Mapgen: "Unfinished" y-slices with num_emerge_threads > 1 #9357
Description
Minetest version
908e762
Bug has been around at least since 0.4.17
OS / Hardware
Operating system: Arch Linux
CPU: Intel(R) Core(TM) i5-3210M CPU @ 2.50GHz
Summary
When setting num_emerge_threads to a value greater than one (default chunksize), the mapgen sometimes generates strange y-slices where biome-specific nodes, ores and caves are missing, and decorations extending into them seem to be cut off above.
I've been able to reproduce this with several different mapgens (v7, valleys, carpathian) and map database backends (SQLite, PostgreSQL).
I am aware that using multiple emerge threads is considered somewhat experimental and advised against in the default config, but still have hope that someone with in-depth knowledge of the mapgen code (@paramat 😉) might be able to spot this one.
Steps to reproduce
Just start a game and fly around for a few minutes. The bug appears using the minimal game, but may be easier to spot in MTG because of the cut-off trees.
Above ground (valleys, minimal game):
Above ground (v7, MTG):
Above ground (valleys, MTG):
Caverns (valleys, MTG):
The slices always seem to appear at mapblock borders (see coordinates):
As far as I understand it, it always happens in (parts of) a mapchunk's topmost (Edit: or lowermost) y-slice.
When other mods try to place additional structures in an on_generated callback, they are cut off as well (in this case https://github.com/FaceDeer/settlements):
