github.com/dtroyer-salad/og2/v2@v2.0.0-20240412154159-c47231610877/internal/syncutil/once.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 ( 19 "context" 20 "sync" 21 "sync/atomic" 22 ) 23 24 // Once is an object that will perform exactly one action. 25 // Unlike sync.Once, this Once allows the action to have return values. 26 type Once struct { 27 result interface{} 28 err error 29 status chan bool 30 } 31 32 // NewOnce creates a new Once instance. 33 func NewOnce() *Once { 34 status := make(chan bool, 1) 35 status <- true 36 return &Once{ 37 status: status, 38 } 39 } 40 41 // Do calls the function f if and only if Do is being called first time or all 42 // previous function calls are cancelled, deadline exceeded, or panicking. 43 // When `once.Do(ctx, f)` is called multiple times, the return value of the 44 // first call of the function f is stored, and is directly returned for other 45 // calls. 46 // Besides the return value of the function f, including the error, Do returns 47 // true if the function f passed is called first and is not cancelled, deadline 48 // exceeded, or panicking. Otherwise, returns false. 49 func (o *Once) Do(ctx context.Context, f func() (interface{}, error)) (bool, interface{}, error) { 50 defer func() { 51 if r := recover(); r != nil { 52 o.status <- true 53 panic(r) 54 } 55 }() 56 for { 57 select { 58 case inProgress := <-o.status: 59 if !inProgress { 60 return false, o.result, o.err 61 } 62 result, err := f() 63 if err == context.Canceled || err == context.DeadlineExceeded { 64 o.status <- true 65 return false, nil, err 66 } 67 o.result, o.err = result, err 68 close(o.status) 69 return true, result, err 70 case <-ctx.Done(): 71 return false, nil, ctx.Err() 72 } 73 } 74 } 75 76 // OnceOrRetry is an object that will perform exactly one success action. 77 type OnceOrRetry struct { 78 done atomic.Bool 79 lock sync.Mutex 80 } 81 82 // OnceOrRetry calls the function f if and only if Do is being called for the 83 // first time for this instance of Once or all previous calls to Do are failed. 84 func (o *OnceOrRetry) Do(f func() error) error { 85 // fast path 86 if o.done.Load() { 87 return nil 88 } 89 90 // slow path 91 o.lock.Lock() 92 defer o.lock.Unlock() 93 94 if o.done.Load() { 95 return nil 96 } 97 if err := f(); err != nil { 98 return err 99 } 100 o.done.Store(true) 101 return nil 102 }