LootTableSeed produces different results from /random reset seed

The bug

Loot tables can be seeded in two different ways: with the “random sequence” seed, a global seed for each loot table that advances with each invocation, or with a one-time seed for a particular loot table invocation.

Each of these seeds can be set to a desired number with /random reset or LootTableSeed, respectively. However, these seed numbers produce different results. This makes it impossible to use a desirable random sequence seed for a one-time chest, or reset the random sequence to produce a desirable one-time seed.

How to reproduce

1. /random reset minecraft:chests/simple_dungeon 12345 false

2. /setblock ~ ~ ~ chest{LootTable:"minecraft:chests/simple_dungeon"}

3. /setblock ~1 ~ ~ chest{LootTable:"minecraft:chests/simple_dungeon",LootTableSeed:12345L}

4. Look in the two chests

Expected result

They should have the same items, since they were given the same seed

Observed results

They have different items

Notes

When a desirable loot table result is found, it must have been created with one of these seeds. Finding out the seed and its kind is not difficult. But then achieving that same loot table result with the other kind of seed proves impossible.

/loot does not support providing a one-time seed like LootTableSeed, so, without interacting with the physical game world, commands only have access to the pool of sequence seeds, using /random reset. Any desired one-time seed can’t be reliably used.

Similarly, chests that are desired to have fixed seeded loot that doesn’t change with opening order can only be seeded with LootTableSeed, so any desired sequence seed also can’t be reliably used.

Motivation

When I read MC-306245, I was curious if I could find a loot table seed with even more water buckets. I made a simple data pack to continually reset the sequence seed and generate loot with loot and random until I found a seed that I liked. But when I tried using that seed in LootTableSeed, the chest didn’t have the results I found. The only way to produce those results was by resetting the sequence and opening an unseeded chest that pulled from the sequence.

Code analysis

RandomSequence is backed by XoroshiroRandomSource, whereas LootContext.withOptionalRandomSeed creates a LegacyRandomSource. The legacy random source is likely still around for good reason that I don’t currently understand, so this may be intended. If keeping existing seeds consistent is a concern, this is obviously impossible to fix, since fixing it requires completely upending one pool of seeds in favor of the other.