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bnoordhuis/await.js

Created Aug 28, 2019
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Await pending promises on Node.js process exit
// Copyright (c) 2019, Ben Noordhuis <info@bnoordhuis.nl>
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
// ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
// ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
// OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
'use strict';
const {EventEmitter} = require('events');
const {Session} = require('inspector');
module.exports = new class extends EventEmitter {
await() {
return wait.call(this);
}
defaults() {
process.on('beforeExit', wait.bind(this));
}
};
// Not written as an async/await-style function because we don't
// want our promise to show up in the list of pending promises,
// it'll deadlock when we try to await ourselves.
function wait() {
const self = this;
// Keep event loop alive.
const keepalive = setInterval(() => self.emit('tick'), 1e3);
const promises = [];
const session = new Session();
session.connect();
step();
// Get reference to Promise.prototype
function step() {
const params = {expression: 'Promise.prototype'};
session.post('Runtime.evaluate', params, step1);
}
// Get ids (as an array) of objects that are instanceof Promise
function step1(err, ok) {
if (err) return done(err);
const {objectId} = ok.result;
const params = {prototypeObjectId: objectId};
const callback = release(objectId, step2);
session.post('Runtime.queryObjects', params, callback);
}
// Get own properties of the array
function step2(err, ok) {
if (err) return done(err);
const {objectId} = ok.objects;
const params = {objectId, ownProperties: true};
const callback = release(objectId, step3);
session.post('Runtime.getProperties', params, callback);
}
// Filter out anything that isn't an element, i.e., length and __proto__
function step3(err, ok) {
if (err) return done(err);
for (const {name, value} of ok.result) {
if (Number.isInteger(+name)) promises.push(value.objectId);
}
step4();
}
// Get internal properties of the promise
function step4(err, ok) {
if (err) return done(err);
if (promises.length === 0) return done();
const objectId = promises.shift();
const params = {objectId, ownProperties: true};
const callback = (err, ok) => step5(objectId, err, ok);
session.post('Runtime.getProperties', params, callback);
}
// Check that the promise is pending and await it if so
function step5(promiseObjectId, err, ok) {
if (err) return done(err);
const loop = release(promiseObjectId, step4); // Go to next promise
for (const {name, value: {value}} of ok.internalProperties) {
if (name !== '[[PromiseStatus]]') continue;
if (value !== 'pending') return loop();
self.emit('await', 1 + promises.length);
session.post('Runtime.awaitPromise', {promiseObjectId}, loop);
return;
}
throw Error('unreachable');
}
function done(err) {
clearInterval(keepalive);
session.disconnect();
if (err) self.emit('error', err);
}
function release(objectId, next) {
return function(err, ok) {
session.post('Runtime.releaseObject', {objectId}, function(err2) {
if (err2) return done(err2);
next(err, ok);
});
};
}
}
// *** FOR TESTING ***
process.once('beforeExit', () => {
new Promise(resolve => setTimeout(() => resolve(1), 1e3));
new Promise(resolve => setTimeout(() => resolve(2), 1e3));
//new Promise((_, reject) => setTimeout(() => reject(Error('BOOM')), 1e3));
//new Promise(r => {global.r = r});
});
{
const that = module.exports;
that.defaults();
that.on('tick', () => console.log('tick'));
that.on('await', (n) => console.log(`${n} pending promises`));
}
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