[Pharo-project] Fuel - class loading issue when superclass changed inst vars

Frank Shearar frank.shearar at gmail.com
Tue Mar 6 16:49:54 EST 2012


On 6 March 2012 18:27, Eliot Miranda <eliot.miranda at gmail.com> wrote:
>
>
> On Tue, Mar 6, 2012 at 3:05 AM, Henrik Johansen
> <henrik.s.johansen at veloxit.no> wrote:
>>
>>
>> On Mar 6, 2012, at 12:03 PM, Henrik Johansen wrote:
>>
>> >>
>> >
>> > Parcels
>> > - Check a hash based on class layout vs equivalent hash for the stored
>> > method's class (stored with the method).
>> > If different:
>> > 1) check for existence of a backwards-compatability reader method, hand
>> > the old instance to it, and expect the old instance to be #become'd into
>> > something current,
>> > 2) raise an error if no such method exists.
>> >
>> > Now, the main problem with this scheme is it's left as an exercise to
>> > the user to come up with a procedure to ensure said backwards-compat reader
>> > method stays up to date as additional changes are made.
>> >
>> > Is it a big problem?
>> > Depends on your tests, and programmer diligence at any given day.
>> >
>> > Added inst vars are not a big deal, as if you forget them, there will
>> > usually be a nil #DNU somewhere down the line. (or you use lazy
>> > initialization, and everything works as expected even for existing
>> > instances)
>> >
>> > Removals/reorderings are a bigger issue, as the detection of failure
>> > (inst vars in wrong slots) are often far removed from the source of problems
>> > (forgetting to update the reader method),
>> > If seldomly used, it may even go unnoticed until the instance is next
>> > saved, at which time you're in real trouble (especially if saved alongside
>> > newly created ones, in which case there is no consistency).
>> >
>> > In general I think it's an ok scheme, but would like to see a solution
>> > more resilient to user-error.
>> > How to achieve that is an interesting topic, which I haven't yet found
>> > time to think through as thoroughly as I had wished/intended some months
>> > ago. :(
>>
>> Errr, BOSS, not Parcels.
>
>
> Right. What Parcels do is shape-change instances *and* rescan methods to fix
> up inst var offsets.  When a parcel is saved the "signature" of classes of
> instances there-in are saved so that the parcel contains all the inst var
> names of the class and its superclass.  If on load the superclass chain has
> a different set of inst vars then lost inst vars are omitted and added inst
> vars are nil in the materialized instances.  If a Parcel contains a full
> class (not just instances) that has methods, and the superclass chains inst
> vars have changed  then these methods are "rescanned" (disassembled, and
> reassembled, not using the compiler) and inst var offsets are fixed up,
> missing inst vars getting changed into undeclared variable references (I
> think; at least this is what *should* happen; its what happens when one
> removes an inst var that is still referenced from methods).
>
> What's missing in the parcel scheme is any hook to allow the user to process
> shape-changing instances with the values of lost inst vars in hand.
>  Presumably there will be cases when those values are essential to any
> schema migration.  Personally I would want to decouple schema migration and
> add it as a post-processing step, which would imply that the materializer
> would offer a service (materialize in a special mode) where it constructed a
> dictionary from materialized and shape-changed instance to in parcel state
> (e.g. an Array of inst var values as they occurred in the parcel).  Then the
> materialized instance could be migrated after the fact, with the default
> behaviour being analogous to what the system does now on class redefinition
> (values of deleted inst vars are lost, aded inst vars are nil).

This sounds a bit like CLOS'  UPDATE-INSTANCE-FOR-REDEFINED-CLASS [1],
which is a generic function (for our purposes just think "something a
bit like a method") that you define and is invoked by the machinery
involved in changing/redefining classes.

A colleague told me about this part of CLOS the other day when
discussing class-based OO: it's the sort of thing we need in at least
the Browser, so one can change the shape of existing instances and
ensure the new class invariants are maintained. (For instance, for
"purely functional" objects, you really don't want to use reflection
to initialise new instvars from outside the object.) So you'd change
the class definition, supply a way of handling the schema migration,
and then the image would change the class definition and run the
migration.

frank

[1] http://clhs.lisp.se/Body/f_upda_1.htm

> This reminds me of a bug with the ClassBuilder/RefactoringBrowser
> combination.  If one refactors pushing an inst var up to a superclass or
> down to subclasses, the values of te inst var in instances are lost because
> the class change is in fact done as two changes, a deletion followed by an
> addition.  This again could be fixed in a wrapper, building a dictionary
> from instance to values, performing the set of class changes, and then
> restoring the inst var state from the dictionary.
>
>
>>
>> My bad.
>>
>> Cheers,
>> Henry
>>
>>
>
>
>
> --
> best,
> Eliot
>




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