inspector: Runtime.evaluate with throwOnSideEffect & contextId of vm.createContext throws when it shouldn't

[rsify]

• Version: v12.1.0, although seems to apply to older versions too
• Platform: macOS
• Subsystem: inspector or vm

Demonstrating the issue from the title:

const inspector = require('inspector')
const vm = require('vm')

const session = new inspector.Session()
session.connect()

const ctx = vm.createContext({
	a: 100
})

a = 100

session.post('Runtime.evaluate', {
	expression: 'a',
	throwOnSideEffect: true,
	contextId: 2 // ctx's id
}, (error, res) => {
	console.log(res)
	if (
		res.exceptionDetails &&
		res.exceptionDetails.exception.description.startsWith('EvalError: Possible side-effect')
	) {
		process.exit(1)
	}
})

^ exits with code 1, even though the provided expression a is clearly causing no side effect.

Weirdly enough, changing the contextId from 2 to 1 (1 being the global one) makes the snippet exit with 0, hence working as expected.

[targos]

@nodejs/v8-inspector

[devsnek]

it actually does cause a side effect. the VM context is a c++ defined proxy-like object.

[rsify]

Is this intended behaviour then @devsnek? If so, are there any workarounds?

[addaleax]

@devsnek Um … what’s the side effect? Yes, it’s a proxy, but accessing variables doesn’t have side effects, right?

I think we’d need to extend NamedPropertyHandlerConfiguration and IndexedPropertyHandlerConfiguration to be able to actually mark the relevant functions as side-effect free in order for this to work, though.

[alexkozy]

I believe that it calls some C++ code, as @addalex mentioned - any C++ callback should be explicitly marked as nosideeffects: https://cs.chromium.org/chromium/src/v8/include/v8.h?rcl=ea50a032a6e5e9c796e408bf2dafc14b767d9896&l=6033 otherwise V8 considers it as side effect.

[addaleax]

Oh, thanks @ak239 – I missed that kHasNoSideEffect exists. That’s nice! :)