Prevent VoxelManipulator size overflow

doc/lua_api.md

@@ -5044,7 +5044,7 @@ inside the VoxelManip.
   can use `core.emerge_area` to make sure that the area you want to
   read/write is already generated.
 
-* Other mods, or the core itself, could possibly modify the area of the map
+* Other mods, or the engine itself, could possibly modify the area of the map
   currently loaded into a VoxelManip object. With the exception of Mapgen
   VoxelManips (see above section), the internal buffers are not updated. For
   this reason, it is strongly encouraged to complete the usage of a particular
@@ -5059,9 +5059,11 @@ inside the VoxelManip.
 Methods
 -------
 
-* `read_from_map(p1, p2)`:  Loads a chunk of map into the VoxelManip object
+* `read_from_map(p1, p2)`: Loads a chunk of map into the VoxelManip object
   containing the region formed by `p1` and `p2`.
     * returns actual emerged `pmin`, actual emerged `pmax`
+    * Note that calling this multiple times will *add* to the area loaded in the
+      VoxelManip, and not reset it.
 * `write_to_map([light])`: Writes the data loaded from the `VoxelManip` back to
   the map.
     * **important**: data must be set using `VoxelManip:set_data()` before
@@ -5120,8 +5122,8 @@ Methods
       generated mapchunk above are propagated down into the mapchunk, defaults
       to `true` if left out.
 * `update_liquids()`: Update liquid flow
-* `was_modified()`: Returns `true` if the data in the voxel manipulator has been modified
-   since it was last read from the map. This means you have to call `get_data` again.
+* `was_modified()`: Returns `true` if the data in the VoxelManip has been modified
+   since it was last read from the map. This means you have to call `get_data()` again.
    This only applies to a `VoxelManip` object from `core.get_mapgen_object`,
    where the engine will keep the map and the VM in sync automatically.
    * Note: this doesn't do what you think it does and is subject to removal. Don't use it!

src/map.cpp

@@ -752,17 +752,12 @@ MMVManip::MMVManip(Map *map):
 	assert(map);
 }
 
-void MMVManip::initialEmerge(v3s16 blockpos_min, v3s16 blockpos_max,
-	bool load_if_inexistent)
+void MMVManip::initialEmerge(v3s16 p_min, v3s16 p_max, bool load_if_inexistent)
 {
 	TimeTaker timer1("initialEmerge", &emerge_time);
 
 	assert(m_map);
 
-	// Units of these are MapBlocks
-	v3s16 p_min = blockpos_min;
-	v3s16 p_max = blockpos_max;
-
 	VoxelArea block_area_nodes
 			(p_min*MAP_BLOCKSIZE, (p_max+1)*MAP_BLOCKSIZE-v3s16(1,1,1));
 
@@ -775,6 +770,7 @@ void MMVManip::initialEmerge(v3s16 blockpos_min, v3s16 blockpos_max,
 		infostream<<std::endl;
 	}
 
+	const bool all_new = m_area.hasEmptyExtent() || block_area_nodes.contains(m_area);
 	addArea(block_area_nodes);
 
 	for(s32 z=p_min.Z; z<=p_max.Z; z++)
@@ -812,16 +808,12 @@ void MMVManip::initialEmerge(v3s16 blockpos_min, v3s16 blockpos_max,
 				setFlags(a, VOXELFLAG_NO_DATA);
 			}
 		}
-		/*else if (block->getNode(0, 0, 0).getContent() == CONTENT_IGNORE)
-		{
-			// Mark that block was loaded as blank
-			flags |= VMANIP_BLOCK_CONTAINS_CIGNORE;
-		}*/
 
 		m_loaded_blocks[p] = flags;
 	}
 
-	m_is_dirty = false;
+	if (all_new)
+		m_is_dirty = false;
 }
 
 void MMVManip::blitBackAll(std::map<v3s16, MapBlock*> *modified_blocks,
@@ -834,6 +826,7 @@ void MMVManip::blitBackAll(std::map<v3s16, MapBlock*> *modified_blocks,
 	/*
 		Copy data of all blocks
 	*/
+	assert(!m_loaded_blocks.empty());
 	for (auto &loaded_block : m_loaded_blocks) {
 		v3s16 p = loaded_block.first;
 		MapBlock *block = m_map->getBlockNoCreateNoEx(p);

src/map.h

@@ -298,9 +298,6 @@ protected:
 		u32 needed_count);
 };
 
-#define VMANIP_BLOCK_DATA_INEXIST     1
-#define VMANIP_BLOCK_CONTAINS_CIGNORE 2
-
 class MMVManip : public VoxelManipulator
 {
 public:
@@ -344,4 +341,8 @@ protected:
 		value = flags describing the block
 	*/
 	std::map<v3s16, u8> m_loaded_blocks;
+
+	enum : u8 {
+		VMANIP_BLOCK_DATA_INEXIST = 1 << 0,
+	};
 };

src/voxel.cpp

@@ -113,6 +113,20 @@ void VoxelManipulator::print(std::ostream &o, const NodeDefManager *ndef,
 	}
 }
 
+static inline void checkArea(const VoxelArea &a)
+{
+	// won't overflow since cbrt(2^64) > 2^16
+	u64 real_volume = static_cast<u64>(a.getExtent().X) * a.getExtent().Y * a.getExtent().Z;
+
+	// Volume limit equal to 8 default mapchunks, (80 * 2) ^ 3 = 4,096,000
+	// Note: the hard limit is somewhere around 2^31 due to s32 type
+	constexpr u64 MAX_ALLOWED = 4096000;
+	if (real_volume > MAX_ALLOWED) {
+		throw BaseException("VoxelManipulator: "
+			"Area volume exceeds allowed value of " + std::to_string(MAX_ALLOWED));
+	}
+}
+
 void VoxelManipulator::addArea(const VoxelArea &area)
 {
 	// Cancel if requested area has zero volume
@@ -124,18 +138,10 @@ void VoxelManipulator::addArea(const VoxelArea &area)
 		return;
 
 	// Calculate new area
-	VoxelArea new_area;
-	// New area is the requested area if m_area has zero volume
-	if(m_area.hasEmptyExtent())
-	{
-		new_area = area;
-	}
-	// Else add requested area to m_area
-	else
-	{
-		new_area = m_area;
-		new_area.addArea(area);
-	}
+	VoxelArea new_area = m_area;
+	new_area.addArea(area);
+
+	checkArea(new_area);
 
 	u32 new_size = new_area.getVolume();