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67 lines
2.3 KiB
67 lines
2.3 KiB
import log from 'app/server/lib/log';
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/**
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* WorkCoordinator is a helper to do work serially. It takes a doWork() callback which may either
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* do some work and return a Promise, or report no work to be done by returning null. After work
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* completes, doWork() will be called again; when idle, ping() should be called to retry doWork().
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*/
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export class WorkCoordinator {
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private _doWorkCB: () => Promise<void>|null;
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private _tryNextStepCB: () => void;
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private _isStepRunning: boolean = false;
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private _isStepScheduled: boolean = false;
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/**
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* The doWork() callback will be called on ping() and whenever previous doWork() promise
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* succeeds. If doWork() had nothing to do, it should return null, and will not be called again
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* until the next ping().
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*
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* Note that doWork() should never fail. If it does, exceptions and rejections will be caught
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* and logged, and WorkCoordinator will not be called again until the next ping().
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*/
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constructor(doWork: () => Promise<void>|null) {
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this._doWorkCB = doWork;
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this._tryNextStepCB = () => this._tryNextStep(); // bound version of _tryNextStep.
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}
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/**
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* Attempt doWork() again. If doWork() is currently running, it will be attempted again on
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* completion even if the current run fails.
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*/
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public ping(): void {
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if (!this._isStepScheduled) {
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this._isStepScheduled = true;
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this._maybeSchedule();
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}
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}
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private async _tryNextStep(): Promise<void> {
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this._isStepScheduled = false;
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if (!this._isStepRunning) {
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this._isStepRunning = true;
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try {
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const work = this._doWorkCB();
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if (work) {
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await work;
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// Only schedule the next step if some work was done. If _doWorkCB() did nothing, or
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// failed, _doWorkCB() will only be called when an external ping() triggers it.
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this._isStepScheduled = true;
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}
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} catch (err) {
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// doWork() should NOT fail. If it does, we log the error here, and stop scheduling work
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// as if there is no more work to be done.
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log.error("WorkCoordinator: error in doWork()", err);
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} finally {
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this._isStepRunning = false;
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this._maybeSchedule();
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}
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}
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}
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private _maybeSchedule() {
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if (this._isStepScheduled && !this._isStepRunning) {
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setImmediate(this._tryNextStepCB);
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}
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}
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}
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