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domain: fix error emit handling #17588

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domain: fix error emit handling #17588

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e4e4e0a
domain: fix error emit handling
apapirovski Dec 10, 2017
d46e020
fixup: use listenerCount instead of _events
apapirovski Dec 10, 2017
f8361f7
fixup: rename doError variable
apapirovski Dec 11, 2017
f401c32
fixup: remove unnecessary null check
apapirovski Dec 12, 2017
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35 changes: 19 additions & 16 deletions lib/domain.js
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Original file line number Diff line number Diff line change
Expand Up @@ -399,32 +399,35 @@ EventEmitter.init = function() {
const eventEmit = EventEmitter.prototype.emit;
EventEmitter.prototype.emit = function emit(...args) {
const domain = this.domain;
if (domain === null || domain === undefined || this === process) {
return Reflect.apply(eventEmit, this, args);
}

const type = args[0];
// edge case: if there is a domain and an existing non error object is given,
// it should not be errorized
// see test/parallel/test-event-emitter-no-error-provided-to-error-event.js
if (type === 'error' && args.length > 1 && args[1] &&
!(args[1] instanceof Error)) {
domain.emit('error', args[1]);
return false;
}
const shouldEmitError = type === 'error' &&
this.listenerCount(type) > 0;

domain.enter();
try {
// Just call original `emit` if current EE instance has `error`
// handler, there's no active domain or this is process
if (shouldEmitError || domain === null || domain === undefined ||
this === process) {
return Reflect.apply(eventEmit, this, args);
} catch (er) {
}

if (type === 'error') {
const er = args.length > 1 && args[1] ?
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args[1] || new errors.Error('ERR_UNHANDLED_ERROR') should be enough. But I think it would be better to stay grammatically closer to the old implementation.

Something like:

er = args[1]
if (!er) {
  const errors = lazyErrors();
  er = new errors.Error('ERR_UNHANDLED_ERROR');
}
if (typeof er === 'object' && er !== null) {
  er.domainEmitter = this;
  er.domain = domain;
  er.domainThrown = false;
}

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@apapirovski apapirovski Dec 11, 2017 •
edited
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That triggers a slow path in V8. We need the if check or a ternary. This version does the same in less lines and we don't need lazyErrors, which is the main reason we didn't just use a ternary before.

args[1] : new errors.Error('ERR_UNHANDLED_ERROR');

if (typeof er === 'object' && er !== null) {
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er can not be null anymore. It is either truthy and in that case it is not null or it is an error and in that case it is also not null. I wonder if the object check is actually necessary either. Since er is always a truthy value, assigning properties to it will never fail (even though it would of course be weird to try to add properties to e.g. a number, but trying would not hurt either and the average case would be faster).

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@BridgeAR Great job noticing that! Seems like we've had that pointless check in this code for a while. Guessing it was useful at some point but clearly left over and unnecessary now.

The only reason I can think of leaving the object check is in case someone is testing for those properties and assuming that they're dealing with an object afterwards... ? But that seems obscure enough that it shouldn't be a real issue.

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That could be solved with a comment as well, right?

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I just mean for the purposes of third-party code... but that seems like a pretty obscure thing to me. We can run CitGM to double-check.

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@apapirovski apapirovski Dec 12, 2017 •
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Although, something to think about: typeof isn't very expensive in V8 these days and setting 3 props on numbers or strings is, I think, a lot more expensive. So from a perf perspective, it might be better to keep this check.

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@BridgeAR BridgeAR Dec 12, 2017 •
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I am not sure what you mean that third-party code relies on that? Trying to set a property on a primitive (not function) is a noop. The outcome will be the same as it is right now.

About the performance implications - I am aware that a typeof is cheap but do we want to optimize for the average case or for obscure cases? Because passing in a non object as error sounds really weird to me...

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@apapirovski apapirovski Dec 12, 2017 •
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Does it? I've definitely seen people use strings. It's tricky... we've allowed it till now so we're kind of stuck with supporting it. Anyway, if we're optimizing then it should be done in another PR. I don't think domain related code is that performance sensitive anyway... there are much worse offenders.

I'll remove the null check tho.

er.domainEmitter = this;
er.domain = domain;
er.domainThrown = false;
}

domain.emit('error', er);
return false;
} finally {
domain.exit();
}

domain.enter();
const ret = Reflect.apply(eventEmit, this, args);
domain.exit();

return ret;
};
20 changes: 20 additions & 0 deletions test/parallel/test-domain-ee-error-listener.js
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Original file line number Diff line number Diff line change
@@ -0,0 +1,20 @@
'use strict';

const common = require('../common');
const assert = require('assert');
const domain = require('domain').create();
const EventEmitter = require('events');

domain.on('error', common.mustNotCall());

const ee = new EventEmitter();

const plainObject = { justAn: 'object' };
ee.once('error', common.mustCall((err) => {
assert.deepStrictEqual(err, plainObject);
}));
ee.emit('error', plainObject);

const err = new Error('test error');
ee.once('error', common.expectsError(err));
ee.emit('error', err);