1056234 - Add support for a method in Component Utils that would check whether an object can be structured cloned or not

Status: REOPENED

(Core :: XPConnect, defect)

Description

[Robert Bindar]

It would be nice to have something like Cu.clonableInto(obj, target) or Cu.isClonable(obj) that would return true if "obj" can be cloned using the structured clone algorithm.

This would help avoiding situations like:
try {
  Cu.cloneInto(obj, {});
} catch(e) {
  obj = undefined;
}

Comment 1

[Boris Zbarsky [:bzbarsky]]

Checking whether something can be structured cloned has side effects, no?  Not only that, but those side effects can change whether the object is structured clonable or not.

So if this is meant for use on untrusted objects you can get situations like this:

  if (Cu.clonableInto(obj, target))
    Cu.cloneInto(obj, target); // oops, throws

Comment 2

[Bobby Holley (:bholley)]

Can you describe in more detail what kinds of objects you're running into that can't be cloned?

Comment 3

[Bobby Holley (:bholley)]

(Note that I've considered implementing a scriptable structured clone callback for Cu.cloneInto, such that people could fill the gaps themselves).

Comment 4

[Michael Henretty [:mikehenrty][:mhenretty]]

For information, this bug came from a review of bug 899585.

https://bugzilla.mozilla.org/show_bug.cgi?id=899585#c14

Comment 5

[Robert Bindar]

bz: How could those side effects change whether the object is structured clonable or not? I'm not aware of the internal implementation of this algorithm, could you be more specific please?

bholley: let's say I have an object that contains some data fields of primitive types and a DOM object. Again, I'm not sure how the algorithm is implemented, but I was thinking maybe a function like that could avoid -potential- useless data copying performed by the algorithm before it throws that exception; and it would also be more intuitive to use "if(clonable(obj)) then clone(obj)".

Comment 6

[Boris Zbarsky [:bzbarsky]]

> How could those side effects change whether the object is structured clonable or not? 

Try this testcase:

  var obj = {};
  Object.defineProperty(obj, "foo", {
    get: function() { obj.doc = document; return 5; },
    enumerable: true
  } )
  window.postMessage(obj, "*");
  window.postMessage(obj, "*");

The first postMessage call succeeds and modifies "obj" so it can no longer be structured cloned.  The second postMessage call fails.

> I'm not aware of the internal implementation of this algorithm

Whether a vanilla object is structured clonable depends on whether the values of all its enumerable own properties are structured clonable.

Comment 7

[Robert Bindar]

Thanks for your thorough answer, I haven't thought of that case. That means it is currently not possible to have such a function, or at least one that can be useful for usecases like that.

The problem is that I used nsIStructuredCloneContainer to serialize some content objects and apparently this interface uses different callbacks than xpc::CloneInto, callbacks that support the cloning of some DOM objects like ImageData; CloneInto doesn't, so I ended up in a situation like Comment 0 because the internal system message for b2g uses CloneInto. Maybe I should've used CloneInto too.

Comment 8

[Boris Zbarsky [:bzbarsky]]

We should consider fixing CloneInto to handle ImageData...

Comment 9

[Robert Bindar]

Can't we make those two use the same callbacks? I have no idea what this would imply, how hard it can be to do this, just asking

Comment 10

[Bobby Holley (:bholley)]

(In reply to Boris Zbarsky [:bz] from comment #8)
> We should consider fixing CloneInto to handle ImageData...

I think this should actually be really easy - NS_DOMWriteStructuredClone is already set on the JSRuntime, and handles this case. We just need to fix up the CloneInto callbacks to play nicely with the runtime-default callbacks.

Comment 11

[BMO Automation]

Per policy at https://wiki.mozilla.org/Bug_Triage/Projects/Bug_Handling/Bug_Husbandry#Inactive_Bugs. If this bug is not an enhancement request or a bug not present in a supported release of Firefox, then it may be reopened.