Regression: FsspecStore over ReferenceFileSystem fails with GroupNotFoundError (path '/' joined as '//key') #3922

Describe the bug

zarr.open_group on an FsspecStore wrapping fsspec.implementations.reference.ReferenceFileSystem raises GroupNotFoundError on current main, even though the reference set contains a valid zarr.json.

This appears to be a regression from #3679 (be0a7b8), which in FsspecStore._get replaced

path = _dereference_path(self.path, key)

with

path = _join_paths([self.path, key])

These differ when self.path == "/" (which is what FsspecStore(fs=ReferenceFileSystem(...)) ends up with):

• _dereference_path("/", "zarr.json") → "zarr.json" (root is rstrip("/")ed to "", so no prefix is added)
• _join_paths(["/", "zarr.json"]) → "//zarr.json" (only empty strings are filtered, so "/" is kept and joined with "/")

ReferenceFileSystem keys its refs by bare "zarr.json", so the lookup misses, _get raises FileNotFoundError, and open_group turns that into GroupNotFoundError.

Local-filesystem-backed FsspecStores aren't affected because their self.path is the full filesystem path (not "/"), so the extra slash never appears.

Steps to reproduce

Save as repro.py and run uv run repro.py:

# /// script
# requires-python = ">=3.11"
# dependencies = [
#     "fsspec",
#     "zarr",
# ]
# ///
import json

import fsspec
import zarr
from zarr.storage import FsspecStore
from zarr.storage._utils import _join_paths

group_json = json.dumps({"zarr_format": 3, "node_type": "group", "attributes": {}})
refs = {"version": 1, "refs": {"zarr.json": group_json}}

fs = fsspec.filesystem("reference", fo=refs, asynchronous=True)
store = FsspecStore(fs=fs, read_only=True)
print(f"zarr version: {zarr.__version__}")
print(f"store: {store!r}")
print(f"store.path: {store.path!r}")
print(f'_join_paths([store.path, "zarr.json"]) = {_join_paths([store.path, "zarr.json"])!r}')

zarr.open_group(store, mode="r")
print("SUCCESS — group opened")

Output on current main (3.1.7.dev45+g866aa8d67)

zarr version: 3.1.7.dev45+g866aa8d67
store: <FsspecStore(ReferenceFileSystem, /)>
store.path: '/'
_join_paths([store.path, "zarr.json"]) = '//zarr.json'
...
zarr.errors.GroupNotFoundError: No group found in store <FsspecStore(ReferenceFileSystem, /)> at path ''

Expected behavior

Same as on zarr 3.1.6: the group is opened successfully.

Environment

• zarr: 3.1.7.dev45+g866aa8d67 (broken), 3.1.6 (works)
• fsspec: 2026.3.0
• Python: 3.13
• OS: macOS / Linux (reproduced on both)

Bisect

Bisected v3.1.6..866aa8d6 to first bad commit be0a7b82 ("feat/memory store registry", #3679).

Downstream impact

Caught while reviewing zarr-developers/VirtualiZarr#957 — 18 kerchunk-roundtrip tests fail against upstream zarr for this reason.