Crash when using a `load` expression within ZPT-Sharp MVC

[craigfowler]

This has been reproduced whilst working on #98, using Profile Three in ZPT mode.

CSF.Zpt.Rendering.ModelEvaluationException: An error was encountered evaluating an expression whilst processing a '[(tal):(http://xml.zope.org/namespaces/tal)]:define' attribute on element 'p', see the inner exception for more details.
Source expression:load:CONTEXTS/Views/Shared/SharedZptComponent/macros/shared ---> System.ArgumentNullException: Value cannot be null.
Parameter name: value
  at CSF.Zpt.MVC.Tales.MvcTalesModel.set_ViewContext (System.Web.Mvc.ViewContext value) [0x00009] in /home/craig/Projects/CSF.Zpt/CSF.Zpt.MVC5/Tales/MvcTalesModel.cs:27 
  at CSF.Zpt.MVC.Tales.MvcTalesModel.CreateChildModel () [0x00001] in /home/craig/Projects/CSF.Zpt/CSF.Zpt.MVC5/Tales/MvcTalesModel.cs:81 
  at CSF.Zpt.Rendering.RenderingContext.<GetChildContexts>b__23_0 (CSF.Zpt.Rendering.IZptElement x) [0x00000] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Rendering/RenderingContext.cs:119 
  at System.Linq.Enumerable+SelectArrayIterator`2[TSource,TResult].ToArray () [0x00012] in <9cf10939da67463f9bbe1799644fd07e>:0 
  at System.Linq.Enumerable.ToArray[TSource] (System.Collections.Generic.IEnumerable`1[T] source) [0x0001f] in <9cf10939da67463f9bbe1799644fd07e>:0 
  at CSF.Zpt.Rendering.RenderingContext.GetChildContexts () [0x00001] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Rendering/RenderingContext.cs:117 
  at CSF.Zpt.Rendering.ContextVisitorBase+<>c.<VisitRecursively>b__1_0 (CSF.Zpt.Rendering.IRenderingContext x) [0x00000] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Rendering/ContextVisitorBase.cs:40 
  at System.Linq.Enumerable+SelectManySingleSelectorIterator`2[TSource,TResult].MoveNext () [0x00051] in <9cf10939da67463f9bbe1799644fd07e>:0 
  at CSF.Zpt.Rendering.ContextVisitorBase.VisitRecursively (CSF.Zpt.Rendering.IRenderingContext context) [0x00060] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Rendering/ContextVisitorBase.cs:41 
  at CSF.Zpt.Rendering.ContextVisitorBase.VisitContext (CSF.Zpt.Rendering.IRenderingContext context) [0x00001] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Rendering/ContextVisitorBase.cs:56 
  at CSF.Zpt.Rendering.ElementRenderer.RenderElement (System.Object model, CSF.Zpt.Rendering.IZptElement element, CSF.Zpt.Rendering.IRenderingSettings options, System.Action`1[T] contextConfigurator) [0x00135] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Rendering/ElementRenderer.cs:71 
  at CSF.Zpt.ZptDocument.Render (System.Object model, System.IO.TextWriter writer, CSF.Zpt.Rendering.IRenderingSettings options, System.Action`1[T] contextConfigurator) [0x00024] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/ZptDocument.cs:125 
  at CSF.Zpt.ZptDocument.Render (System.Object model, CSF.Zpt.Rendering.IRenderingSettings options, System.Action`1[T] contextConfigurator) [0x00024] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/ZptDocument.cs:87 
  at CSF.Zpt.ZptDocument.Render (CSF.Zpt.Rendering.IRenderingSettings options, System.Action`1[T] contextConfigurator) [0x00001] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/ZptDocument.cs:49 
  at CSF.Zpt.ExpressionEvaluators.LoadExpressions.LoadExpressionEvaluator.Render (CSF.Zpt.IZptDocument document, CSF.Zpt.Rendering.IRenderingContext context) [0x0003b] in /home/craig/Projects/CSF.Zpt/ExpressionEvaluators/CSF.Zpt.ExpressionEvaluators.LoadExpressions/LoadExpressionEvaluator.cs:159 
  at CSF.Zpt.ExpressionEvaluators.LoadExpressions.LoadExpressionEvaluator.Render (CSF.Zpt.Rendering.IZptElement element, CSF.Zpt.Rendering.IRenderingContext context) [0x00021] in /home/craig/Projects/CSF.Zpt/ExpressionEvaluators/CSF.Zpt.ExpressionEvaluators.LoadExpressions/LoadExpressionEvaluator.cs:208 
  at CSF.Zpt.ExpressionEvaluators.LoadExpressions.LoadExpressionEvaluator.RenderDocument (System.Object targetToRender, CSF.Zpt.Rendering.IRenderingContext context) [0x0009b] in /home/craig/Projects/CSF.Zpt/ExpressionEvaluators/CSF.Zpt.ExpressionEvaluators.LoadExpressions/LoadExpressionEvaluator.cs:132 
  at CSF.Zpt.ExpressionEvaluators.LoadExpressions.LoadExpressionEvaluator.Evaluate (CSF.Zpt.Tales.Expression expression, CSF.Zpt.Rendering.IRenderingContext context, CSF.Zpt.Tales.ITalesModel model) [0x00067] in /home/craig/Projects/CSF.Zpt/ExpressionEvaluators/CSF.Zpt.ExpressionEvaluators.LoadExpressions/LoadExpressionEvaluator.cs:74 
  at CSF.Zpt.Tales.TalesModel.Evaluate (CSF.Zpt.Tales.Expression talesExpression, CSF.Zpt.Rendering.IRenderingContext context) [0x00036] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Tales/TalesModel.cs:97 
  at CSF.Zpt.Tales.TalesModel.Evaluate (System.String expression, CSF.Zpt.Rendering.IRenderingContext context) [0x00022] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Tales/TalesModel.cs:77 
  at CSF.Zpt.Tal.DefineAttributeHandler.Handle (CSF.Zpt.Rendering.IRenderingContext context) [0x00151] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Tal/DefineAttributeHandler.cs:79 
   --- End of inner exception stack trace ---
  at CSF.Zpt.Tal.DefineAttributeHandler.Handle (CSF.Zpt.Rendering.IRenderingContext context) [0x001a1] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Tal/DefineAttributeHandler.cs:88 
  at CSF.Zpt.Tal.TalVisitor.Visit (CSF.Zpt.Rendering.IRenderingContext context) [0x00090] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Tal/TalVisitor.cs:56 
  at CSF.Zpt.Rendering.ContextVisitorBase.VisitRecursively (CSF.Zpt.Rendering.IRenderingContext context) [0x00015] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Rendering/ContextVisitorBase.cs:38 
  at CSF.Zpt.Tal.TalVisitor.VisitRecursively (CSF.Zpt.Rendering.IRenderingContext context) [0x0004b] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Tal/TalVisitor.cs:102 
  at CSF.Zpt.Rendering.ContextVisitorBase.VisitRecursively (CSF.Zpt.Rendering.IRenderingContext context) [0x00056] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Rendering/ContextVisitorBase.cs:43 
  at CSF.Zpt.Tal.TalVisitor.VisitRecursively (CSF.Zpt.Rendering.IRenderingContext context) [0x0004b] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Tal/TalVisitor.cs:102 
  at CSF.Zpt.Rendering.ContextVisitorBase.VisitRecursively (CSF.Zpt.Rendering.IRenderingContext context) [0x00056] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Rendering/ContextVisitorBase.cs:43 
  at CSF.Zpt.Tal.TalVisitor.VisitRecursively (CSF.Zpt.Rendering.IRenderingContext context) [0x0004b] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Tal/TalVisitor.cs:102 
  at CSF.Zpt.Rendering.ContextVisitorBase.VisitRecursively (CSF.Zpt.Rendering.IRenderingContext context) [0x00056] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Rendering/ContextVisitorBase.cs:43 
  at CSF.Zpt.Tal.TalVisitor.VisitRecursively (CSF.Zpt.Rendering.IRenderingContext context) [0x0004b] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Tal/TalVisitor.cs:102 
  at CSF.Zpt.Rendering.ContextVisitorBase.VisitRecursively (CSF.Zpt.Rendering.IRenderingContext context) [0x00056] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Rendering/ContextVisitorBase.cs:43 
  at CSF.Zpt.Tal.TalVisitor.VisitRecursively (CSF.Zpt.Rendering.IRenderingContext context) [0x0004b] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Tal/TalVisitor.cs:102 
  at CSF.Zpt.Rendering.ContextVisitorBase.VisitRecursively (CSF.Zpt.Rendering.IRenderingContext context) [0x00056] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Rendering/ContextVisitorBase.cs:43 
  at CSF.Zpt.Tal.TalVisitor.VisitRecursively (CSF.Zpt.Rendering.IRenderingContext context) [0x0004b] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Tal/TalVisitor.cs:102 
  at CSF.Zpt.Rendering.ContextVisitorBase.VisitContext (CSF.Zpt.Rendering.IRenderingContext context) [0x00001] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Rendering/ContextVisitorBase.cs:56 
  at CSF.Zpt.Rendering.ElementRenderer.RenderElement (System.Object model, CSF.Zpt.Rendering.IZptElement element, CSF.Zpt.Rendering.IRenderingSettings options, System.Action`1[T] contextConfigurator) [0x00135] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/Rendering/ElementRenderer.cs:71 
  at CSF.Zpt.ZptDocument.Render (System.Object model, System.IO.TextWriter writer, CSF.Zpt.Rendering.IRenderingSettings options, System.Action`1[T] contextConfigurator) [0x00024] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/ZptDocument.cs:125 
  at CSF.Zpt.ZptDocument.Render (System.IO.TextWriter writer, CSF.Zpt.Rendering.IRenderingSettings options, System.Action`1[T] contextConfigurator) [0x00001] in /home/craig/Projects/CSF.Zpt/CSF.Zpt/ZptDocument.cs:62 
  at CSF.Zpt.MVC.ZptView.Render (System.Web.Mvc.ViewContext viewContext, System.IO.TextWriter writer) [0x00009] in /home/craig/Projects/CSF.Zpt/CSF.Zpt.MVC5/ZptView.cs:59 

[craigfowler]

I believe that the root cause of this is because - when a rendering context CopyTo method is called, it naïvely copies all of the TALES definitions to/from the TAL and METAL models.

In the case of an MVC model though, this isn't enough, they have an extra ViewContext property which needs to be copied. This means that it ends up null.

Solution to investigate

Try moving the copy-to functionality downwards into the models themselves.

The rendering context would still have such a method, but the default implementation would just call the one from the models. The base model classes may then do the definition-copying, but the MVC version can override to also copy the appropriate view context property.

[craigfowler]

This is fixed in the branch for #98.