The ifAbsentBlock is being evaluated in the critical section of this method.  As a result, this block cannot modify the btreeDictionary.  If we move the ifAbsentBlock evaluation to outside of the critical section would this method still be "correct"?  Here is how it would read:

valueAt: aKey ifAbsent: aBlock
        "Private - Answer correct version of value holder at aKey, 
        evaluate aBlock if absent."

    | value |
    changed == nil ifFalse: [
        value := changed at: aKey.
        value == nil ifFalse: [ ^value ]
    ].
    dataBaseObject == nil ifTrue: [ ^aBlock value ].
    "critical section begin"
    mutex critical: [
        (value := dataBaseObject iterator goTo: aKey; getCurrent)     ].
    "critical section end"
    "evaluate aBlock outside the critical section"
    value == nil
            ifTrue: [^aBlock value].
    value := ODBValueHolder createOn: value value.
    [transaction versionDescriptor versionOK: value versionNumber]
        whileFalse: [value loadPreviousVersionFrom: transaction oldHolders].
    ^value


Thanks.
Derek Brans
Nerd on a Wire
Web design that's anything but square
http://www.nerdonawire.com 
mailto: brans at nerdonawire.com
phone: 604.874.6463
toll-free: 1-877-NERD-ON-A-WIRE
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