| /* |
| Copyright 2012 Google Inc. |
| |
| Licensed under the Apache License, Version 2.0 (the "License"); |
| you may not use this file except in compliance with the License. |
| You may obtain a copy of the License at |
| |
| http://www.apache.org/licenses/LICENSE-2.0 |
| |
| Unless required by applicable law or agreed to in writing, software |
| distributed under the License is distributed on an "AS IS" BASIS, |
| WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| See the License for the specific language governing permissions and |
| limitations under the License. |
| */ |
| |
| // Package singleflight provides a duplicate function call suppression |
| // mechanism. |
| package singleflight |
| |
| import "sync" |
| |
| // call is an in-flight or completed Do call |
| type call struct { |
| wg sync.WaitGroup |
| val interface{} |
| err error |
| } |
| |
| // Group represents a class of work and forms a namespace in which |
| // units of work can be executed with duplicate suppression. |
| type Group struct { |
| mu sync.Mutex // protects m |
| m map[string]*call // lazily initialized |
| } |
| |
| // Do executes and returns the results of the given function, making |
| // sure that only one execution is in-flight for a given key at a |
| // time. If a duplicate comes in, the duplicate caller waits for the |
| // original to complete and receives the same results. |
| func (g *Group) Do(key string, fn func() (interface{}, error)) (interface{}, error) { |
| g.mu.Lock() |
| if g.m == nil { |
| g.m = make(map[string]*call) |
| } |
| if c, ok := g.m[key]; ok { |
| g.mu.Unlock() |
| c.wg.Wait() |
| return c.val, c.err |
| } |
| c := new(call) |
| c.wg.Add(1) |
| g.m[key] = c |
| g.mu.Unlock() |
| |
| c.val, c.err = fn() |
| c.wg.Done() |
| |
| g.mu.Lock() |
| delete(g.m, key) |
| g.mu.Unlock() |
| |
| return c.val, c.err |
| } |