[Vm-dev] Weird rehashing problem

[Eliot Miranda]

Hi Max,

On Fri, Oct 24, 2014 at 7:29 AM, Max Leske <maxleske at gmail.com> wrote:

>
> Hi
>
> I have a rather strange problem with rehasing that occurred simultaneously
> multiple times on the same day but not since. Luckily I’ve saved an image
> with the debugger in it. The mean thing is: that doesn’t help me at all
> because from what I can tell either the Debugger is broken or there’s a VM
> problem.
>

I'm interested in taking a look at that image...  (and I'm relieved you saw
it on both the Cog and the Interpreter VMs ;-) )

Note:
> The image is a Pharo 1.3 image. The problem occurred on two different VMs
> independently, an NBCog VM (Croquet Closure Cog VM [CoInterpreter
> VMMaker-oscog-EstebanLorenzano.166] 21.0) and a Squeak VM (Squeak3.10.2 of
> '5 June 2008' [latest update: #7179]).
> The operation during which this happened was the loading of an
> ImageSegment (in both instances).
>
>
> Here’s what happened. I got a SubscriptOutOfBounds error during rehashing
> of a Dictionary. When I look at the debugger it’s clear why the exception
> got signaled but not how that situation could ever happen.
>
> These are the top three contexts on the stack:
>
> Array(Object)>>errorSubscriptBounds:
> Array(Object)>>at:put:
> CBConfiguration(Dictionary)>>noCheckAdd:
>
> This is the #noCheckAdd: method”
>
> noCheckAdd: anObject
>         "Must be defined separately for Dictionary because (self
> findElementOrNil:) expects a key, not an association.  9/7/96 tk"
>
>         array at: (self findElementOrNil: anObject key) put: anObject.
>         tally := tally + 1
>
> and here’s #at:put: where the exception is signaled:
>
> at: index put: value
>         <primitive: 61>
>         index isInteger ifTrue:
>                 [self class isVariable
>                         ifTrue: [(index >= 1 and: [index <= self size])
>                                         ifTrue: [self errorImproperStore]
>                                         ifFalse: [self
> errorSubscriptBounds: index]] <——— signaled
>                         ifFalse: [self errorNotIndexable]].
>         index isNumber
>                 ifTrue: [^self at: index asInteger put: value]
>                 ifFalse: [self errorNonIntegerIndex]
>
> Here’s the weird thing: according to the debugger ‘array' in #noCheckAdd:
> IS NOT THE SAME INSTANCE as ‘self’ in #at:put:!!!!!! ????????? When I look
> at the pointers to both arrays, the correct one is pointed to by the
> dictionary while the other is only being referenced by the exception
> context:
>
> Bad array:
> 1: SubscriptOutOfBounds: 88
> 2: Array(Object)>>errorSubscriptBounds:
> 3: Array(Object>>at:put:
> 4: an Inspector
> …
>
> Good array:
> 1: aCBConfiguration(size 107)
>
>
> I really have no clue how this could have happened. I’d appreciate any
> thoughts you might have…
>
> Cheers,
> Max




-- 
best,
Eliot
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