github.com/opcr-io/oras-go/v2@v2.0.0-20231122155130-eb4260d8a0ae/internal/syncutil/pool.go (about)

     1  /*
     2  Copyright The ORAS Authors.
     3  Licensed under the Apache License, Version 2.0 (the "License");
     4  you may not use this file except in compliance with the License.
     5  You may obtain a copy of the License at
     6  
     7  http://www.apache.org/licenses/LICENSE-2.0
     8  
     9  Unless required by applicable law or agreed to in writing, software
    10  distributed under the License is distributed on an "AS IS" BASIS,
    11  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  See the License for the specific language governing permissions and
    13  limitations under the License.
    14  */
    15  
    16  package syncutil
    17  
    18  import "sync"
    19  
    20  // poolItem represents an item in Pool.
    21  type poolItem[T any] struct {
    22  	value    T
    23  	refCount int
    24  }
    25  
    26  // Pool is a scalable pool with items identified by keys.
    27  type Pool[T any] struct {
    28  	// New optionally specifies a function to generate a value when Get would
    29  	// otherwise return nil.
    30  	// It may not be changed concurrently with calls to Get.
    31  	New func() T
    32  
    33  	lock  sync.Mutex
    34  	items map[any]*poolItem[T]
    35  }
    36  
    37  // Get gets the value identified by key.
    38  // The caller should invoke the returned function after using the returned item.
    39  func (p *Pool[T]) Get(key any) (*T, func()) {
    40  	p.lock.Lock()
    41  	defer p.lock.Unlock()
    42  
    43  	item, ok := p.items[key]
    44  	if !ok {
    45  		if p.items == nil {
    46  			p.items = make(map[any]*poolItem[T])
    47  		}
    48  		item = &poolItem[T]{}
    49  		if p.New != nil {
    50  			item.value = p.New()
    51  		}
    52  		p.items[key] = item
    53  	}
    54  	item.refCount++
    55  
    56  	return &item.value, func() {
    57  		p.lock.Lock()
    58  		defer p.lock.Unlock()
    59  		item.refCount--
    60  		if item.refCount <= 0 {
    61  			delete(p.items, key)
    62  		}
    63  	}
    64  }