605470 - Variables defined in web console can disappear with no warning

Status: RESOLVED WORKSFORME

(DevTools :: General, defect)

Description

[Josh Matthews [:jdm]]

Open a blank page, and open the web console.  Type |var f = document.createElement("iframe")|, then |f| to verify that f contains a value, and sit back and wait for (presumably) a GC to occur.  Type |f| in the console again, note that you now have "ReferenceError: f is not defined" instead of the previous value.

Comment 1

[Josh Matthews [:jdm]]

This is a surprising and unfortunate user experience; I suspect blocking is in order.

Comment 2

[Rob Campbell [:rc] (:robcee)]

It's unfortunate for a lot of reasons!

Comment 3

[Rob Campbell [:rc] (:robcee)]

(In reply to comment #0)
> Open a blank page, and open the web console.  Type |var f =
> document.createElement("iframe")|, then |f| to verify that f contains a value,
> and sit back and wait for (presumably) a GC to occur.  Type |f| in the console
> again, note that you now have "ReferenceError: f is not defined" instead of the
> previous value.

Could you retest with today's nightly? There were some changes made to evalInSandbox (oustanding wrapper+compartments fixes) last night that may have fixed this.

Comment 4

[Josh Matthews [:jdm]]

The error message has changed to "10:44:27.213: Error: Permission denied to access property 'f'" in today's nightly.

Comment 5

[Rob Campbell [:rc] (:robcee)]

curious to hear if this is still happening in current nightlies. I'm hoping the modifications to the evaluation code in bug 599940 have cured this.

Comment 6

[Josh Matthews [:jdm]]

I tried to reproduce and no longer can.

Comment 7

[Rob Campbell [:rc] (:robcee)]

great success!

thanks for the follow-up.

Comment 8

[Rob Campbell [:rc] (:robcee)]

For future generations, my theory as to what was happening, was that 

evalInSandbox(with(__helpers__) { with(__window__) { var a = Object; } });

Was executing | var a = Object | inside a separate scope from the sandbox itself. This allowed the garbage collector to detect that these variables were disconnected and clean them up.

The magic sauce was removing the always awful with() statements.

Comment 9

[Rob Campbell [:rc] (:robcee)]

That should be quoted, as in:

evalInSandbox("with(__helpers__) { with(__window__) { var a = Object; } }");

(bugzilla needs a js parser)

Comment 10

[David Dahl :ddahl]

let this be a lesson to us! 'with' is one of the "bad parts" of JS:)

Comment 11

[Rob Campbell [:rc] (:robcee)]

(In reply to comment #10)
> let this be a lesson to us! 'with' is one of the "bad parts" of JS:)

I'm pretty sure I read that in a book somewhere. hmm...