Bug fixes in BacktraceCommand

Summary: In recent changes, forgot to properly check for root nodes in two places. This would cause us not to output trivial lifelines to roots, or crash when outputting backtraces for such situations.

Differential Revision: D55224461

fbshipit-source-id: 157ca71db3e16ee216fa39d26f5f31ed4d26f5fa

Commands/BacktraceCommand.cs

@@ -177,21 +177,24 @@ bool DumpBacktraceLine(int nodeIndex, Dictionary<int, int>? perTypeCounts, HashS
             sb.Append(CurrentBacktracer.DescribeNodeIndex(nodeIndex, Output, FullyQualified));
 
             int postorderIndex = CurrentBacktracer.NodeIndexToPostorderIndex(nodeIndex);
-            int typeIndex = CurrentTracedHeap.PostorderTypeIndexOrSentinel(postorderIndex);
-            if (typeIndex != -1)
+            if (postorderIndex != -1)
             {
-                if (perTypeCounts != null)
+                int typeIndex = CurrentTracedHeap.PostorderTypeIndexOrSentinel(postorderIndex);
+                if (typeIndex != -1)
                 {
-                    sb.AppendFormat(" x{0}", perTypeCounts[typeIndex]);
-                }
+                    if (perTypeCounts != null)
+                    {
+                        sb.AppendFormat(" x{0}", perTypeCounts[typeIndex]);
+                    }
 
-                if (Fields)
-                {
-                    AppendFields(nodeIndex, successorNodeIndex, sb);
-                }
+                    if (Fields)
+                    {
+                        AppendFields(nodeIndex, successorNodeIndex, sb);
+                    }
 
-                NativeWord address = CurrentTracedHeap.PostorderAddress(nodeIndex);
-                AppendTags(address, sb);
+                    NativeWord address = CurrentTracedHeap.PostorderAddress(nodeIndex);
+                    AppendTags(address, sb);
+                }
             }
 
             Output.AddDisplayStringLineIndented(indent, sb.ToString());
@@ -239,7 +242,7 @@ bool IsDestination(int nodeIndex)
 
         bool ShouldIgnoreNode(int nodeIndex)
         {
-            if (IgnoreIfAncestorHasTag != null)
+            if (IgnoreIfAncestorHasTag != null && CurrentBacktracer.IsLiveObjectNode(nodeIndex))
             {
                 // If a tag was given on the command line, stop if the object has that tag.
                 int postorderIndex = CurrentBacktracer.NodeIndexToPostorderIndex(nodeIndex);