bump cecil to get scope start fix

[radekdoulik]

commit 82a2fe15984df2b697c42b2fef5ccc17370007e3
Author: Radek Doulik <[email protected]>
Date:   Mon Jul 3 22:41:43 2017 +0200

    do not set scope.Start when start_instruction is null

     - this gets us to the original state how scope start was handled
       before
       https://github.com/jbevain/cecil/commit/7c8e0f767d7b2f652a430e4e26f1d98a20f9e125

     - fixes #57889

     - without this change we get excpetion when writing mdb like this:

        The "LinkAssemblies" task failed unexpectedly. [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
        System.NotSupportedException: Specified method is not supported. [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.Cil.InstructionOffset.get_Offset() [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.Mdb.MdbWriter.WriteScope(ScopeDebugInformation scope, MethodDebugInformation info) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.Mdb.MdbWriter.WriteScopes(Collection`1 scopes, MethodDebugInformation info) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.Mdb.MdbWriter.WriteScope(ScopeDebugInformation scope, MethodDebugInformation info) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.Mdb.MdbWriter.WriteScopes(Collection`1 scopes, MethodDebugInformation info) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.Mdb.MdbWriter.WriteRootScope(ScopeDebugInformation scope, MethodDebugInformation info) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.Mdb.MdbWriter.Write(MethodDebugInformation info) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.Cil.CodeWriter.WriteResolvedMethodBody(MethodDefinition method) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.Cil.CodeWriter.WriteMethodBody(MethodDefinition method) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.MetadataBuilder.AddMethod(MethodDefinition method) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.MetadataBuilder.AddMethods(TypeDefinition type) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.MetadataBuilder.AddType(TypeDefinition type) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.MetadataBuilder.AddTypes() [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.MetadataBuilder.BuildTypes() [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.MetadataBuilder.BuildModule() [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.MetadataBuilder.BuildMetadata() [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.ModuleWriter.<>c.b__2_0(MetadataBuilder builder, MetadataReader _) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.ModuleDefinition.Read[TItem,TRet](TItem item, Func`3 read) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.ModuleWriter.BuildMetadata(ModuleDefinition module, MetadataBuilder metadata) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.ModuleWriter.Write(ModuleDefinition module, Disposable`1 stream, WriterParameters parameters) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.ModuleWriter.WriteModule(ModuleDefinition module, Disposable`1 stream, WriterParameters parameters) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.ModuleDefinition.Write(String fileName, WriterParameters parameters) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Linker.Steps.OutputStep.OutputAssembly(AssemblyDefinition assembly) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Linker.Steps.OutputStep.ProcessAssembly(AssemblyDefinition assembly) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           ...

[jonpryor]

As with the prior attempted fix for Bug #57889, I fear that this is just hiding symptoms.

Some quick gripping, and MdbWriter.WriteScope() makes me uncomfortable:

		void WriteScope (ScopeDebugInformation scope, MethodDebugInformation info)
		{
			writer.OpenScope (scope.Start.Offset);

			WriteScopeVariables (scope);

			if (scope.HasScopes)
				WriteScopes (scope.Scopes, info);

			writer.CloseScope (scope.End.IsEndOfMethod ? info.code_size : scope.End.Offset);
		}

This is where your listed stack trace is hitting:

        System.NotSupportedException: Specified method is not supported. [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.Cil.InstructionOffset.get_Offset() [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]
           at Mono.Cecil.Mdb.MdbWriter.WriteScope(ScopeDebugInformation scope, MethodDebugInformation info) [C:\xms\xamarin-forms-samples\Templates\DataTemplateSelector\Droid\Selector.Droid.csproj]

Instead, then, the likely result is that scope.Start.Offset may be 0, as it's not always set -- e.g. if the Instruction instance is created by the Instruction(OpCode, object) constructor.

My quick static analysis doesn't otherwise make me convinced that this approach will continue to work, as I don't understand why jbevain made that change in the first place. Perhaps he was fixing another bug at the same time?

[radekdoulik]

Both of these bug fixes were intended to fix the regression and bring us back to the state after the original bugfix jbevain/cecil@01a695f

So with this second fix we get to the state where cecil handles the broken mdb's again without crashing.

I think the basic question here is whether we want cecil to be able to cope with broken mdb's or not and rather reject them in some way, like throwing appropriate exception. Which is the question of jbevain/cecil#424

[jonpryor]

After my week of (bad) explorations...

I don't think we want Cecil to deal with broken mdb's, but that's jbevain's call.

The more "interesting" thing is that we shouldn't be asking Cecil to deal with broken mdbs in the first place. Bug #57889 is an exception from the <LinkAssemblies/> task, and the <LinkAssemblies/> task should not be loading debug symbols:

https://github.com/xamarin/xamarin-android/blob/4b00475/src/Xamarin.Android.Build.Tasks/Tasks/LinkAssemblies.cs#L88

Thus, the "real" question: Why is the <LinkAssemblies/> task loading debug symbols when it's explicitly asking to not load debug symbols? If it never loaded the debug symbols, we wouldn't care if the debug symbols were corrupted.

The problem here is the loading of debug symbols at all, not Cecil.

[radekdoulik]

I agree with that. Handling broken mdb's is necessary as well though as it can happen in Debug configuration too. It is much less likely, because XamlC task will not cause trouble in Debug. Still I saw at least one report of this issue for third party assembly coming from nuget package IIRC.

I will try to fix assembly loading without symbols as well.

This PR should get us to the state we were for many months before the regression and fix the actual problem, so if we are in hurry it should be ok for d15-3.

[radekdoulik]

Forgot to comment on this

Instead, then, the likely result is that scope.Start.Offset may be 0, as it's not always set -- e.g. if the Instruction instance is created by the Instruction(OpCode, object) constructor.

in this case the Instruction instance is not created and thus the start has the original int value from mdb, which is later written back. https://github.com/jbevain/cecil/blob/1530a21d652992e9adb749d207a4f4fd44ad8024/Mono.Cecil.Cil/Symbols.cs#L183