cached value is not set.
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// Only one caller will call this function at any time, others will be blocking.
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// The returned value can no longer be modified once returned.
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// Should be set before calling Get().
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Update func() (item I, err error)
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// TTL for a cached value.
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// If not set 1 second TTL is assumed.
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// Should be set before calling Get().
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TTL time.Duration
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// When set to true, return the last cached value
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// even if updating the value errors out
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Relax bool
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// Once can be used to initialize values for lazy initialization.
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// Should be set before calling Get().
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Once sync.Once
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// Managed values.
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value I // our cached value
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valueSet bool // 'true' if the value 'I' has a value
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lastUpdate time.Time // indicates when value 'I' was updated last, used for invalidation.
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mu sync.RWMutex
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}
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// New initializes a new cached value instance.
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func New[I any]() *Cache[I] {
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return &Cache[I]{}
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}
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// Get will return a cached value or fetch a new one.
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// If the Update function returns an error the value is forwarded as is and not cached.
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func (t *Cache[I]) Get() (item I, err error) {
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item, ok := t.get(t.ttl())
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if ok {
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return item, nil
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}
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item, err = t.Update()
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if err != nil {
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if t.Relax {
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// if update fails, return current
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// cached value along with error.
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//
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// Let the caller decide if they want
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// to use the returned value based
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// on error.
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item, ok = t.get(0)
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if ok {
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return item, err
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}
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}
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return item, err
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}
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t.update(item)
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return item, nil
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}
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func (t *Cache[_]) ttl() time.Duration {
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ttl := t.TTL
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if ttl <= 0 {
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ttl = time.Second
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}
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return ttl
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}
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func (t *Cache[I]) get(ttl time.Duration) (item I, ok bool) {
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t.mu.RLock()
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defer t.mu.RUnlock()
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if t.valueSet {
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item = t.value
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if ttl <= 0 {
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return item, true
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}
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if time.Since(t.lastUpdate) < ttl {
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return item, true
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}
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}
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return item, false
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}
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func (t *Cache[I]) update(item I) {
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t.mu.Lock()
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defer t.mu.Unlock()
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t.value = item
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t.valueSet = true
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t.lastUpdate = time.Now()
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}
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