Implement VoxelManipulator-based transactions.

Rather than calling out through the Lua-to-C API for each node that
needs to be read or written, a group of map reads (and optionally
writes) can be grouped into a transaction. Access to map data within the
transaction is provided at high speed by means of VoxelManipulators.
Once the reads and writes are finished, the transaction can be committed
or aborted.

mesecons/util.lua

@@ -236,8 +236,100 @@ local function unhash_blockpos(hash)
 	return vector.multiply(minetest.get_position_from_hash(hash), BLOCKSIZE)
 end
 
--- get node and force-load area
+-- Maps from a hashed mapblock position (as returned by hash_blockpos) to a
+-- table.
+--
+-- Contents of the table are:
+-- “vm” → the VoxelManipulator
+-- “va” → the VoxelArea
+-- “data” → the data array
+-- “param1” → the param1 array
+-- “param2” → the param2 array
+-- “dirty” → true if data has been modified
+--
+-- Nil if no VM-based transaction is in progress.
+local vm_cache = nil
+
+-- Starts a VoxelManipulator-based transaction.
+--
+-- During a VM transaction, calls to vm_get_node and vm_swap_node operate on a
+-- cached copy of the world loaded via VoxelManipulators. That cache can later
+-- be committed to the real map by means of vm_commit or discarded by means of
+-- vm_abort.
+function mesecon.vm_begin()
+	vm_cache = {}
+end
+
+-- Finishes a VoxelManipulator-based transaction, freeing the VMs and map data
+-- and writing back any modified areas.
+function mesecon.vm_commit()
+	for hash, tbl in pairs(vm_cache) do
+		if tbl.dirty then
+			local vm = tbl.vm
+			vm:set_data(tbl.data)
+			vm:write_to_map()
+			vm:update_map()
+		end
+	end
+	vm_cache = nil
+end
+
+-- Finishes a VoxelManipulator-based transaction, freeing the VMs and throwing
+-- away any modified areas.
+function mesecon.vm_abort()
+	vm_cache = nil
+end
+
+-- Gets the cache entry covering a position, populating it if necessary.
+local function vm_get_or_create_entry(pos)
+	local hash = hash_blockpos(pos)
+	local tbl = vm_cache[hash]
+	if not tbl then
+		local vm = minetest.get_voxel_manip(pos, pos)
+		local min_pos, max_pos = vm:get_emerged_area()
+		local va = VoxelArea:new{MinEdge = min_pos, MaxEdge = max_pos}
+		tbl = {vm = vm, va = va, data = vm:get_data(), param1 = vm:get_light_data(), param2 = vm:get_param2_data(), dirty = false}
+		vm_cache[hash] = tbl
+	end
+	return tbl
+end
+
+-- Gets the node at a given position during a VoxelManipulator-based
+-- transaction.
+function mesecon.vm_get_node(pos)
+	local tbl = vm_get_or_create_entry(pos)
+	local index = tbl.va:indexp(pos)
+	local node_value = tbl.data[index]
+	if node_value == core.CONTENT_IGNORE then
+		return nil
+	else
+		local node_param1 = tbl.param1[index]
+		local node_param2 = tbl.param2[index]
+		return {name = minetest.get_name_from_content_id(node_value), param1 = node_param1, param2 = node_param2}
+	end
+end
+
+-- Sets a node’s name during a VoxelManipulator-based transaction.
+--
+-- Existing param1, param2, and metadata are left alone.
+function mesecon.vm_swap_node(pos, name)
+	local tbl = vm_get_or_create_entry(pos)
+	local index = tbl.va:indexp(pos)
+	tbl.data[index] = minetest.get_content_id(name)
+	tbl.dirty = true
+end
+
+-- Gets the node at a given position, regardless of whether it is loaded or
+-- not, respecting a transaction if one is in progress.
+--
+-- Outside a VM transaction, if the mapblock is not loaded, it is pulled into
+-- the server’s main map data cache and then accessed from there.
+--
+-- Inside a VM transaction, the transaction’s VM cache is used.
 function mesecon.get_node_force(pos)
+	if vm_cache then
+		return mesecon.vm_get_node(pos)
+	else
 		local node = minetest.get_node_or_nil(pos)
 		if node == nil then
 			-- Node is not currently loaded; use a VoxelManipulator to prime
@@ -246,6 +338,29 @@ function mesecon.get_node_force(pos)
 			node = minetest.get_node_or_nil(pos)
 		end
 		return node
+	end
+end
+
+-- Swaps the node at a given position, regardless of whether it is loaded or
+-- not, respecting a transaction if one is in progress.
+--
+-- Outside a VM transaction, if the mapblock is not loaded, it is pulled into
+-- the server’s main map data cache and then accessed from there.
+--
+-- Inside a VM transaction, the transaction’s VM cache is used.
+--
+-- This function can only be used to change the node’s name, not its parameters
+-- or metadata.
+function mesecon.swap_node_force(pos, name)
+	if vm_cache then
+		return mesecon.vm_swap_node(pos, name)
+	else
+		-- This serves to both ensure the mapblock is loaded and also hand us
+		-- the old node table so we can preserve param2.
+		local node = mesecon.get_node_force(pos)
+		node.name = name
+		minetest.swap_node(pos, node)
+	end
 end
 
 -- Un-forceload any forceloaded mapblocks from older versions of Mesecons which