constants: freeze the constants object

[bengl]

Constants ought to be constant. The primary goal of this commit is to
make constants exposed in require('constants') immutable, as they were
prior to node@7.0.0, and as the constants exposed on fs.constants,
crypto.constants, etc. are.

Since this is implemented by using Object.freeze, it also has the side
effect of making the entire exports of require('constants') immutable,
so no new constants can be defined on the object in userland.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

[bengl]

CI: https://ci.nodejs.org/job/node-test-pull-request/14059/

[Trott]

(Will need a CITGM run, of course.)

[Trott]

LGTM if there are no horrible ecosystem repercussions.

[mscdex]

Does a frozen object still cause performance issues when accessing properties?

[mscdex]

Although it seems like a nice idea, I think I am -1 on it as we don't freeze other parts of our API and I don't think the inconsistency is a good thing.

Also, I'm not sure I understand the comment about constants being frozen prior to node v7. I'm able to write to the constants object just fine prior to v7. I can also overwrite existing property values in node v6.x (likely due to the use of Object.assign() in lib/constants.js).

[bengl]

@mscdex Right, sorry, the change (to using Object.assign) was in 6.0.0 in commit dcccbfd. It first appeared in 7.0.0 but then was backported to 6.x in v6.3.0, so v6.3.0 is the first version (by semver ordering, not release date) that had that change. I can change that in the commit message if you'd like.

Prior to dcccbfd, constants in require('constants') are immutable:

$ nvm use 6.0.0
$ node -p "Object.getOwnPropertyDescriptor(require('constants'), 'R_OK')"
{ value: 4,
  writable: false,
  enumerable: true,
  configurable: false }

This is because prior to dcccbfd, require('constants') === process.binding('constants'). In the binding, NODE_DEFINE_CONSTANT is used:

node/src/node_constants.cc

Line 1116 in 091abb3

NODE_DEFINE_CONSTANT(target, R_OK);

Which makes the properties immutable:

node/src/node.h

Lines 233 to 234 in 091abb3

	v8::PropertyAttribute constant_attributes = \
	static_cast<v8::PropertyAttribute>(v8::ReadOnly | v8::DontDelete); \

So this is a return to the original behavior, prior to v6.3.0.

I used Object.freeze for simplicity. If desired, I can instead replace the Object.assign usage with something that copies by property descriptor, since I'm pretty sure the properties on process.binding('constants').* are still immutable. That way it wouldn't be a frozen object, but we'd still have that existing properties would be immutable, consistent with pre- dcccbfd behavior. I'd make the appropriate change to the test, of course.

[bengl]

CITGM: https://ci.nodejs.org/view/Node.js-citgm/job/citgm-smoker/1355/

[bengl]

@mscdex Ping.

Note also that fs.constants, crypto.constants, etc., all also have immutable properties.

[addaleax]

Taking this off the 10.0.0 milestone, the deadline has passed anyway … and ping @mscdex re: the above

Dismissing review due to missing response. PTAL

[BridgeAR]

@nodejs/tsc PTAL. I dismissed the review from @mscdex

[BridgeAR]

I am going to land this in ~ 48 hours if there is no objection from anyone until then.

[mcollina]

LGTM

[BridgeAR]

Landed in 90e8f79 🎉