River generator for valleys mapgen places c_stone instead of the defined stone

[kromka-chleba]

Luanti version

Luanti 5.10.0 (Linux)
Using LuaJIT 2.1.ROLLING
Built by GCC 11.4
Running on Linux/6.11.11 x86_64
BUILD_TYPE=RelWithDebInfo
RUN_IN_PLACE=0
USE_CURL=1
USE_GETTEXT=1
USE_SOUND=1
STATIC_SHAREDIR="/gnu/store/4dbiiflxqzx52lzl1nkc00njz64ajrad-minetest-5.10.0/share/luanti"
STATIC_LOCALEDIR="/gnu/store/4dbiiflxqzx52lzl1nkc00njz64ajrad-minetest-5.10.0/share/locale"

Operating system and version

Guix System

CPU model

No response

GPU model

No response

Active renderer

No response

Summary

Similar to this bug #9357 but appears to have a different cause, because it doesn't disappear after applying this fix: #15637
Basically mapgen places c_stone nodes at the bottom of a river even though it's not in the biome definition.

Pink c_stone nodes being placed in a river:

Looks like it can get pretty ugly for oceans too:

There's some c_water in caves too for some reason (the yellow nodes).
EDIT: c_water disappears from caves after setting node_cave_liquid to the proper water.

Steps to reproduce

1. Download mapgen_test
2. fly around, find a river, witness pink c_stone nodes that are placed at the bottom

[kromka-chleba]

Surprisingly the bug also happens with only 1 emerge thread:

But it's way more common with more emerge threads.

[cosin15]

My theory:

The thing is that the value for stone is 0. And somewhere in the code memory is used after free, as soon this memory gets recycled and initialized you get stones. This could also happen with one thread. When memory gets allocated.