Sorry, complicated explanation... XPCOMUtils.defineLazyGetter defines an accessor property that's supposed to replace itself with a data property the first time it's accessed. But it does so using a delete followed by a set: get: function () { delete aObject[aName]; return aObject[aName] = aLambda.apply(aObject); } Now sometimes after aObject[aName] is deleted, there's *another* lazy getter property on aObject's prototype chain. Seems weird; but I have a test case where a Sandbox object with a "Services" lazy getter inherits from a ChromeWindow object that also has a "Services" lazy getter. Maybe it's normal. Anyway, because the inherited property has no setter, the assignment fails. In strict mode, it would be a TypeError. This means the property, instead of turning into a data property, simply disappears; and from then on aObject[aName] is inherited. This is going to be an "extra warning" from now on (side effect of bug 1113369 causing a *ton* of error handling to be more consistent) and it fires way too often. In fact it causes a devtools test to fail because the test expects no warnings to happen during the test.

Comment on attachment 8570918 [details] [diff] [review] Tweak XPCOMUtils.defineLazyGetter behavior to avoid weird behavior in weird cases Review of attachment 8570918 [details] [diff] [review]: ----------------------------------------------------------------- wrong reviewer, oops

Comment on attachment 8570927 [details] [diff] [review] Tweak XPCOMUtils.defineLazyGetter behavior to avoid weird behavior in weird cases Review of attachment 8570927 [details] [diff] [review]: ----------------------------------------------------------------- lgtm! ::: js/xpconnect/loader/XPCOMUtils.jsm @@ +196,2 @@ > delete aObject[aName]; > + var value = aLambda.apply(aObject); nit: we usually use let in newer code. @@ +198,5 @@ > + Object.defineProperty(aObject, aName, { > + value, > + writable: true, > + configurable: true, > + enumerable: true}); nit: maybe add whitespace before } for readability or indent }); with Object