[cDAC] Restore Frame-walk fallback in Thread.GetContext

docs/design/datacontracts/Thread.md

@@ -403,10 +403,21 @@ byte[] IThread.GetContext(TargetPointer threadPointer, ThreadContextSource conte
         return bytes;
     }
 
-    // Fall back to the OS thread context
+    // Try to read the OS thread context from the data target
     ulong osId = target.ReadNUInt(threadPointer + /* Thread::OSId offset */);
-    target.GetThreadContext(osId, contextFlags, bytes);
-    return bytes;
+    if (target.TryGetThreadContext(osId, contextFlags, bytes))
+        return bytes;
+
+    // Derive a context from the explicit Frame chain stored in the Thread (sufficient for managed
+    // debugging stack walks). Walk the Frame chain and pick the first
+    // frame that yields a usable context:
+    //   * If it is an InterpreterFrame, fill the context from the top
+    //     InterpMethodContextFrame.
+    //   * Otherwise, update the context from the frame and accept it when both
+    //     StackPointer and InstructionPointer are non-zero. Mark the resulting
+    //     context flags as full so callers know SP/PC/FP are valid.
+    // If no frame supplies a usable context (thread is not running managed code),
+    // return a zeroed context.
 }
 
 ```

src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/Thread_1.cs

@@ -326,11 +326,48 @@ byte[] IThread.GetContext(TargetPointer threadPointer, ThreadContextSource conte
             return bytes;
         }
 
-        if (!_target.TryGetThreadContext(thread.OSId.Value, contextFlags, buffer))
+        if (_target.TryGetThreadContext(thread.OSId.Value, contextFlags, buffer))
         {
-            throw new InvalidOperationException($"GetThreadContext failed for thread {thread.OSId.Value}");
+            return bytes;
+        }
+
+        // Fall back to deriving a context from the explicit Frame chain stored in the Thread object.
+        return GetContextFromFrames(threadPointer);
+    }
+
+    private byte[] GetContextFromFrames(TargetPointer threadPointer)
+    {
+        IPlatformAgnosticContext context = IPlatformAgnosticContext.GetContextForPlatform(_target);
+
+        ThreadData threadData = ((IThread)this).GetThreadData(threadPointer);
+        FrameIterator iterator = new FrameIterator(_target, threadData);
+        while (iterator.IsValid())
+        {
+            // For InterpreterFrame, fill the context from the top InterpMethodContextFrame
+            // (matches native InterpreterFrame::SetContextToInterpMethodContextFrame).
+            if (iterator.GetCurrentFrameType() == FrameType.InterpreterFrame)
+            {
+                context.Clear();
+                iterator.UpdateContextFromCurrentFrame(context);
+                return context.GetBytes();
+            }
+
+            // For other frames, look for the first (deepest) frame that yields a context
+            // with both SP and PC set (e.g. RedirectedThreadFrame, InlinedCallFrame,
+            // DynamicHelperFrame).
+            context.Clear();
+            iterator.UpdateContextFromCurrentFrame(context);
+            if (context.StackPointer.Value != 0 && context.InstructionPointer.Value != 0)
+            {
+                context.RawContextFlags = context.FullContextFlags;
+                return context.GetBytes();
+            }
+
+            iterator.Next();
         }
 
-        return bytes;
+        // The thread is not running managed code: return a zeroed context.
+        context.Clear();
+        return context.GetBytes();
     }
 }