The 'tag' NBTTagCompound field of the ItemStack assumes that it is OK to
save a reference to an NBT supplied via load() and assumes it is OK to
supply a reference to the internal field during a save(). Neither is true,
as Chunk NBT structures are required to be read-only once created (due to
being written asynchronously off the server thread AND due to the potential
to be passed to a new Chunk if the same chunk is reloaded before the
writing of the NBT is completed by the File I/O thread). Keeping a live
reference to the NBT copy passed in, or to the NBT value passed back
during saving, creates serious thread safety issues which can result in
corrupted data being written to the world data files.
The specific issue here was uncovered by the recent change to use
setName("") on the ItemStack.tag object. When a chunk is being loaded
again before its save is completed, this results in name of the field
in the NBT being set to "". This causes it to be saved as "" instead
of "tag" resulting in it not being properly reloaded in the future which
results in the itemstack losing all of its metadata.
An ItemStack gains the tag name "tag" when the stack is serialized
to NBT, however items don't have a tag *until* they are serialized at
least once. So to solve this, we remove the tag name when loading the
NBT data.
Another problem with NBT are TagLists, when transferring tag lists
between the server and the client the names are lost, and so we
simply don't add a name to the tag.
Filtering item data is usually a good idea to make sure we don't have
invalid data or data on items that shouldn't have it. However, anvils
use item data in slightly different way and so running its code for
filtering here causes the data to be corrupted.
We know these updates (can) break plugins bypassing Bukkit. They are needed for
smooth updates however. There will be another one right before before 1.1-R1.
