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- /*
- Copyright 2014 The Camlistore Authors
-
- 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 internal
-
- import (
- "sync"
- "sync/atomic"
- )
-
- // A Once will perform a successful action exactly once.
- //
- // Unlike a sync.Once, this Once's func returns an error
- // and is re-armed on failure.
- type Once struct {
- m sync.Mutex
- done uint32
- }
-
- // Do calls the function f if and only if Do has not been invoked
- // without error for this instance of Once. In other words, given
- // var once Once
- // if once.Do(f) is called multiple times, only the first call will
- // invoke f, even if f has a different value in each invocation unless
- // f returns an error. A new instance of Once is required for each
- // function to execute.
- //
- // Do is intended for initialization that must be run exactly once. Since f
- // is niladic, it may be necessary to use a function literal to capture the
- // arguments to a function to be invoked by Do:
- // err := config.once.Do(func() error { return config.init(filename) })
- func (o *Once) Do(f func() error) error {
- if atomic.LoadUint32(&o.done) == 1 {
- return nil
- }
- // Slow-path.
- o.m.Lock()
- defer o.m.Unlock()
- var err error
- if o.done == 0 {
- err = f()
- if err == nil {
- atomic.StoreUint32(&o.done, 1)
- }
- }
- return err
- }
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