Determine metadata length without an extra ostringstream

src/mapblock.cpp

@@ -415,12 +415,13 @@ void MapBlock::serialize(std::ostream &os_compressed, u8 version, bool disk, int
  /*
  Node metadata
  */
+ u32 metadata_offset = 0;
+ u32 metadata_length = 0;
  if (version >= 29) {
- std::ostringstream metadata(std::ios_base::binary);
- m_node_metadata.serialize(metadata, version, disk);
- std::string metadata_str = metadata.str();
- writeU32(os, metadata_str.size());
- os.write(metadata_str.c_str(), metadata_str.size());
+ metadata_offset = os.tellp();
+ writeU32(os, 0); // placeholder for length
+ m_node_metadata.serialize(os, version, disk);
+ metadata_length = (u32)os.tellp() - metadata_offset - sizeof(u32);
  } else {
  // use os_raw from above to avoid allocating another stream object
  m_node_metadata.serialize(os_raw, version, disk);
@@ -454,8 +455,11 @@ void MapBlock::serialize(std::ostream &os_compressed, u8 version, bool disk, int
  }
 
  if(version >= 29) {
+ // write the metadata length
+ std::string output = os_raw.str();
+ writeU32(reinterpret_cast<u8*>(&output[metadata_offset]), metadata_length);
  // now compress the whole thing
- compress(os_raw.str(), os_compressed, version, compression_level);
+ compress(output, os_compressed, version, compression_level);
  }
 }