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60 lines
1.7 KiB
60 lines
1.7 KiB
/*
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Copyright 2014 The Camlistore Authors
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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*/
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package internal
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import (
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"sync"
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"sync/atomic"
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)
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// A Once will perform a successful action exactly once.
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//
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// Unlike a sync.Once, this Once's func returns an error
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// and is re-armed on failure.
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type Once struct {
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m sync.Mutex
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done uint32
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}
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// Do calls the function f if and only if Do has not been invoked
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// without error for this instance of Once. In other words, given
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// var once Once
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// if once.Do(f) is called multiple times, only the first call will
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// invoke f, even if f has a different value in each invocation unless
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// f returns an error. A new instance of Once is required for each
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// function to execute.
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//
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// Do is intended for initialization that must be run exactly once. Since f
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// is niladic, it may be necessary to use a function literal to capture the
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// arguments to a function to be invoked by Do:
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// err := config.once.Do(func() error { return config.init(filename) })
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func (o *Once) Do(f func() error) error {
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if atomic.LoadUint32(&o.done) == 1 {
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return nil
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}
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// Slow-path.
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o.m.Lock()
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defer o.m.Unlock()
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var err error
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if o.done == 0 {
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err = f()
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if err == nil {
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atomic.StoreUint32(&o.done, 1)
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}
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}
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return err
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}
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