Extend core.generate_decorations to generate biomes respecting the biome map (#16397)

Large structures which are generated in on_generated callbacks
independently by Lua cannot influence decoration placement. This
change enables such a callback to assume responsibility for generating
decorations itself, presumably after structures are placed, by
disabling decorations in mg_flags and executing
core.generate_decorations.

---------

Co-authored-by: Po Lu <luangruo@yahoo.com>

builtin/game/features.lua

@@ -50,6 +50,7 @@ core.features = {
 	object_guids = true,
 	on_timer_four_args = true,
 	particlespawner_exclude_player = true,
+	generate_decorations_biomes = true,
 }
 
 function core.has_feature(arg)

doc/lua_api.md

@@ -5847,6 +5847,8 @@ Utilities
       on_timer_four_args = true,
       -- `ParticleSpawner` definition supports `exclude_player` field (5.14.0)
       particlespawner_exclude_player = true,
+      -- core.generate_decorations() supports `use_mapgen_biomes` parameter (5.14.0)
+      generate_decorations_biomes = true,
   }
   ```
 
@@ -6739,10 +6741,14 @@ Environment access
     * Generate all registered ores within the VoxelManip `vm` and in the area
       from `pos1` to `pos2`.
     * `pos1` and `pos2` are optional and default to mapchunk minp and maxp.
-* `core.generate_decorations(vm[, pos1, pos2])`
+* `core.generate_decorations(vm[, pos1, pos2, [use_mapgen_biomes]])`
     * Generate all registered decorations within the VoxelManip `vm` and in the
       area from `pos1` to `pos2`.
     * `pos1` and `pos2` are optional and default to mapchunk minp and maxp.
+    * `use_mapgen_biomes` (optional boolean). For use in on_generated callbacks only.
+       If set to true, decorations are placed in respect to the biome map of the current chunk.
+       `pos1` and `pos2` must match the positions of the current chunk, or an error will be raised.
+       default: `false`
 * `core.clear_objects([options])`
     * Clear all objects in the environment
     * Takes an optional table as an argument with the field `mode`.

src/script/lua_api/l_mapgen.cpp

@@ -1611,7 +1611,7 @@ int ModApiMapgen::l_generate_ores(lua_State *L)
 }
 
 
-// generate_decorations(vm, p1, p2)
+// generate_decorations(vm, p1, p2, use_mapgen_biomes)
 int ModApiMapgen::l_generate_decorations(lua_State *L)
 {
 	NO_MAP_LOCK_REQUIRED;
@@ -1621,26 +1621,56 @@ int ModApiMapgen::l_generate_decorations(lua_State *L)
 		return 0;
 
 	DecorationManager *decomgr;
-	if (auto mg = getMapgen(L))
+	Mapgen mg, *mgp = nullptr;
+	bool use_mapgen_biomes = readParam<bool>(L, 4, false);
+	MMVManip *oldvm = nullptr, *vm = checkObject<LuaVoxelManip>(L, 1)->vm;
+	if (auto mg = getMapgen(L)) {
 		decomgr = mg->m_emerge->decomgr;
-	else
+		if (use_mapgen_biomes) {
+			mgp = mg;
+			oldvm = mgp->vm;
+			if (!oldvm) {
+				goto no_vm;
+			}
+		}
+	} else {
+		if (use_mapgen_biomes) {
+			no_vm:
+			throw LuaError("use_mapgen_biomes specified outside a "
+				       "map generation context");
+		}
 		decomgr = emerge->decomgr;
+	}
+	if (!mgp) {
+		mgp = &mg;
+		// Intentionally truncates to s32, see Mapgen::Mapgen()
+		mg.seed = (s32)emerge->mgparams->seed;
+		mg.ndef = emerge->ndef;
+	}
 
-	Mapgen mg;
-	// Intentionally truncates to s32, see Mapgen::Mapgen()
-	mg.seed = (s32)emerge->mgparams->seed;
-	mg.vm   = checkObject<LuaVoxelManip>(L, 1)->vm;
-	mg.ndef = emerge->ndef;
-
-	v3s16 pmin = lua_istable(L, 2) ? check_v3s16(L, 2) :
-			mg.vm->m_area.MinEdge + v3s16(1,1,1) * MAP_BLOCKSIZE;
-	v3s16 pmax = lua_istable(L, 3) ? check_v3s16(L, 3) :
-			mg.vm->m_area.MaxEdge - v3s16(1,1,1) * MAP_BLOCKSIZE;
+	const v3s16 default_pmin = vm->m_area.MinEdge + MAP_BLOCKSIZE,
+				default_pmax = vm->m_area.MaxEdge - MAP_BLOCKSIZE;
+	v3s16 pmin = lua_istable(L, 2) ? check_v3s16(L, 2) : default_pmin;
+	v3s16 pmax = lua_istable(L, 3) ? check_v3s16(L, 3) : default_pmax;
 	sortBoxVerticies(pmin, pmax);
+	if (use_mapgen_biomes) {
+		assert(oldvm);
+		const v3s16 required_pmin = oldvm->m_area.MinEdge + MAP_BLOCKSIZE,
+			required_pmax = oldvm->m_area.MaxEdge - MAP_BLOCKSIZE;
+		if (pmin != required_pmin || pmax != required_pmax)
+			throw LuaError("use_mapgen_biomes requires extents matching chunk area");
+	}
 
 	u32 blockseed = Mapgen::getBlockSeed(pmin, mg.seed);
 
-	decomgr->placeAllDecos(&mg, blockseed, pmin, pmax);
+	mgp->vm = vm;
+	try {
+		decomgr->placeAllDecos(mgp, blockseed, pmin, pmax);
+	} catch (...) {
+		mgp->vm = oldvm;
+		throw;
+	}
+	mgp->vm = oldvm;
 
 	return 0;
 }