Ephemeron bug

> Javier,
> Re-reading your posting I guess you get a crash not because the Ephemerons
> are broken in the way you test - maybe because you set the
> methodDictionaryArray of the EphemeronWRescueMemory key to nil to detect
> whether it is rescued. Setting the mda of any Object to nil should crash the
> VSE-VM:
>
> | a | a := Object new. a methodDictionaryArray: nil. a class
>
> Frank
>
> -----Ursprüngliche Nachricht-----
> Von: Using Visual Smalltalk for Windows/Enterprise
> [mailto:[hidden email]] Im Auftrag von Javier Burroni
> Gesendet: Dienstag, 26. April 2011 18:39
> An: [hidden email]
> Betreff: Ephemeron bug
>
> Consider the following code:
>
> testNestedEphemeronsWEphemeronKey
>   | toBeRescued |
>   toBeRescued := EphemeronWRescueMemory
>       key: Object new
>       value: (EphemeronWRescueMemory key: Object new).
>   Smalltalk unusedMemory.
>   Ephemeron processRescuedEphemerons.
>   Smalltalk unusedMemory.
>   self
>       assert: toBeRescued hasBeenRescued;
>       deny: toBeRescued value key methodDictionaryArray isNil;
>       assert: toBeRescued value hasBeenRescued
>
> Where EphemeronWRescueMemory is an Ephemeron's subclass which can
> answer if it has been already rescued.
> This code will break smalltalk.
> The reason for this test to fail, is the existence of an ephemeron (B)
> as other ephemeron's value (A).
> Following Hayes, whenever ephemeron A rescued, it should be traced as
> any other object -loosing the idea of ephemerons, neither in a direct
> or indirect way-.
> However, the apparent behavior when following object A and finding
> ephemerons, is to mark and queue the just found ephemeron -B in this
> case-; but never trace the ephemeron -B-. This makes B's reference to
> be out of date, and (potentially) point to non valid objects.
> In Hayes code:
>
> Heap::markPhase3
>   EphemeronQueue enumerate:[:ephemeron|
>           ephemeron deref signal: almostCollectable.
>           ephemeron deref keyField tracePointerQueueingEphemerons.
>           ephemeron deref valueField tracePointerQueueingEphemerons].
>
> When the correct code should be:
> Heap::markPhase3
>   EphemeronQueue enumerate:[:ephemeron|
>           ephemeron deref signal: almostCollectable.
>           ephemeron deref keyField tracePointer.
>           ephemeron deref valueField tracePointer].
>
> There are an interesting point regarding Hayes work: the first time a
> GC is run, the ephemeron B will be handle as a non-ephemeron ("Mark
> all of the objects reachable from this pointer, paying no attention to
> ephemerons" p. 183 [1]).
> With a small modification, the algorithm may be able to handle object
> B as an ephemeron in the first run. To do so, we have to focus on the
> fundamental idea of ephemerons: the topological relation between
> objects and the ephemeron's key/value. The relation between objects
> and the ephemeron itself shall be of no importance.
>
> So, suppose we have this:
> VMGarbageCollector>>#rescueEphemerons
>   | unknowns rescan rescued |
>   rescued := rescan := false.
>   unknowns := ephemeronsStack.
>   [ephemerons isEmpty] whileFalse: [
>       rescan := self followEphemeronsCollectingIn: unknowns.
>       rescan ifTrue: [ephemerons addAll: unknowns] ifFalse: [
>           unknowns do: [:ephemeron |
>               self rescueEphemeron: ephemeron.
>               rescued := true]].
>       unknowns reset].
>   rescued ifTrue: [self someEphemeronsRescued]
>
>
> VMGarbageCollector>>#followEphemeronsCollectingIn: unknowns
>   | rescan |
>   rescan := false.
>   [ephemerons isEmpty] whileFalse: [| ephemeron |
>       ephemeron := ephemerons pop.
>       ephemeron == nil ifFalse: [
>           (self checkReachablePropertyOf: ephemeron)
>               ifTrue: [
>                   self follow: ephemeron count: ephemeron
> _extendedSize startingAt: 1.
>                   rescan := true]
>               ifFalse: [unknowns add: ephemeron]]].
>   ^rescan
>
> VMGarbageCollector>>#rescueEphemeron: ephemeron
>   ephemeron _haveNoWeaks
>   self follow: ephemeron count: ephemeron _extendedSize startingAt: 1.
>   rescuedEphemerons add: ephemeron
>
> This implementation may fail to detect object as 'almost collectable',
> whenever a rescued ephemeron refers to an object, when this object is
> the key of a non yet seen ephemeron. But the set of correctly rescued
> ephemerons is big than the Hayes set (which is a subset)
>
> /jb
>
> [1]: Hayes, B. 1997. Ephemerons: a new finalization mechanism.
> Proceedings of the 12th ACM SIGPLAN conference on Object-oriented
> programming, systems, languages, and applications.
> http://portal.acm.org/citation.cfm?id=263733
>
> --
> " To be is to do " ( Socrates )
> " To be or not to be " ( Shakespeare )
> " To do is to be " ( Sartre )
> " Do be do be do " ( Sinatra )
>
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> VSE-VM:
>
> | a | a := Object new. a methodDictionaryArray: nil. a class
>
> Frank
>
> -----Ursprüngliche Nachricht-----
> Von: Using Visual Smalltalk for Windows/Enterprise
> [mailto:[hidden email]] Im Auftrag von Javier Burroni
> Gesendet: Dienstag, 26. April 2011 18:39
> An: [hidden email]
> Betreff: Ephemeron bug
>
> Consider the following code:
>
> testNestedEphemeronsWEphemeronKey
>   | toBeRescued |
>   toBeRescued := EphemeronWRescueMemory
>       key: Object new
>       value: (EphemeronWRescueMemory key: Object new).
>   Smalltalk unusedMemory.
>   Ephemeron processRescuedEphemerons.
>   Smalltalk unusedMemory.
>   self
>       assert: toBeRescued hasBeenRescued;
>       deny: toBeRescued value key methodDictionaryArray isNil;
>       assert: toBeRescued value hasBeenRescued
>
> Where EphemeronWRescueMemory is an Ephemeron's subclass which can
> answer if it has been already rescued.
> This code will break smalltalk.
> The reason for this test to fail, is the existence of an ephemeron (B)
> as other ephemeron's value (A).
> Following Hayes, whenever ephemeron A rescued, it should be traced as
> any other object -loosing the idea of ephemerons, neither in a direct
> or indirect way-.
> However, the apparent behavior when following object A and finding
> ephemerons, is to mark and queue the just found ephemeron -B in this
> case-; but never trace the ephemeron -B-. This makes B's reference to
> be out of date, and (potentially) point to non valid objects.
> In Hayes code:
>
> Heap::markPhase3
>   EphemeronQueue enumerate:[:ephemeron|
>           ephemeron deref signal: almostCollectable.
>           ephemeron deref keyField tracePointerQueueingEphemerons.
>           ephemeron deref valueField tracePointerQueueingEphemerons].
>
> When the correct code should be:
> Heap::markPhase3
>   EphemeronQueue enumerate:[:ephemeron|
>           ephemeron deref signal: almostCollectable.
>           ephemeron deref keyField tracePointer.
>           ephemeron deref valueField tracePointer].
>
> There are an interesting point regarding Hayes work: the first time a
> GC is run, the ephemeron B will be handle as a non-ephemeron ("Mark
> all of the objects reachable from this pointer, paying no attention to
> ephemerons" p. 183 [1]).
> With a small modification, the algorithm may be able to handle object
> B as an ephemeron in the first run. To do so, we have to focus on the
> fundamental idea of ephemerons: the topological relation between
> objects and the ephemeron's key/value. The relation between objects
> and the ephemeron itself shall be of no importance.
>
> So, suppose we have this:
> VMGarbageCollector>>#rescueEphemerons
>   | unknowns rescan rescued |
>   rescued := rescan := false.
>   unknowns := ephemeronsStack.
>   [ephemerons isEmpty] whileFalse: [
>       rescan := self followEphemeronsCollectingIn: unknowns.
>       rescan ifTrue: [ephemerons addAll: unknowns] ifFalse: [
>           unknowns do: [:ephemeron |
>               self rescueEphemeron: ephemeron.
>               rescued := true]].
>       unknowns reset].
>   rescued ifTrue: [self someEphemeronsRescued]
>
>
> VMGarbageCollector>>#followEphemeronsCollectingIn: unknowns
>   | rescan |
>   rescan := false.
>   [ephemerons isEmpty] whileFalse: [| ephemeron |
>       ephemeron := ephemerons pop.
>       ephemeron == nil ifFalse: [
>           (self checkReachablePropertyOf: ephemeron)
>               ifTrue: [
>                   self follow: ephemeron count: ephemeron
> _extendedSize startingAt: 1.
>                   rescan := true]
>               ifFalse: [unknowns add: ephemeron]]].
>   ^rescan
>
> VMGarbageCollector>>#rescueEphemeron: ephemeron
>   ephemeron _haveNoWeaks
>   self follow: ephemeron count: ephemeron _extendedSize startingAt: 1.
>   rescuedEphemerons add: ephemeron
>
> This implementation may fail to detect object as 'almost collectable',
> whenever a rescued ephemeron refers to an object, when this object is
> the key of a non yet seen ephemeron. But the set of correctly rescued
> ephemerons is big than the Hayes set (which is a subset)
>
> /jb
>
> [1]: Hayes, B. 1997. Ephemerons: a new finalization mechanism.
> Proceedings of the 12th ACM SIGPLAN conference on Object-oriented
> programming, systems, languages, and applications.
> http://portal.acm.org/citation.cfm?id=263733
>
> --
> " To be is to do " ( Socrates )
> " To be or not to be " ( Shakespeare )
> " To do is to be " ( Sartre )
> " Do be do be do " ( Sinatra )
>
> ***           this signature added by listserv             ***
> *** Visit  http://www.listserv.dfn.de/archives/vswe-l.html ***
> *** for archive browsing and VSWE-L membership management  ***
>
> ***           this signature added by listserv             ***
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> *** for archive browsing and VSWE-L membership management  ***
>