Fix NullReferenceException in SegmentedList.Add during large heap snapshots

[Copilot]

Taking a heap snapshot on large heaps (100M+ objects) throws System.NullReferenceException inside SegmentedList<T>.Add via Graph.SetNodeTypeAndSize → Node.WriteCompressedInt.

Root Cause

SegmentedMemoryStreamWriter.Clear() was replacing the backing bytes list with a brand-new empty SegmentedList<byte>:

// Before
public virtual void Clear() { bytes = new SegmentedList<byte>(131_072); }

Graph.ClearWorker() creates the writer with a large initial capacity (m_expectedNodeCount * 8, up to ~1 GB for 128M-node graphs) and then immediately calls m_writer.Clear(), discarding that entire pre-allocation. The replacement list starts with items = null. When Write(byte) is JIT-inlined into WriteCompressedInt, calling bytes.Add(value) on this fresh null-items list under memory pressure results in a NullReferenceException inside SegmentedList.Add. There's also a segment-size inconsistency: the constructor uses segmentSize = 65_536; Clear() was creating a new list with segmentSize = 131_072.

Fix

Reset the count of the existing bytes list rather than replacing it:

// After
public virtual void Clear() { bytes.Count = 0; }

This preserves the pre-allocated buffer (eliminating the wasted ~1 GB allocation and the items = null state), keeps the segment size consistent, and removes the NullReferenceException risk.

Changes

• src/FastSerialization/SegmentedMemoryStreamWriter.cs — one-line fix to Clear()
• src/FastSerialization.Tests/SegmentedMemoryStreamWriterTests.cs — new tests covering Clear() semantics, write/read round-trips, multiple clear cycles, and cross-segment-boundary writes

Original prompt

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This section details on the original issue you should resolve

<issue_title>System.NullReferenceException while taking heap snapshot</issue_title>
<issue_description>Version: P2.0.42 (64 bit)
Max Dump K objects: 250
Freeze: No
Save ETL: No

Tail of log:

1070.1s: Dumped 114,354,847 objects, max_dump_limit 134,217,716 Dumper heap Size 7,384MB
HeapDump Error: System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Collections.Generic.SegmentedList`1.Add(T item)
   at Graphs.Node.WriteCompressedInt(SegmentedMemoryStreamWriter writer, Int32 value)
   at Graphs.Graph.SetNodeTypeAndSize(NodeIndex nodeIndex, NodeTypeIndex typeIndex, Int32 sizeInBytes)
   at Graphs.Graph.SetNode(NodeIndex nodeIndex, NodeTypeIndex typeIndex, Int32 sizeInBytes, GrowableArray`1 children)
   at GCHeapDumper.DumpAllSegments()
   at GCHeapDumper.DumpDotNetHeapDataWorker(ClrHeap heap, ICorDebugProcess& debugProcess, Boolean isDump, Double retryScale)
   at GCHeapDumper.DumpDotNetHeapData(ClrHeap heap, ICorDebugProcess& debugProcess, Boolean isDump)
   at GCHeapDumper.TryGetDotNetDump(Int32 processID)
   at GCHeapDumper.CaptureLiveHeapDump(Int32 processID)
   at GCHeapDumper.DumpLiveHeap(Int32 processID)
   at Program.MainWorker(String[] args)


Completed: Dumping GC Heap to c:\temp\20190613.gcDump   (Elapsed Time: 1071.709 sec)
Error: HeapDump failed with exit code 1

</issue_description>

Comments on the Issue (you are @copilot in this section)

Custom agent used: Issue Triager
Investigates, reproduces, and fixes issues in PerfView and TraceEvent.

• Fixes System.NullReferenceException while taking heap snapshot #951

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