fix(storage): preserve leading slashes in FsspecStore.path

src/zarr/storage/_fsspec.py

@@ -16,7 +16,7 @@
 )
 from zarr.core.buffer import Buffer
 from zarr.errors import ZarrUserWarning
-from zarr.storage._utils import _join_paths, normalize_path
+from zarr.storage._utils import _dereference_path
 
 if TYPE_CHECKING:
     from collections.abc import AsyncIterator, Iterable
@@ -127,7 +127,7 @@ def __init__(
     ) -> None:
         super().__init__(read_only=read_only)
         self.fs = fs
-        self.path = normalize_path(path)
+        self.path = path
         self.allowed_exceptions = allowed_exceptions
 
         if not self.fs.async_impl:
@@ -282,7 +282,7 @@ async def get(
         # docstring inherited
         if not self._is_open:
             await self._open()
-        path = _join_paths([self.path, key])
+        path = _dereference_path(self.path, key)
 
         try:
             if byte_range is None:
@@ -329,7 +329,7 @@ async def set(
             raise TypeError(
                 f"FsspecStore.set(): `value` must be a Buffer instance. Got an instance of {type(value)} instead."
             )
-        path = _join_paths([self.path, key])
+        path = _dereference_path(self.path, key)
         # write data
         if byte_range:
             raise NotImplementedError
@@ -338,7 +338,7 @@ async def set(
     async def delete(self, key: str) -> None:
         # docstring inherited
         self._check_writable()
-        path = _join_paths([self.path, key])
+        path = _dereference_path(self.path, key)
         try:
             await self.fs._rm(path)
         except FileNotFoundError:
@@ -354,14 +354,14 @@ async def delete_dir(self, prefix: str) -> None:
             )
         self._check_writable()
 
-        path_to_delete = _join_paths([self.path, prefix])
+        path_to_delete = _dereference_path(self.path, prefix)
 
         with suppress(*self.allowed_exceptions):
             await self.fs._rm(path_to_delete, recursive=True)
 
     async def exists(self, key: str) -> bool:
         # docstring inherited
-        path = _join_paths([self.path, key])
+        path = _dereference_path(self.path, key)
         exists: bool = await self.fs._exists(path)
         return exists
 
@@ -378,7 +378,7 @@ async def get_partial_values(
             starts: list[int | None] = []
             stops: list[int | None] = []
             for key, byte_range in key_ranges:
-                paths.append(_join_paths([self.path, key]))
+                paths.append(_dereference_path(self.path, key))
                 if byte_range is None:
                     starts.append(None)
                     stops.append(None)
@@ -429,7 +429,7 @@ async def list_prefix(self, prefix: str) -> AsyncIterator[str]:
             yield onefile.removeprefix(f"{self.path}/")
 
     async def getsize(self, key: str) -> int:
-        path = _join_paths([self.path, key])
+        path = _dereference_path(self.path, key)
         info = await self.fs._info(path)
 
         size = info.get("size")

src/zarr/storage/_utils.py

@@ -185,6 +185,53 @@ def _join_paths(paths: Iterable[str]) -> str:
     return "/".join(filter(lambda v: v != "", paths))
 
 
+def _dereference_path(root: str, path: str) -> str:
+    """
+    Combine a store-side root with a key into a single fully-qualified path.
+
+    Unlike `_join_paths`, this is purpose-built for the case where `root` is
+    an opaque backend-side prefix that may use `"/"` as a sentinel for "root
+    of the filesystem" (notably for fsspec's `ReferenceFileSystem`). A
+    trailing `"/"` is stripped from `root` before joining; if `root` is then
+    empty, the bare `path` is returned so that joining `"/"` with `"key"`
+    yields `"key"` rather than `"//key"`. A trailing `"/"` on the result is
+    also stripped.
+
+    Leading slashes on `root` are preserved -- a backend-side path like
+    `"/home/foo/data.zarr"` is an absolute filesystem path for
+    `LocalFileSystem` and must not lose its leading separator.
+
+    Parameters
+    ----------
+    root : str
+        The backend-side root of a store. May be `""`, `"/"`, an absolute
+        filesystem path, or a backend-specific prefix.
+    path : str
+        The key within the store, typically a zarr key like `"zarr.json"`
+        or `"a/b/c/zarr.json"`.
+
+    Returns
+    -------
+    str
+        `root` and `path` joined by a single `"/"`, with the `"/"` sentinel
+        collapsed and trailing slashes removed.
+
+    Examples
+    --------
+    ```python
+    from zarr.storage._utils import _dereference_path
+    _dereference_path("/", "zarr.json")          # 'zarr.json'
+    _dereference_path("", "zarr.json")           # 'zarr.json'
+    _dereference_path("/home/foo", "zarr.json")  # '/home/foo/zarr.json'
+    _dereference_path("/home/foo/", "zarr.json") # '/home/foo/zarr.json'
+    _dereference_path("bucket/p", "zarr.json")   # 'bucket/p/zarr.json'
+    ```
+    """
+    root = root.rstrip("/")
+    path = f"{root}/{path}" if root else path
+    return path.rstrip("/")
+
+
 def _relativize_path(*, path: str, prefix: str) -> str:
     """
     Make a "/"-delimited path relative to some prefix. If the prefix is '', then the path is

tests/test_store/test_fsspec.py

@@ -17,7 +17,6 @@
 from zarr.errors import ZarrUserWarning
 from zarr.storage import FsspecStore
 from zarr.storage._fsspec import _make_async
-from zarr.storage._utils import normalize_path
 from zarr.testing.store import StoreTests
 
 if TYPE_CHECKING:
@@ -287,25 +286,114 @@ def array_roundtrip(store: FsspecStore) -> None:
     np.testing.assert_array_equal(arr[:], data)
 
 
-@pytest.mark.skipif(
-    parse_version(fsspec.__version__) < parse_version("2024.12.0"),
-    reason="No AsyncFileSystemWrapper",
+@pytest.mark.parametrize(
+    ("root", "key", "expected"),
+    [
+        # `"/"` as root collapses so that bare-key backends (notably
+        # ReferenceFileSystem) get the right key. Regression test for
+        # https://github.com/zarr-developers/zarr-python/issues/3922 .
+        ("/", "zarr.json", "zarr.json"),
+        ("", "zarr.json", "zarr.json"),
+        # Trailing slashes on the root are stripped before joining.
+        ("foo/", "zarr.json", "foo/zarr.json"),
+        ("foo", "zarr.json", "foo/zarr.json"),
+        # Leading slashes on the root are preserved -- absolute filesystem
+        # paths must stay absolute. Regression test for the titiler-xarray
+        # breakage that #3924 introduced when `normalize_path` was applied to
+        # `FsspecStore.path`.
+        ("/home/runner/data.zarr", "zarr.json", "/home/runner/data.zarr/zarr.json"),
+        ("/home/runner/data.zarr/", "zarr.json", "/home/runner/data.zarr/zarr.json"),
+        # Multi-segment keys.
+        ("/home/foo", "a/b/zarr.json", "/home/foo/a/b/zarr.json"),
+        ("", "a/b/zarr.json", "a/b/zarr.json"),
+        # Trailing slash on the result is stripped (relevant when key is "").
+        ("/home/foo", "", "/home/foo"),
+    ],
 )
-@pytest.mark.parametrize("path", ["", "/", "//", "foo", "foo/", "/foo", "/foo/", "//foo//"])
-def test_fsspec_store_path_normalization(path: str) -> None:
-    """`FsspecStore.path` is normalized to the canonical form, matching
-    `normalize_path`, regardless of the surface representation the caller
-    supplies.
-
-    Regression test for https://github.com/zarr-developers/zarr-python/issues/3922
-    -- when a caller passed `path="/"` the leading slash flowed through
-    unmodified to subsequent `_join_paths([self.path, key])` calls, producing
-    `"//key"` and missing the underlying object.
+def test_dereference_path(root: str, key: str, expected: str) -> None:
+    """Verify the contract `_dereference_path` provides for `FsspecStore`.
+
+    `FsspecStore.path` is stored verbatim; the join with a key must collapse a
+    sentinel `"/"` root, strip trailing slashes, and preserve leading
+    slashes on absolute paths.
+    """
+    from zarr.storage._utils import _dereference_path
+
+    assert _dereference_path(root, key) == expected
+
+
+async def test_fsspec_store_open_group_via_reference_filesystem() -> None:
+    """End-to-end regression test for
+    https://github.com/zarr-developers/zarr-python/issues/3922 .
+
+    ``ReferenceFileSystem`` keys its refs by bare strings like ``"zarr.json"``.
+    The bug was that ``FsspecStore(fs=ref_fs, path="/")`` produced
+    ``"//zarr.json"`` at the join site and failed to find the entry, raising
+    ``GroupNotFoundError``. This test pins ``path="/"`` explicitly to keep
+    coverage even if the default value changes later.
+    """
+    import json
+
+    from fsspec.implementations.reference import ReferenceFileSystem
+
+    group_json = json.dumps({"zarr_format": 3, "node_type": "group", "attributes": {}})
+    fs = ReferenceFileSystem(
+        fo={"version": 1, "refs": {"zarr.json": group_json}},
+        asynchronous=True,
+    )
+    store = FsspecStore(fs=fs, path="/", read_only=True)
+    group = await zarr.api.asynchronous.open_group(store, mode="r")
+    assert group.metadata.zarr_format == 3
+
+
+async def test_fsspec_store_read_array_chunk_via_reference_filesystem() -> None:
+    """End-to-end regression test that exercises the byte-range read path
+    against ``ReferenceFileSystem``.
+
+    Beyond opening a group (covered by
+    ``test_fsspec_store_open_group_via_reference_filesystem``), this test
+    constructs a small zarr v3 array whose chunk lives in the refs dict and
+    reads it through the store. Path-handling bugs on the byte-range
+    fetch path (used by kerchunk-style virtualization) would surface here
+    rather than at metadata-open time.
     """
-    sync_fs = fsspec.filesystem("memory")
-    fs = _make_async(sync_fs)
-    store = FsspecStore(fs=fs, path=path)
-    assert store.path == normalize_path(path)
+    import json
+
+    import numpy as np
+    from fsspec.implementations.reference import ReferenceFileSystem
+
+    # Construct a minimal v3 zarr: a single 1-D uint8 array of length 4 with
+    # one chunk of size 4. The chunk bytes are little-endian uint8s 1..4.
+    array_meta = json.dumps(
+        {
+            "zarr_format": 3,
+            "node_type": "array",
+            "shape": [4],
+            "chunk_grid": {"name": "regular", "configuration": {"chunk_shape": [4]}},
+            "data_type": "uint8",
+            "chunk_key_encoding": {"name": "default", "configuration": {"separator": "/"}},
+            "fill_value": 0,
+            "codecs": [{"name": "bytes", "configuration": {"endian": "little"}}],
+            "attributes": {},
+        }
+    )
+    chunk_bytes = bytes([1, 2, 3, 4])
+
+    refs: dict[str, str] = {
+        "zarr.json": array_meta,
+        # ReferenceFileSystem accepts raw bytes via base64 encoding or
+        # latin-1-decoded strings; latin-1 round-trips bytes 1:1.
+        "c/0": chunk_bytes.decode("latin-1"),
+    }
+
+    fs = ReferenceFileSystem(
+        fo={"version": 1, "refs": refs},
+        asynchronous=True,
+    )
+    store = FsspecStore(fs=fs, path="/", read_only=True)
+    array = await zarr.api.asynchronous.open_array(store=store, mode="r")
+    data = await array.getitem(slice(None))
+    np.testing.assert_array_equal(data, np.array([1, 2, 3, 4], dtype="uint8"))
 
 
 @pytest.mark.skipif(