Elia Battiston | aa7a048 | 2022-08-17 12:24:02 +0000 | [diff] [blame^] | 1 | /* |
| 2 | Copyright 2014 The Camlistore Authors |
| 3 | |
| 4 | Licensed under the Apache License, Version 2.0 (the "License"); |
| 5 | you may not use this file except in compliance with the License. |
| 6 | You may obtain a copy of the License at |
| 7 | |
| 8 | http://www.apache.org/licenses/LICENSE-2.0 |
| 9 | |
| 10 | Unless required by applicable law or agreed to in writing, software |
| 11 | distributed under the License is distributed on an "AS IS" BASIS, |
| 12 | WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| 13 | See the License for the specific language governing permissions and |
| 14 | limitations under the License. |
| 15 | */ |
| 16 | |
| 17 | package internal |
| 18 | |
| 19 | import ( |
| 20 | "sync" |
| 21 | "sync/atomic" |
| 22 | ) |
| 23 | |
| 24 | // A Once will perform a successful action exactly once. |
| 25 | // |
| 26 | // Unlike a sync.Once, this Once's func returns an error |
| 27 | // and is re-armed on failure. |
| 28 | type Once struct { |
| 29 | m sync.Mutex |
| 30 | done uint32 |
| 31 | } |
| 32 | |
| 33 | // Do calls the function f if and only if Do has not been invoked |
| 34 | // without error for this instance of Once. In other words, given |
| 35 | // var once Once |
| 36 | // if once.Do(f) is called multiple times, only the first call will |
| 37 | // invoke f, even if f has a different value in each invocation unless |
| 38 | // f returns an error. A new instance of Once is required for each |
| 39 | // function to execute. |
| 40 | // |
| 41 | // Do is intended for initialization that must be run exactly once. Since f |
| 42 | // is niladic, it may be necessary to use a function literal to capture the |
| 43 | // arguments to a function to be invoked by Do: |
| 44 | // err := config.once.Do(func() error { return config.init(filename) }) |
| 45 | func (o *Once) Do(f func() error) error { |
| 46 | if atomic.LoadUint32(&o.done) == 1 { |
| 47 | return nil |
| 48 | } |
| 49 | // Slow-path. |
| 50 | o.m.Lock() |
| 51 | defer o.m.Unlock() |
| 52 | var err error |
| 53 | if o.done == 0 { |
| 54 | err = f() |
| 55 | if err == nil { |
| 56 | atomic.StoreUint32(&o.done, 1) |
| 57 | } |
| 58 | } |
| 59 | return err |
| 60 | } |