Fix seg fault if popping from empty stack (L-system trees)

See: https://github.com/minetest/minetest/issues/1525

Background
Wuzzy2: If you attempt to spawn a L-system tree with minetest.spawn_tree, you can make Minetest crash if it is attempted to pop an empty stack.

ShadowNinja: This shouldn't cause a segmentation fault, but it should throw a Lua error

Commit Description
This commit throws a Lua error instead of causing a segmentation fault. The server will still "crash" but will include a Lua backtrace.

L-Systems fix randomness
Unless a random seed is provided (via Lua treedef) seed the PRNG with a different seed for each tree
Resolves: https://github.com/minetest/minetest/issues/1469

Fix l-system crash when treedef random_level not set by Lua

1 files changed, 13 insertions, 3 deletions

diff --git a/src/script/lua_api/l_env.cpp b/src/script/lua_api/l_env.cpp
index e6ca846d8..deeb5966f 100644
--- a/src/script/lua_api/l_env.cpp
+++ b/src/script/lua_api/l_env.cpp
@@ -747,7 +747,8 @@ int ModApiEnvMod::l_spawn_tree(lua_State *L)
 		}
 		getintfield(L, 2, "angle", tree_def.angle);
 		getintfield(L, 2, "iterations", tree_def.iterations);
-		getintfield(L, 2, "random_level", tree_def.iterations_random_level);
+		if (!getintfield(L, 2, "random_level", tree_def.iterations_random_level))
+			tree_def.iterations_random_level = 0;
 		getstringfield(L, 2, "trunk_type", tree_def.trunk_type);
 		getboolfield(L, 2, "thin_branches", tree_def.thin_branches);
 		tree_def.fruit_chance=0;
@@ -757,11 +758,20 @@ int ModApiEnvMod::l_spawn_tree(lua_State *L)
 			tree_def.fruitnode=ndef->getId(fruit);
 			getintfield(L, 2, "fruit_chance",tree_def.fruit_chance);
 		}
-		getintfield(L, 2, "seed", tree_def.seed);
+		tree_def.explicit_seed = getintfield(L, 2, "seed", tree_def.seed);
 	}
 	else
 		return 0;
-	treegen::spawn_ltree (env, p0, ndef, tree_def);
+
+	treegen::error e;
+	if ((e = treegen::spawn_ltree (env, p0, ndef, tree_def)) != treegen::SUCCESS) {
+		if (e == treegen::UNBALANCED_BRACKETS) {
+			luaL_error(L, "spawn_tree(): closing ']' has no matching opening bracket");
+		} else {
+			luaL_error(L, "spawn_tree(): unknown error");
+		}
+	}
+
 	return 1;
 }