Add ManifestStore support for empty chunks. (#668)

Author: sharkinsspatial

virtualizarr/manifests/manifest.py

@@ -48,8 +48,8 @@ def with_validation(
         """
 
         # note: we can't just use `__init__` or a dataclass' `__post_init__` because we need `fs_root` to be an optional kwarg
-
-        path = validate_and_normalize_path_to_uri(path, fs_root=fs_root)
+        if path != "":
+            path = validate_and_normalize_path_to_uri(path, fs_root=fs_root)
         validate_byte_range(offset=offset, length=length)
         return ChunkEntry(path=path, offset=offset, length=length)
 

virtualizarr/manifests/store.py

@@ -252,7 +252,10 @@ async def get(
         chunk_indexes = parse_manifest_index(
             key, marr.metadata.chunk_key_encoding.separator
         )
+
         path = manifest._paths[chunk_indexes]
+        if path == "":
+            return None
         offset = manifest._offsets[chunk_indexes]
         length = manifest._lengths[chunk_indexes]
         # Get the configured object store instance that matches the path

virtualizarr/tests/test_manifests/test_store.py

@@ -126,6 +126,31 @@ def s3_store(minio_bucket):
     )
 
 
+@pytest.fixture()
+def empty_memory_store():
+    import obstore as obs
+
+    store = obs.store.MemoryStore()
+    prefix = get_store_prefix("")
+    chunk_dict = {
+        "0.0": {"path": "", "offset": 0, "length": 4},
+    }
+    manifest = ChunkManifest(entries=chunk_dict)
+    codecs = [{"configuration": {"endian": "little"}, "name": "bytes"}]
+    array_metadata = create_v3_array_metadata(
+        shape=(1, 1),
+        chunk_shape=(1, 1),
+        data_type=np.dtype("int32"),
+        codecs=codecs,
+        chunk_key_encoding={"name": "default", "separator": "."},
+        fill_value=0,
+    )
+    manifest_array = ManifestArray(metadata=array_metadata, chunkmanifest=manifest)
+    manifest_group = ManifestGroup(arrays={"foo": manifest_array})
+    registry = ObjectStoreRegistry({prefix: store})
+    return ManifestStore(store_registry=registry, group=manifest_group)
+
+
 @requires_obstore
 class TestManifestStore:
     def test_manifest_store_properties(self, local_store):
@@ -135,6 +160,16 @@ def test_manifest_store_properties(self, local_store):
         assert not local_store.supports_writes
         assert not local_store.supports_partial_writes
 
+    @pytest.mark.asyncio
+    @pytest.mark.parametrize(
+        "manifest_store",
+        ["empty_memory_store"],
+    )
+    async def test_get_empty_chunk(self, manifest_store, request):
+        store = request.getfixturevalue(manifest_store)
+        observed = await store.get("foo/c/0.0", prototype=default_buffer_prototype())
+        assert observed is None
+
     @pytest.mark.asyncio
     @pytest.mark.parametrize(
         "manifest_store",