[Javapathfinder-devel] ElementInfo:pinDown() question

[Vika F. <ru...@cs...>]

Hi everybody!
I have a question about using the pinDown() method in the ElementInfo  
class. According to the method's documentation, this method allows to  
keep an object in the heap and not be garbage collected "no matter  
what". However, I ran into a problem that some of the objects that I  
pinned down using this method were still garbage collected after the  
heap was restored using CollapsingRestorer. As far as I can tell, the  
problem is the following. If an object was created on the heap as an  
ordinary object (not an internal one) and then later it was pinned  
down using pinDown() method, this fact can go unnoticed by  
CollapsingRestorer, which  keeps the cache of seen objects and updates  
it only if an object has been changed. One of the methods that marks  
an object as changed is analyzeHeap() in DynamicArea. This method  
marks an object as changed if the object's attributes lastAttrs are  
not equal to the current value of attributes. However, lastAttrs are  
set at the beginning of analyzeHeap() to the current  value of objects  
attributes. Thus, for the objects which were pinned down recently, the  
comparison of lastAttrs to current attributes will always result in  
equality and these objects will not be marked as changed.  In this  
case, CollapsingRestorer will not update its cache if the value of the  
pinned down object didn't change (only its attributes) and, when the  
object is restored later, it will not be pinned down in heap anymore.  
As a temporary fix, I put a call to markChanged() in analyzeHeap() for  
all objects whose ATTR_PINDOWN is set and it looks like it solved the  
problem for me.
My question is, is it a bug in JPF or did I use the pinDown() method  
in unintended way? If it's a bug, what would be a good fix for it? If  
the method shouldn't be used like that, what would be a good way to  
force JPF to temporarily  keep some objects in memory even when there  
are no references to these objects left?
Thank you in advance!
-- Vika