go.chromium.org/luci@v0.0.0-20240309015107-7cdc2e660f33/common/sync/promise/map.go (about)

     1  // Copyright 2018 The LUCI Authors.
     2  //
     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  package promise
    16  
    17  import (
    18  	"context"
    19  	"sync"
    20  )
    21  
    22  // Map is a map from some key to a promise that does something associated
    23  // with this key.
    24  //
    25  // First call to Get initiates a new promise. All subsequent calls return exact
    26  // same promise (even if it has finished).
    27  type Map struct {
    28  	mu sync.RWMutex
    29  	m  map[any]*Promise
    30  }
    31  
    32  // Get either returns an existing promise for the given key or creates and
    33  // immediately launches a new promise.
    34  func (pm *Map) Get(ctx context.Context, key any, gen Generator) *Promise {
    35  	pm.mu.RLock()
    36  	p := pm.m[key]
    37  	pm.mu.RUnlock()
    38  
    39  	if p != nil {
    40  		return p
    41  	}
    42  
    43  	pm.mu.Lock()
    44  	defer pm.mu.Unlock()
    45  
    46  	if p = pm.m[key]; p == nil {
    47  		p = New(ctx, gen)
    48  		if pm.m == nil {
    49  			pm.m = make(map[any]*Promise, 1)
    50  		}
    51  		pm.m[key] = p
    52  	}
    53  	return p
    54  }