Node 6.5 fails when using --harmony-proxies flag

[colbycheeze]

$ node --version                  
v6.5.0
$ node --harmony_proxies          
node: bad option: --harmony_proxies
$ nvm use 6.4
Now using node v6.4.0 (npm v3.10.3)
$ node --version        
v6.4.0
$ node --harmony_proxies
> 

I originally found this while doing a Travis build.
https://travis-ci.org/IBM-Bluemix/logistics-wizard-webui/builds/156878342

[cjihrig]

v6.5.0 updated V8, so I'm assuming that flag (which is now unnecessary) has been removed there.

[silverwind]

Yep, no such option there. See node --v8-options for valid options.

[silverwind]

BTW, I get that this is a pain point, but unfortunately we can't really keep v8 options in sync with the semver policy, especially for experimental flags like the --harmony ones.

Reopening for possible documentation update to mention these flags are to be considered experimental.

[colbycheeze]

Gotcha. I just had to use the flag because a package in my tests was using global.Proxy so it forced me to pass it to run. Is that method built into v8 now?

[silverwind]

Yes, looks like it:

$ node -p Proxy
[Function: Proxy]

[Fishrock123]

Hmm, not good. We should probably parse it as a no-op to maintain 100% backwards compat.

[MylesBorins]

+1 @Fishrock123

I know that in the past we have not treated V8 flags as "semver'y", but we should not be breaking code that was working before.

Add this to the list of things to think about for updated V8 mid release in the future

[colbycheeze]

@Fishrock123 that would be cool, would allow us to keep using the latest Node in prod without having the annoyance of explaining to everyone doing dev that they need to all update their Node to 6.5 (can just leave in the flag for them, and in prod it would just ignore it right?)

[dnalborczyk]

I would advise against taking any action.

The flags were always advertised as a playground to try and test new features - explicitly not production ready! Anything can break during v8 staging, features might change or they might get scratched all together from the ECMA spec - following the removal from v8 and eventually Node itself.

https://nodejs.org/en/docs/es6/

Please note that these are incomplete and possibly broken features of V8, so use them at your own risk:

[MylesBorins]

I would say that it makes no sense to add these flags on master. whether
or not it is officially supported, it is a behavior change on v6. floating
a patch on v8 of node 6 seems reasonable to me. it is a one time thing with
minimal overhead.

/cc @nodejs/v8 for their thoughts

On Fri, Sep 2, 2016, 9:14 PM dnalborczyk notifications@github.com wrote:

I would advise against taking any action.

The flags were always advertised as a playground to try and test new
features - explicitly not production ready! Anything can break during
v8 staging, features might change or they might get scratched all together
from the ECMA spec - following the removal from v8 and eventually Node
itself.

https://nodejs.org/en/docs/es6/

Please note that these are incomplete and possibly broken features of V8,
so use them at your own risk:

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[dnalborczyk]

I would say that it makes no sense to have these flags on
master.

@thealphanerd Do you want to disallow any flags in master? That would be very unfortunate.

[MylesBorins]

@dnalborczyk I've edited my comment above

I meant that it doesn't make sense to add these specific flags to master

edit: updated v8 on v6.5.0 resulted in the flags being removed. I am suggesting that we should add no-ops for those flags to avoid breaking behavior, but to not apply that patch to master.

[dnalborczyk]

Gotcha. I can totally see your reasoning behind it, though it feels like a waste of resources. If proxies were to have the slightest change in behavior it could smoke up the app nonetheless (with or without no-ops).

I also want to remind that this kind of thing can (and likely will) happen over and over again.

Just my 2 cents.

[MylesBorins]

@dnalborczyk this is the first time we have ever bumped a version of V8 in an already released branch and it brought up a whole bunch of interested / unexpected edge cases. Fixing this would likely take less than an hour of total effort. I imagine that it would likely happen less than twice a year, even less if we consider this is really only going to happen on LTS release lines. An hour or two a year seems pretty reasonable to stop people's stuff from breaking... especially considering that things are arguable more stable when the flag has been dropped.

[silverwind]

One concern with making the flag graceful is that we're making the impression that 6.4.0's experminal Proxy with the flag is 100% compatible with 6.5.0's Proxy without it. Do we know for sure?

[targos]

There is a slight misunderstanding here. V8 shipped proxies without a flag in 4.9: http://v8project.blogspot.ch/2016/01/v8-release-49.html
Using the flag in v6.x has always been a no-op and the feature was not considered experimental anymore. It was only here to possibly opt out with --no-harmony-proxies in case of an unexpected issue with the feature.
The flag was removed in V8 5.1 beacause nothing bad happened during the lifecycle of two Chrome versions.

[ofrobots]

--harmony flags are an experimental feature. By using experimental features, e.g. AsyncWrap, harmony etc., you opt into the risk that backwards incompatible changes can happen.

Still, removal of this flag was not an intentional change in 6.5.0. It is something that slipped through the cracks as part of the V8 update. I'd be in favor of adding back a nop flag if someone wants to put together a PR.

EDIT: grammar.

[Fishrock123]

--harmony flags are an experimental feature. By using experimental features, e.g. AsyncWrap, harmony etc., you opt into the risk that backwards incompatible changes can happen.

I do agree that this is very much true.

[thetutlage]

We should maintain the backward compatibility, since projects are written for multiple versions of Node.js. It is a pain for me to run CI tests for multiple versions too.

[ofrobots]

I will try to put together a patch adding back the removed harmony flags (probably tomorrow).

[ofrobots]

Fixed by #8445.