EnC update on method with custom attribute duplicates the attribute

[vritant24]

Given the following method:

[Custom]
public int Method()
{
    return 3;
}
public class CustomAttribute : System.Attribute {}

if there is an edit to change the return statement to return 2, applying the updates from the EnC service leads to 2 [Custom] attributes on Method() during runtime.
Every successive edit adds another instance of the attribute to the CustomAttributes list.

This is an issue in testing because xunit only accepts 1 [Fact] attribute on a method and throws if multiple exist.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[vritant24]

tagging @tmat

[davidwengier]

Looks like this is a bug in how we emit attributes. For example, given a generation 0:

Method (0x06, 0x1C):
==============================================================================================================================================================================================================================
   Name           Signature        RVA         Parameters  GenericParameters  Attributes                                                                ImplAttributes  ImportAttributes  ImportName  ImportModule     
==============================================================================================================================================================================================================================
	<... snip ...>
2: 'F' (#31)      string () (#21)  0x00002054  nil         nil                0x00000091 (PrivateScope, Private, Static, HideBySig)                     0               0                 nil         nil (ModuleRef)  
	<... snip ...>

CustomAttribute (0x0c):
====================================================================================================================================================
   Parent                  Constructor             Value                                                                                            
====================================================================================================================================================
	<... snip ...>
4: 0x06000002 (MethodDef)  0x0a000004 (MemberRef)  01-00-00-00 (#56)    

When method F() is modified we get the follow emitting:

CustomAttribute (0x0c):
=========================================================================
   Parent                  Constructor             Value                 
=========================================================================
1: 0x06000002 (MethodDef)  0x0a000006 (MemberRef)  01-00-00-00 (#75/19)  

EnC Log (0x1e):
===========================================
   Entity                        Operation  
===========================================
	<... snip ...>
9: 0x0c000005 (CustomAttribute)  0          

That CustomAttribute entry in the EnC Log is inserting a new row in CustomAttribute (entity 5) rather than updating the existing row (should entity 4).

For attributes in general we should emit rows that edit any attribute that existed in the old compilation, and then rows for new attributes.

Deletes are TBD, but might be unsupported will be emitted by issuing an update that sets the Parent of a CustomAttribute row to a parent of the same kind, but with row id 0.