[mapguide-trac] #510: Potential stylization crash with composite styles + expressions

[MapGuide Open Source]

#510: Potential stylization crash with composite styles + expressions
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   Reporter:  waltweltonlair  |       Owner:  waltweltonlair
       Type:  defect          |      Status:  new           
   Priority:  high            |   Milestone:  2.1           
  Component:  General         |     Version:  2.0.0         
   Severity:  critical        |    Keywords:                
External_id:  1022456         |  
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 An interaction between the expression engine and the feature reader can
 lead to a stylization crash.  Here’s the scenario:

  * Stylization creates an RS_FeatureReader, and this guy maintains a
 reference to an FdoIFeatureReader.
  * Stylization creates an expression engine, passing it the
 FdoIFeatureReader.  The expression engine only maintains a weak reference
 to this reader (non-addref'ed).
  * With composite styles stylization will reset the RS_FeatureReader.  The
 reset call releases the FdoIFeatureReader and creates a new one.  Since
 the expression engine does not addref the FdoIFeatureReader, the ref count
 of the original FdoIFeatureReader drops to zero and it’s destroyed.
  * After the RS_FeatureReader has been reset stylization starts processing
 the next style in the composite.  A crash can then occur as soon as we
 attempt to evaluate an expression.  That’s because the expression engine
 is still pointing to the original FdoIFeatureReader that was destroyed in
 step 3.

 To fix this we simply need to create a new expression engine for each
 style in a composite.  This also applies to rendering passes with the new
 stylization.

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Ticket URL: <https://trac.osgeo.org/mapguide/ticket/510>
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