Bug 114594 – HEISENBUG ONLY BITES WITHOUT A DEBUGGER

• Status: RESOLVED
• Resolution: WORKSFORME
• Component: JavaScriptCore

[Jorge]

Created attachment 198007 [details]
.html with a JS script that runs in a loop and demonstrates the bug

The function()s works() and fails() ought to produce the same result, but fails() produces incorrect results every now and then.

Notes:

-THIS IS A PROBLEM OF THE JAVASCRIPT ENGINE: THE SAME FUNCTION REWRITTEN IN A SLIGHTLY DIFFERENT STYLE WORKS FINE.
-IT WON'T BITE WHEN THE DEBUGGER IS OPEN
-IT BITES ON iOS 6.x TOO (tested on an iPhone 4)
-ON PUMA (MOUNTAIN LION) TOO.
-PERHAPS IN LION TOO (but I didn't test it)
-NOT IN SNOW LEOPARD
-SEE THE COMMENTS IN THE ATTACHMENT

The attached file "HeisenBugSafari.html" runs both functions in a loop and compares the results, as soon as they don't match it will print the results to document.body.

Sometimes opening and closing the debugger window forces it to bite more often.

This bug broke my webapp :-(

Cheers,
-- 
Jorge.

[Jorge]

Created attachment 198009 [details]
start for( ; ; ) loop from 0

Looks like it also fails in Snow Leopard (but much less often)

[Jorge]

Created attachment 198016 [details]
Loops faster, use this one.

[Alexey Proskuryakov]

How often are you usually seeing this? I had this test open until ctr reached 3 million, and there wasn't any failure output. Testing with Safari 6.0.3.

[Jorge]

Created attachment 198041 [details]
This one comes with "instructions" :-)

Alexey, try this one and let me know.

Tested in Safari Version 6.0.3 (8536.28.10)/OSX 10.8.3 and Snow Leopard 10.6.8/Safari Version 5.1.8 (6534.58.2)

[Juan Falgueras]

Created attachment 198115 [details]
Bug n++ inside an expression in Webkit JS

[Juan Falgueras]

It seems that the only difference between both functions is the function which gives a wrong answer does a n++ inside an expression, while the one that does it ok, does the n++ outside the expression.  It seems that the n++ is the culprit.  If you run this code in Chrome or FF, it works well, that is, both functions do the same, as it should.

[Jorge]

@JuanFalgueras:

Also when fails(s) fails, what happens is that some characters in 'a' are duplicates of the next character, so it would seem that the (n++) were *sometimes* being evaluated as (++n)s, right?

-- 
( Jorge )();

[Juan Falgueras]

(In reply to comment #7)
> @JuanFalgueras:
> 
> Also when fails(s) fails, what happens is that some characters in 'a' are duplicates of the next character, so it would seem that the (n++) were *sometimes* being evaluated as (++n)s, right?
> 
> -- 
> ( Jorge )();

@Jorge

what seems more apparent is that the uncomfortable (to compile) integer++  is badly compile in JS and returns the wrong value, not the previous, or does the post-increment wrongly.  That would also justify that repetition of values between iterations.  Funnily it doesn't happy when the n++ is made out of any expression.

I find this bug really intriguing and way dangerous!

- jf

[Jorge]

Alexey, can you confirm this bug?

[Geoffrey Garen]

I can reproduce this in Safari 6 but not TOT. I believe it has been fixed.

Can you re-try with the latest build @ nightly.webkit.org?

[Juan Falgueras]

(In reply to comment #10)
> I can reproduce this in Safari 6 but not TOT. I believe it has been fixed.
> 
> Can you re-try with the latest build @ nightly.webkit.org?

I'have tried it with the latest build.  No fails.

It was(?) a really funny bug.

-

[Jorge]

Ok, so this bug was fixed a while ago, or so it seems, because I've been testing some more (randomly chosen) nightlies from r148962 further and further back to r146575 2013-03-22 09:58:34 GMT, in a Mac, and it runs fine in every one I tried !

The pity is that in both 6.0.4 and 5.1.9, the ones that came out last week via software update, it still fails.

Thank you!

[Alexey Proskuryakov]

Marking resolved per the above discussion.