events: add ability to prepend event listeners #6032

jasnell · 2016-04-04T03:04:02Z

Pull Request check-list

Does make -j8 test (UNIX) or vcbuild test nosign (Windows) pass with
this change (including linting)?
Is the commit message formatted according to [CONTRIBUTING.md][0]?
If this change fixes a bug (or a performance problem), is a regression
test (or a benchmark) included?
Is a documentation update included (if this change modifies
existing APIs, or introduces new ones)?

NOTE: these things are not required to open a PR and can be done
afterwards / while the PR is open.

Affected core subsystem(s)

events

Description of change

A handful of modules (including readable-streams) make inappropriate use of the internal _events property. One such use is to prepend an event listener to the front of the array of listeners.

To address part of the issue, this adds a new optional bitwise flag to the addListener/on/once methods that, when set, causes the listener to be prepended.

Doc update and test case is included.

Fixes: #1817

/cc @ChALkeR

mscdex · 2016-04-04T03:16:37Z

I'm not sure something like this should be documented since the whole point of Readable's use of _events is to make sure its error handler gets executed before any user error event handler. If end users now have a documented way or prepending event handlers, then Readable's error event handler could easily not be first anymore.

Yes, it's still possible for people to do this even if left undocumented, but it's less likely to happen and if it does, the end user should know they could end up shooting themselves in the foot.

jasnell · 2016-04-04T04:03:43Z

Another option: Readable can implement an override for on/once/addListener that emits a process warning if the flag is used to prepend error handlers.

jasnell · 2016-04-04T04:05:07Z

Or... a similar on/once/addListener override on listener could remove it's listener and prepend it again whenever the flag is used to prepend a new error listener.

ChALkeR · 2016-04-04T10:03:55Z

@mscdex Could this be solved by throwing an exception if a listener with EventEmitter.F_PREPEND flag was added a second time to the same event name when there is already one present?

mscdex · 2016-04-04T13:06:33Z

@ChALkeR That would mean storing more state somehow and AFAIK the way that would have to be implemented (storing flags for existing events in a separate object) would cause noticeable performance regressions.

jasnell · 2016-04-04T13:45:42Z

I'd considered something along those lines also but I'd prefer not to have
to track that additional state. Having an emitter subclass override on to
ensure that it's special error handlers are always first seems to be the
most elegant solution. Wouldn't be that difficult.
On Apr 4, 2016 3:04 AM, "Сковорода Никита Андреевич" <
[email protected]> wrote:

@mscdex https://github.com/mscdex Could this be solved by throwing an
exception if a listener with EventEmitter.F_PREPEND flag was added a
second time to the same event name when there is already one?

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#6032 (comment)

vkurchatkin · 2016-04-04T13:54:40Z

-1, it makes no sense to make it public.:

making it public doesn't make it less of a hack, the fact that order matter means that abstraction is bad; we can make at an internal helper, though;
bitwise flags seem to be antithetical to javascript;

A couple of ideas:

error event is special, maybe reserve a special system handler that is always called first, if exists;
or maybe error handlers in contrast to other event handlers should be called in reverse order.

Fishrock123 · 2016-04-04T14:02:11Z

Refs: #5833 (comment) -- I think it should be an options object over a flag.

jasnell · 2016-04-04T14:13:12Z

@vkurchatkin ... I'm fine if this doesn't make it in but the motivation for making this public is to discourage module developers from hacking against internal fields like _events. While this particular hack (prepending an event handler) appears to be relatively rare, it still happens and adding the ability to prepend turns out to be quite trivial. There's only one instance where we do this internally so an internal utility seems to be overkill.

jasnell · 2016-04-04T14:13:34Z

@Fishrock123 ... I'm good with the options object.

Fishrock123 · 2016-04-04T14:15:56Z

Note: if this ends up internal, we can just use a true/false parameter

ChALkeR · 2016-04-04T14:23:52Z

@vkurchatkin We have an external module inside the org atm that misuses the internal API: nodejs/readable-stream.

vkurchatkin · 2016-04-04T14:40:19Z

@ChALkeR true, not sure what to do with this. Maybe we can just add listener normally? Is there an explanation, why it's really necessary to prepend the listener?

ChALkeR · 2016-04-04T15:09:42Z

/cc @nodejs/streams

calvinmetcalf · 2016-04-04T15:42:38Z

so what streams is really trying to do (and other modules might need to do as well) is set an error listener that doesn't prevent the default throwing behavior from also happening (in the case of streams it's to do resource cleanup), some other way we could handle it include

events accepting an onerror function in the options object that gets called whenever there is an error event without counting against error events
an _error event (or maybe a symbol or something) that gets called for error events as well without counting towards the limit

jasnell · 2016-04-04T16:10:57Z

Looking at the readable streams case in more detail, the error handler that is being prepended is getting set on the destination object that is being passed to the pipe method. In other words, Readable is modifying the internal private state of an object that it does not own. The only way to do this correctly would be via a public API.

I suppose that we could introduce a new pre-error event that is called immediately before the normal set of error listeners but that seems a bit hackish to me as well.

mscdex · 2016-04-04T16:26:45Z

What about just monkey-patching dest.emit() (proxying other events) until eventName === 'error' at which point onerror() is called and the old dest.emit() is put back into place? I'm not sure what kind of performance impact that might have, but it seems like that would be the least intrusive and most backwards compatible?

jasnell · 2016-04-04T16:42:55Z

@mscdex ... that seems a bit too hackish also, and given that we've made the decision not to support modules that monkey patch node internals we likely shouldn't be relying on that kind of behavior either.

Again, just continuing the brainstorming here: Another thought came to mind: an undocumented pre-{eventName} event. For instance,

const myEE = new EventEmitter();
myEE.on('pre-error', (er) => { /* ... */ });
myEE.on('pre-foo', (a,b,c) => { /* ... */ });

Essentially, prefix any event name with pre- and that event will be triggered immediately before the actual event handlers.

ChALkeR · 2016-04-04T16:54:53Z

@jasnell Seems good to me, but why keep it undocumented?

If those events would be added, at some point someone would want to document them. Why not document those from the beginning?

jasnell · 2016-04-04T16:55:33Z

Personally I'm good with documenting it. I suggested that given the feedback here questioning whether it should be documented.

mscdex · 2016-04-04T17:11:30Z

For a solution like that I'd be very careful about reserving such a prefix since I could easily see someone already implementing similarly-named events.

calvinmetcalf · 2016-04-04T17:16:56Z

if we were to add a before method to the event object that would also help with versioning, if it was a pre event or an options object there would be no way to know if the event emitter supported that api.

ChALkeR · 2016-04-04T17:17:16Z

@mscdex I couldn't find pre-error event anywhere, not sure about the other ones.

jasnell · 2016-04-04T17:20:08Z

@calvinmetcalf makes a very good point. Having something like a before('eventName', fn) would make it a bit more explicit in terms of whether this new functionality is supported or not.

jasnell · 2016-04-04T17:27:19Z

If we went with before(), would it work like on where I could attach multiple functions before or should it be limited to a single function?

e.g. what should happen in the following case:

const myEE = new EventEmitter();
myEE.before('foo', () => console.log('a'));
myEE.before('foo', () => console.log('b'));
myEE.on('foo', () => console.log('c'));
myEE.emit('foo');

mafintosh · 2016-04-04T17:28:59Z

Should we name it something more explicit than before to avoid breaking userland apis that extend EventEmitter/Stream?

calvinmetcalf · 2016-04-04T17:37:39Z

the other option would be just have an addErrorListener which works exactly like addEventListener but doesn't prevent an error event from being thrown due to no listeners.

mafintosh · 2016-04-04T17:39:12Z

@calvinmetcalf i like that. error listeners are already special anyway

jasnell · 2016-04-19T03:33:39Z

Done! PTAL

jasnell · 2016-04-19T03:39:25Z

New CI: https://ci.nodejs.org/job/node-test-pull-request/2307/

jasnell · 2016-04-19T04:05:48Z

CI is green

ChALkeR · 2016-04-19T05:08:38Z

Yes, those sound better to me, thanks!

ChALkeR · 2016-04-19T05:13:46Z

LGTM

mscdex · 2016-04-19T05:22:33Z

doc/api/events.markdown

+* `listener` {Function} The callback function
+
+Adds a **one time** `listener` function for the event named `eventName` to the
+beginning of the listeners array. This listener is invoked only the next time 


Should "beginning" be emphasized too for consistency?

Fishrock123 · 2016-04-19T05:26:59Z

I'd like to still take a look at this if possible. May be a day or so.

mscdex · 2016-04-19T05:27:18Z

test/parallel/test-event-emitter-prepend.js

+myEE.emit('foo');
+
+
+// Test fail-back if prependListener is undefined


s/fail-back/fallback ?

jasnell · 2016-04-19T05:31:55Z

@mscdex ... updated to address nits

mscdex · 2016-04-19T05:38:38Z

LGTM. Hopefully once this change has been around for a long while I will finally be able to land my .once() optimizations ;-)

jasnell · 2016-04-19T05:39:58Z

Yeah I was going to ping you about that. Would definitely like to get those in soon.
@Fishrock123 ... certainly possible! I'd definitely like to get this in for v6 tho

mscdex · 2016-04-19T05:49:35Z

@jasnell The problem with modules using old readable-stream module versions still exists though, so AFAIK we'd have to wait for the changes in this PR to circulate in the ecosystem for awhile before landing the .once() optimizations.

mcollina · 2016-04-19T07:48:04Z

lib/_stream_readable.js

+    emitter._events[event].unshift(fn);
+  else
+    emitter._events[event] = [fn, emitter._events[event]];
+}


Thanks for including the monkeypatch here.

Maybe we can even avoid that, and just move it to readable-stream.
I don't see any case these lines will be executed in core.

cc @nodejs/streams

mcollina · 2016-04-19T07:50:27Z

Anyway, we should land this asap and include it in a release asap, so we can start integrating the change into readable-stream.

jasnell · 2016-04-20T17:23:58Z

@mcollina ... yep, @Fishrock123 asked for another day or two to review. My plan is to get this landed on Thursday if there are no objections between now and then

A handful of modules (including readable-streams) make inappropriate use of the internal _events property. One such use is to prepend an event listener to the front of the array of listeners. This adds EE.prototype.prependListener() and EE.prototype.prependOnceListener() methods to add handlers to the *front* of the listener array. Doc update and test case is included. Fixes: nodejs#1817

jasnell · 2016-04-21T19:16:00Z

Commits squashed and rebased. Planning to get this landed later on today.

jasnell · 2016-04-22T15:42:59Z

One final CI run: https://ci.nodejs.org/job/node-test-pull-request/2367/

A handful of modules (including readable-streams) make inappropriate use of the internal _events property. One such use is to prepend an event listener to the front of the array of listeners. This adds EE.prototype.prependListener() and EE.prototype.prependOnceListener() methods to add handlers to the *front* of the listener array. Doc update and test case is included. Fixes: #1817 PR-URL: #6032 Reviewed-By: Сковорода Никита Андреевич <[email protected]> Reviewed-By: Brian White <[email protected]>

jasnell · 2016-04-22T16:31:59Z

Landed in 0e7d57a

A handful of modules (including readable-streams) make inappropriate use of the internal _events property. One such use is to prepend an event listener to the front of the array of listeners. This adds EE.prototype.prependListener() and EE.prototype.prependOnceListener() methods to add handlers to the *front* of the listener array. Doc update and test case is included. Fixes: nodejs#1817 PR-URL: nodejs#6032 Reviewed-By: Сковорода Никита Андреевич <[email protected]> Reviewed-By: Brian White <[email protected]>

A handful of modules (including readable-streams) make inappropriate use of the internal _events property. One such use is to prepend an event listener to the front of the array of listeners. This adds EE.prototype.prependListener() and EE.prototype.prependOnceListener() methods to add handlers to the *front* of the listener array. Doc update and test case is included. Fixes: #1817 PR-URL: #6032 Reviewed-By: Сковорода Никита Андреевич <[email protected]> Reviewed-By: Brian White <[email protected]>

jasnell added events Issues and PRs related to the events subsystem / EventEmitter. semver-minor PRs that contain new features and should be released in the next minor version. labels Apr 4, 2016

jasnell force-pushed the events-prepend branch from 15cf90d to 6cba1e6 Compare April 19, 2016 03:37

mscdex reviewed Apr 19, 2016
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ChALkeR mentioned this pull request Apr 19, 2016

EventEmitter API #1817

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mcollina reviewed Apr 19, 2016
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jasnell mentioned this pull request Apr 21, 2016

events: improve eventEmitter.once() performance #5127

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jasnell force-pushed the events-prepend branch from 26ef578 to d5c1072 Compare April 21, 2016 19:14

jasnell closed this Apr 22, 2016

		myEE.emit('foo');


		// Test fail-back if prependListener is undefined

events: add ability to prepend event listeners #6032

events: add ability to prepend event listeners #6032

Conversation

jasnell commented Apr 4, 2016

Pull Request check-list

Affected core subsystem(s)

Description of change

mscdex commented Apr 4, 2016

jasnell commented Apr 4, 2016

jasnell commented Apr 4, 2016

ChALkeR commented Apr 4, 2016

mscdex commented Apr 4, 2016

jasnell commented Apr 4, 2016

vkurchatkin commented Apr 4, 2016

Fishrock123 commented Apr 4, 2016

jasnell commented Apr 4, 2016

jasnell commented Apr 4, 2016

Fishrock123 commented Apr 4, 2016

ChALkeR commented Apr 4, 2016

vkurchatkin commented Apr 4, 2016

ChALkeR commented Apr 4, 2016

calvinmetcalf commented Apr 4, 2016

jasnell commented Apr 4, 2016

mscdex commented Apr 4, 2016

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ChALkeR commented Apr 4, 2016

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