mirror of
https://github.com/ceph/ceph-csi.git
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84 lines
2.2 KiB
Go
84 lines
2.2 KiB
Go
/*
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Copyright 2023 The Kubernetes Authors.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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*/
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package synctrack
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import (
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"sync"
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"sync/atomic"
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)
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// Lazy defers the computation of `Evaluate` to when it is necessary. It is
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// possible that Evaluate will be called in parallel from multiple goroutines.
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type Lazy[T any] struct {
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Evaluate func() (T, error)
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cache atomic.Pointer[cacheEntry[T]]
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}
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type cacheEntry[T any] struct {
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eval func() (T, error)
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lock sync.RWMutex
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result *T
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}
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func (e *cacheEntry[T]) get() (T, error) {
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if cur := func() *T {
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e.lock.RLock()
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defer e.lock.RUnlock()
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return e.result
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}(); cur != nil {
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return *cur, nil
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}
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e.lock.Lock()
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defer e.lock.Unlock()
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if e.result != nil {
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return *e.result, nil
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}
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r, err := e.eval()
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if err == nil {
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e.result = &r
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}
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return r, err
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}
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func (z *Lazy[T]) newCacheEntry() *cacheEntry[T] {
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return &cacheEntry[T]{eval: z.Evaluate}
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}
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// Notify should be called when something has changed necessitating a new call
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// to Evaluate.
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func (z *Lazy[T]) Notify() { z.cache.Swap(z.newCacheEntry()) }
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// Get should be called to get the current result of a call to Evaluate. If the
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// current cached value is stale (due to a call to Notify), then Evaluate will
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// be called synchronously. If subsequent calls to Get happen (without another
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// Notify), they will all wait for the same return value.
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//
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// Error returns are not cached and will cause multiple calls to evaluate!
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func (z *Lazy[T]) Get() (T, error) {
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e := z.cache.Load()
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if e == nil {
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// Since we don't force a constructor, nil is a possible value.
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// If multiple Gets race to set this, the swap makes sure only
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// one wins.
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z.cache.CompareAndSwap(nil, z.newCacheEntry())
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e = z.cache.Load()
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}
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return e.get()
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}
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