MemoryStore.list_dir incorrectly includes non-immediate children

[TomAugspurger]

Zarr version

v3

Numcodecs version

n/a

Python Version

n/a

Operating System

n/a

Installation

n/a

Description

MemoryStore.list_dir incorrectly includes an entry for each nested child of a group. This differs from the behavior of LocalStore.list_dir

Steps to reproduce

This program sets up a store with some nested objects:

import zarr.store
import zarr.core.buffer


async def main():
    local = zarr.store.LocalStore("/tmp/foo", mode="w")
    mem = zarr.store.MemoryStore({}, mode="w")

    paths = [
        'root/a0/zarr.json',
        'root/g0/zarr.json',
        'root/g0/a1/zarr.json',
        'root/g0/g1/zarr.json',
        'root/g0/g1/a2/zarr.json',
        'root/g0/g1/g2/zarr.json',
    ]

    for store in [local, mem]:
        for path in paths:
            await store.set(path, zarr.core.buffer.Buffer.from_bytes(b""))

        print(type(store).__name__)
        print([x async for x in store.list_dir("root")])
        print("-" * 80)


if __name__ == "__main__":
    import asyncio
    asyncio.run(main())

Here's the output:

LocalStore
['g0', 'a0']
--------------------------------------------------------------------------------
MemoryStore
['a0', 'g0', 'g0', 'g0', 'g0', 'g0']
--------------------------------------------------------------------------------

The g0 group is repeated once per child.

Additional output

I'll have a PR shortly.