[Pharo-dev] Weird rehashing problem

[Max Leske]

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.

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