Differentiate chunks with empty callstacks from those without callstacks

* This is a minor distinction but helps ensure that conversions to/from other
  formats are binary identical.

renderdoc/api/replay/structured_data.h

@@ -206,11 +206,17 @@ DOCUMENT(R"(Bitfield flags that could be applied to an :class:`SDChunk`.
 
   This chunk wasn't supported for decoding or was skipped for another reason and was detailed as
   an opaque byte stream. It should be preserved as-is and will remain in native RDC format.
+
+.. data:: HasCallstack
+
+  This chunk has a callstack. Used to indicate the presence of a callstack even if it's empty
+  (perhaps due to failure to collect the stack frames).
 )");
 enum class SDChunkFlags : uint64_t
 {
   NoFlags = 0x0,
   OpaqueChunk = 0x1,
+  HasCallstack = 0x2,
 };
 
 BITMASK_OPERATORS(SDChunkFlags);

renderdoc/serialise/codecs/xml_codec.cpp

@@ -414,7 +414,7 @@ static ReplayStatus Structured2XML(const char *filename, const RDCFile &file, ui
       xChunk.append_attribute("timestamp") = chunk->metadata.timestampMicro;
     if(chunk->metadata.durationMicro >= 0)
       xChunk.append_attribute("duration") = chunk->metadata.durationMicro;
-    if(!chunk->metadata.callstack.empty())
+    if(chunk->metadata.flags & SDChunkFlags::HasCallstack)
     {
       pugi::xml_node stack = xChunk.append_child("callstack");
 
@@ -746,6 +746,8 @@ static ReplayStatus XML2Structured(const char *xml, const ThumbTypeAndData &thum
     pugi::xml_node callstack = xChunk.child("callstack");
     if(callstack)
     {
+      chunk->metadata.flags |= SDChunkFlags::HasCallstack;
+
       size_t i = 0;
       for(pugi::xml_node address = callstack.first_child(); address; address = address.next_sibling())
       {

renderdoc/serialise/serialiser.cpp

@@ -85,6 +85,8 @@ uint32_t Serialiser<SerialiserMode::Reading>::BeginChunk(uint32_t, uint32_t)
       uint32_t numFrames = 0;
       m_Read->Read(numFrames);
 
+      m_ChunkMetadata.flags |= SDChunkFlags::HasCallstack;
+
       m_ChunkMetadata.callstack.resize((size_t)numFrames);
       m_Read->Read(m_ChunkMetadata.callstack.data(), m_ChunkMetadata.callstack.byteSize());
     }
@@ -297,6 +299,8 @@ uint32_t Serialiser<SerialiserMode::Writing>::BeginChunk(uint32_t chunkID, uint3
           }
         }
 
+        m_ChunkMetadata.flags |= SDChunkFlags::HasCallstack;
+
         uint32_t numFrames = (uint32_t)m_ChunkMetadata.callstack.size();
         m_Write->Write(numFrames);
 
@@ -433,7 +437,7 @@ void Serialiser<SerialiserMode::Writing>::WriteStructuredFile(const SDFile &file
 
     m_ChunkFlags = 0;
 
-    if(!m_ChunkMetadata.callstack.empty())
+    if(m_ChunkMetadata.flags & SDChunkFlags::HasCallstack)
       m_ChunkFlags |= ChunkCallstack;
 
     if(m_ChunkMetadata.threadID != 0)