1431568 - WebExtensions: Dead object exception when trying to access bg object from popup

Status: RESOLVED WONTFIX

(WebExtensions :: Untriaged, defect)

Description

[Andrey Vyrvich]

Attachment: extension.zip

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36

Steps to reproduce:

I have an extension with a popup; 
There's a Backbone's view on it which should listen Model's changes from background and re-render itself

popup:
````
var app = browser.extension.getBackgroundPage().app;

var AppView = Backbone.View.extend({
	el: 'body',
	template: function() {
		return `<h1>App View: ${this.model.get('time')}</h1>`;
	},
	initialize: function() {
		this.listenTo(this.model, 'change', this.render);
		window.addEventListener('unload', () => {
			this.remove();
		});
	},
	render: function() {
		this.$el.html(this.template());
	}
});

window.addEventListener('DOMContentLoaded', () => {
	new AppView({
		model: app
	}).render();
});
````

background:

````
var AppModel = Backbone.Model.extend({
	initialize: function() {
		window.setInterval(() => {
			this.set({
				time: +(new Date())
			});
		}, 1000);
	}
});

window.app = new AppModel();
````


(source code attached)


Actual results:

Everything works perfectly when I open popup for the first time

When I try to open it again nothing happens and error in console appears:
`TypeError: can't access dead object`



Expected results:

This works well without any errors in older Firefox versions (<57)
Also works fine in Chrome

Comment 1

[Kris Maglione [:kmag]]

This means that you're leaving references to objects from the closed popup in your background page. That's something we intentionally don't support for extension pages, since it's a common cause of serious leaks. When an extension page closes, all references to objects belonging to it become invalid.

Comment 2

[Andrey Vyrvich]

(In reply to Kris Maglione [:kmag] (long backlog; ping on IRC if you're blocked) from comment #1)
> This means that you're leaving references to objects from the closed popup
> in your background page. That's something we intentionally don't support for
> extension pages, since it's a common cause of serious leaks. When an
> extension page closes, all references to objects belonging to it become
> invalid.

I checked backbone's code - looks like it stores all the references in _listeners object in background; When we remove view from popup it removes references from that object (so everything should be clean), however entire _listeners becomes 'dead'

Comment 3

[Kris Maglione [:kmag]]

(In reply to Andrey Vyrvich from comment #2)
> I checked backbone's code - looks like it stores all the references in
> _listeners object in background; When we remove view from popup it removes
> references from that object (so everything should be clean), however entire
> _listeners becomes 'dead'

Yup, that's not surprising.

You're loading two separate copies of Backbone, one in the popup, one in the background page. You're attaching an AppView created using the copy in the popup to the AppModel instance created by the copy in the background page. The _listeners object is created the first time it's necessary, and since it winds up being created by the Backbone copy loaded into the popup, the object itself also belongs to the popup, and can't outlive it.