Isolating execution via separate contexts

[mashdragon]

Describe your feature request here.

It does not seem like there is a feature in PythonMonkey to run JavaScript scripts or evals in isolation after importing the module. This means that I have to keep track of the JavaScript state myself and perform any cleanup manually on objects that pollute the globalThis space.

It would be nice if there were a way to isolate execution so that separate scripts run in separate environments, just like how scripts in the web are isolated to their pages.

Maybe this would appear as a pm.context() function to get a new JavaScript execution context.

Code example

import pythonmonkey as pm

# Potential implementation of this context feature
context = pm.context()
context.eval("x = 4")

# A different context
context2 = pm.context()
context2.eval("x = 5")

print(context.eval("x")) # Prints 4
print(context2.eval("x")) # Prints 5

[wiwichips]

Hi @mashdragon, thanks for taking the time to check out PythonMonkey! This is definitely on our roadmap and something we will get to in the future

[hjdhnx]

I think it's a good idea, quickjs did it
see here

[wesgarland]

This is definitely an interesting idea.

It does raise some complex questions like - what happens when a JS variable is shared from one context to another via the underlying Python code? The GC requirements in PythonMonkey can get very interesting because we have to interoperate with both interpreters at the same time.

One of the questions that pops is -- do we care about complete isolation, meaning a new SpiderMonkey JSContext *cx, or is symbol-level isolation enough?

Providing a new global object and standard classes (Array, Object, etc) would be pretty easy, along with an isolated module context via ctx-module.

But this type of incomplete isolation might have interesting prototype-walking attacks, like [].constructor.prototype ... might be outside of the symbol isolation context because there is only one struct JSClass * per standard class in a JSContext *cx even when different instances of the standard classes are available in JavaScript.

I like the QuickJS interface. If we do this, we will also make it possible to emulate the NodeJS vm module calls somehow.

[wesgarland]

I think what we want is separate realms. We can call 'em contexts from the users' POV.

http://www.carolinecullen.com/spidermonkeynotes/notes/runtime.html