480773 - Preference observers stop working after a few observations

Status: RESOLVED INVALID

(Core :: Preferences: Backend, defect)

Description

[Jason Barnabe (np)]

Tested in Firefox 3.0.6 and 3.1b2 on Ubuntu.

The attached test extension adds a pref observer to browser windows that alerts whenever extensions.preftest.test changes.

1. Install the extension and close Firefox
2. Open one Firefox window
3. Go to about:config and toggle extensions.preftest.test a few times. You can see that it alerts the name of the pref and the value.
4. Open a second Firefox window
5. In the first window, toggle extensions.preftest.test some more. It should be alerting twice (once for each window).
6. Keep toggling

Expected results:

You keep getting two alerts every time you toggle the pref.

Actual Results: 

As you keep toggling, eventually one or both windows stop alerting. This usually happens around the 4th toggle with both windows open for me. This is blocking me from adding a new feature to my extension, so requesting blocking.

The relevant code in the extension is simply:

var prefTestOverlay = {

	init: function() {
		Components.classes["@mozilla.org/preferences-service;1"].getService(Components.interfaces.nsIPrefService).getBranch("extensions.preftest.").QueryInterface(Components.interfaces.nsIPrefBranch2).addObserver("", prefTestOverlay, false);
	},

	observe: function(subject, topic, data) {
		//Components.utils.reportError(data + " " + subject.QueryInterface(Components.interfaces.nsIPrefBranch2).getBoolPref(data));
		alert(data + " " + subject.QueryInterface(Components.interfaces.nsIPrefBranch2).getBoolPref(data));
	}

}

addEventListener("load", prefTestOverlay.init, false);

Comment 1

[Mike Beltzner [:beltzner, not reading bugmail]]

Not gonna block this release since it's not a regression.

Comment 2

[:Gavin Sharp [email: [e-mail]]]

The preference branch object that you added your observer to is being GCed, since you're not keeping a reference to it. You need to keep a reference to the preference branch to keep it alive. This should perhaps be better documented on the interface...

Comment 3

[:Gavin Sharp [email: [e-mail]]]

Alternatively, you could add your observer on the root branch, which is kept alive by the preferences service (you can QI the preferences service to nsIPrefBranch[2], it forwards those calls to the root branch).