[cDAC] Fix stale Continuation MT bug (#129109)

_continuationMethodTablePointer and _continuationSingletonEEClassPointer
are not filled on startup, but on the first creation of a Continuation
without metadata. Therefore, on a live target we may read a null MT if
we only read upon contract creation. Update this such that if these are
still null, we try again to read them.

Author: rcj1

src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/RuntimeTypeSystem_1.cs

@@ -19,8 +19,8 @@ internal partial struct RuntimeTypeSystem_1 : IRuntimeTypeSystem
     private readonly Target _target;
     private readonly TargetPointer _freeObjectMethodTablePointer;
     private readonly TargetPointer _objectMethodTablePointer;
-    private readonly TargetPointer _continuationMethodTablePointer;
-    private readonly TargetPointer _continuationSingletonEEClassPointer;
+    private TargetPointer _continuationMethodTablePointer;
+    private TargetPointer _continuationSingletonEEClassPointer;
     private readonly TargetPointer _multicastDelegateMethodTablePointer;
     private readonly ulong _methodDescAlignment;
     private readonly TypeValidation _typeValidation;
@@ -436,7 +436,29 @@ internal RuntimeTypeSystem_1(Target target)
 
     internal TargetPointer FreeObjectMethodTablePointer => _freeObjectMethodTablePointer;
     internal TargetPointer ObjectMethodTablePointer => _objectMethodTablePointer;
-    internal TargetPointer ContinuationMethodTablePointer => _continuationMethodTablePointer;
+    internal TargetPointer ContinuationMethodTablePointer
+    {
+        get
+        {
+            if (_continuationMethodTablePointer != TargetPointer.Null)
+                return _continuationMethodTablePointer;
+            _continuationMethodTablePointer = _target.ReadPointer(
+                _target.ReadGlobalPointer(Constants.Globals.ContinuationMethodTable));
+            return _continuationMethodTablePointer;
+        }
+    }
+
+    internal TargetPointer ContinuationSingletonEEClassPointer
+    {
+        get
+        {
+            if (_continuationSingletonEEClassPointer != TargetPointer.Null)
+                return _continuationSingletonEEClassPointer;
+            _continuationSingletonEEClassPointer = _target.ReadPointer(
+                _target.ReadGlobalPointer(Constants.Globals.ContinuationSingletonEEClass));
+            return _continuationSingletonEEClassPointer;
+        }
+    }
 
     internal ulong MethodDescAlignment => _methodDescAlignment;
 
@@ -571,10 +593,10 @@ public bool IsObjRef(TypeHandle typeHandle)
     public bool ContainsGCPointers(TypeHandle typeHandle) => !typeHandle.IsMethodTable() ? false : _methodTables[typeHandle.Address].Flags.ContainsGCPointers;
     public bool RequiresAlign8(TypeHandle typeHandle) => !typeHandle.IsMethodTable() ? false : _methodTables[typeHandle.Address].Flags.RequiresAlign8;
     public bool IsContinuationWithoutMetadata(TypeHandle typeHandle) => typeHandle.IsMethodTable()
-        && _continuationMethodTablePointer != TargetPointer.Null
-        && _methodTables[typeHandle.Address].ParentMethodTable == _continuationMethodTablePointer
-        && _continuationSingletonEEClassPointer != TargetPointer.Null
-        && GetClassPointer(typeHandle) == _continuationSingletonEEClassPointer;
+        && ContinuationMethodTablePointer != TargetPointer.Null
+        && _methodTables[typeHandle.Address].ParentMethodTable == ContinuationMethodTablePointer
+        && ContinuationSingletonEEClassPointer != TargetPointer.Null
+        && GetClassPointer(typeHandle) == ContinuationSingletonEEClassPointer;
 
     IEnumerable<(uint Offset, uint Size)> IRuntimeTypeSystem.GetGCDescSeries(TypeHandle typeHandle, uint numComponents)
     {

src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/RuntimeTypeSystemHelpers/TypeValidation.cs

@@ -9,8 +9,8 @@ namespace Microsoft.Diagnostics.DataContractReader.RuntimeTypeSystemHelpers;
 internal sealed class TypeValidation
 {
     private readonly Target _target;
-    private readonly TargetPointer _continuationMethodTablePointer;
-    private readonly TargetPointer _continuationSingletonEEClassPointer;
+    private TargetPointer _continuationMethodTablePointer;
+    private TargetPointer _continuationSingletonEEClassPointer;
 
     internal TypeValidation(Target target, TargetPointer continuationMethodTablePointer, TargetPointer continuationSingletonEEClassPointer)
     {
@@ -19,6 +19,30 @@ internal TypeValidation(Target target, TargetPointer continuationMethodTablePoin
         _continuationSingletonEEClassPointer = continuationSingletonEEClassPointer;
     }
 
+    private TargetPointer ContinuationMethodTablePointer
+    {
+        get
+        {
+            if (_continuationMethodTablePointer != TargetPointer.Null)
+                return _continuationMethodTablePointer;
+            _continuationMethodTablePointer = _target.ReadPointer(
+                _target.ReadGlobalPointer(Constants.Globals.ContinuationMethodTable));
+            return _continuationMethodTablePointer;
+        }
+    }
+
+    private TargetPointer ContinuationSingletonEEClassPointer
+    {
+        get
+        {
+            if (_continuationSingletonEEClassPointer != TargetPointer.Null)
+                return _continuationSingletonEEClassPointer;
+            _continuationSingletonEEClassPointer = _target.ReadPointer(
+                _target.ReadGlobalPointer(Constants.Globals.ContinuationSingletonEEClass));
+            return _continuationSingletonEEClassPointer;
+        }
+    }
+
     // This doesn't need as many properties as MethodTable because we don't want to be operating on
     // a NonValidatedMethodTable for too long
     internal struct NonValidatedMethodTable
@@ -235,10 +259,10 @@ private TargetPointer GetClassThrowing(NonValidatedMethodTable methodTable)
     // duplicate the check using the raw ParentMethodTable read from target memory.
     private bool IsContinuationWithoutMetadata(NonValidatedMethodTable methodTable)
     {
-        return _continuationMethodTablePointer != TargetPointer.Null
-            && methodTable.ParentMethodTable == _continuationMethodTablePointer
-            && _continuationSingletonEEClassPointer != TargetPointer.Null
-            && GetClassThrowing(methodTable) == _continuationSingletonEEClassPointer;
+        return ContinuationMethodTablePointer != TargetPointer.Null
+            && methodTable.ParentMethodTable == ContinuationMethodTablePointer
+            && ContinuationSingletonEEClassPointer != TargetPointer.Null
+            && GetClassThrowing(methodTable) == ContinuationSingletonEEClassPointer;
     }
 
     internal bool TryValidateMethodTablePointer(TargetPointer methodTablePointer)