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86 lines
2.9 KiB
Go
86 lines
2.9 KiB
Go
package utils
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// This file is forked from https://github.com/golang/go/blob/bc593eac2dc63d979a575eccb16c7369a5ff81e0/src/sync/once.go.
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import (
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"sync"
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"sync/atomic"
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)
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// OnceAgain is an object that will perform only one action "in flight". It's
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// basically the same as sync.Once, but is automatically reused when the
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// function was executed and everyone who waited has left.
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// Important: This is somewhat racy when used heavily as it only resets _after_
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// everyone who waited has left. So, while some goroutines are waiting to be
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// activated again to leave the waiting state, other goroutines will call Do()
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// without executing the function again.
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type OnceAgain struct {
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// done indicates whether the action has been performed.
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// It is first in the struct because it is used in the hot path.
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// The hot path is inlined at every call site.
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// Placing done first allows more compact instructions on some architectures (amd64/x86),
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// and fewer instructions (to calculate offset) on other architectures.
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done uint32
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// Number of waiters waiting for the function to finish. The last waiter resets done.
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waiters int32
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m sync.Mutex
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}
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// Do calls the function f if and only if Do is being called for the
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// first time for this instance of Once. In other words, given
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//
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// var once Once
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//
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// if once.Do(f) is called multiple times, only the first call will invoke f,
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// even if f has a different value in each invocation. A new instance of
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// Once is required for each 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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//
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// config.once.Do(func() { config.init(filename) })
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//
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// Because no call to Do returns until the one call to f returns, if f causes
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// Do to be called, it will deadlock.
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//
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// If f panics, Do considers it to have returned; future calls of Do return
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// without calling f.
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func (o *OnceAgain) Do(f func()) {
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// Note: Here is an incorrect implementation of Do:
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//
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// if atomic.CompareAndSwapUint32(&o.done, 0, 1) {
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// f()
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// }
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//
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// Do guarantees that when it returns, f has finished.
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// This implementation would not implement that guarantee:
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// given two simultaneous calls, the winner of the cas would
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// call f, and the second would return immediately, without
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// waiting for the first's call to f to complete.
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// This is why the slow path falls back to a mutex, and why
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// the atomic.StoreUint32 must be delayed until after f returns.
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if atomic.LoadUint32(&o.done) == 0 {
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// Outlined slow-path to allow inlining of the fast-path.
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o.doSlow(f)
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}
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}
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func (o *OnceAgain) doSlow(f func()) {
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atomic.AddInt32(&o.waiters, 1)
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defer func() {
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if atomic.AddInt32(&o.waiters, -1) == 0 {
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atomic.StoreUint32(&o.done, 0) // reset
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}
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}()
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o.m.Lock()
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defer o.m.Unlock()
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if o.done == 0 {
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defer atomic.StoreUint32(&o.done, 1)
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f()
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}
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}
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