43 lines
1.6 KiB
TypeScript
43 lines
1.6 KiB
TypeScript
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// Based on https://github.com/jsoendermann/semaphore-async-await/blob/master/src/Semaphore.ts
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export class Lock {
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private permits: number = 1
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private promiseResolverQueue: Array<(v: boolean) => void> = []
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/**
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* Returns a promise used to wait for a permit to become available. This method should be awaited on.
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* @returns A promise that gets resolved when execution is allowed to proceed.
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*/
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public async acquire(): Promise<boolean> {
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if (this.permits > 0) {
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this.permits -= 1
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return Promise.resolve(true)
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}
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// If there is no permit available, we return a promise that resolves once the semaphore gets
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// signaled enough times that permits is equal to one.
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return new Promise<boolean>((resolver) => this.promiseResolverQueue.push(resolver))
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}
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/**
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* Increases the number of permits by one. If there are other functions waiting, one of them will
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* continue to execute in a future iteration of the event loop.
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*/
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public release(): void {
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this.permits += 1
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if (this.permits > 1 && this.promiseResolverQueue.length > 0) {
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// eslint-disable-next-line no-console
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console.warn('Lock.permits should never be > 0 when there is someone waiting.')
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} else if (this.permits === 1 && this.promiseResolverQueue.length > 0) {
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// If there is someone else waiting, immediately consume the permit that was released
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// at the beginning of this function and let the waiting function resume.
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this.permits -= 1
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const nextResolver = this.promiseResolverQueue.shift()
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if (nextResolver) {
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nextResolver(true)
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}
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}
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}
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}
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