Fix unreleased bug in DataTree.to_zarr() with mode='r+'

Writing to zarr with `mode='r+'` was broken by pydata#10625, due to a Zarr
bug (zarr-developers/zarr-python#3428). This
add a work-around that works on older versions of Zarr.

The PR that introduced this bug has not yet appeared in an xarray
release, so there's no need for a release note.

Author: shoyer

xarray/core/datatree_io.py

@@ -211,12 +211,13 @@ def _datatree_to_zarr(
     writer = ArrayWriter()
 
     try:
-        for node in dt.subtree:
+        for rel_path, node in dt.subtree_with_keys:
             at_root = node is dt
             dataset = node.to_dataset(inherit=write_inherited_coords or at_root)
-            node_store = (
-                root_store if at_root else root_store.get_child_store(node.path)
-            )
+            # Use a relative path for group, because absolute paths are broken
+            # with consolidated metadata in zarr 3.1.2 and earlier:
+            # https://github.com/zarr-developers/zarr-python/pull/3428
+            node_store = root_store if at_root else root_store.get_child_store(rel_path)
 
             dataset = node_store._validate_and_autodetect_region(dataset)
             node_store._validate_encoding(encoding)

xarray/tests/test_backends_datatree.py

@@ -804,6 +804,16 @@ def assert_expected_zarr_files_exist(
         with open_datatree(str(storepath), engine="zarr") as written_dt:
             assert_identical(written_dt, original_dt)
 
+    def test_rplus_mode(
+        self, tmp_path: Path, simple_datatree: DataTree, zarr_format: Literal[2, 3]
+    ) -> None:
+        storepath = tmp_path / "test.zarr"
+        original_dt = simple_datatree.chunk()
+        original_dt.to_zarr(storepath, compute=False, zarr_format=zarr_format)
+        original_dt.to_zarr(storepath, mode="r+")
+        with open_datatree(str(storepath), engine="zarr") as written_dt:
+            assert_identical(written_dt, original_dt)
+
     @requires_dask
     def test_to_zarr_no_redundant_computation(self, tmpdir, zarr_format) -> None:
         import dask.array as da