Initialize store in local temporary directory in ZarrWriter

fme/core/test_writer.py

@@ -391,7 +391,14 @@ def test_ZarrWriter_already_initialized(tmp_path):
         chunks={"time": 2},
     )
 
+    def _mock_initialize_zarr(**kwargs):
+        # The logic of initialize_store requires _initialize_zarr to at least
+        # create a dummy directory at the path, so we mock that aspect here.
+        os.makedirs(kwargs["path"])
+
     with patch("fme.core.writer._initialize_zarr") as mock_initialize:
+        mock_initialize.side_effect = _mock_initialize_zarr
+
         # Initialize the store for the first time
         writer.initialize_store(data_dtype="f4", data_vars=["var"])
 
@@ -443,3 +450,12 @@ def test_ZarrWriter_initialize_error_when_no_data_vars(tmp_path):
 
     with pytest.raises(ValueError, match="data_vars must be provided"):
         writer_no_vars.initialize_store(data_dtype="f4")
+
+
+def test_ZarrWriter_raises_FileExistsError_when_path_exists_and_mode_w_minus(tmp_path):
+    """ZarrWriter raises FileExistsError if path exists and mode is 'w-'."""
+    path = os.path.join(tmp_path, "test.zarr")
+    os.makedirs(path)
+
+    with pytest.raises(FileExistsError, match="already exists"):
+        _create_writer(path=path, n_times=4, chunks={"time": 2}, mode="w-")

fme/core/writer.py

@@ -1,9 +1,12 @@
 import logging
+import os
+import tempfile
 from collections.abc import Mapping
 from typing import Literal
 
 import cftime
 import fsspec
+import fsspec.generic
 import numpy as np
 import xarray as xr
 import zarr
@@ -282,6 +285,11 @@ def __init__(
 
         if mode == "a" or mode == "r+":
             self._store_initialized = True if self._path_exists() else False
+        elif self._dist.is_root() and mode == "w-" and self._path_exists():
+            raise FileExistsError(
+                f"Path {self._path!r} already exists and cannot be overwritten "
+                f"since mode is {mode!r}."
+            )
         else:
             self._store_initialized = False
 
@@ -365,22 +373,28 @@ def initialize_store(
                     "data_vars must be provided either to ZarrWriter or to "
                     "initialize()"
                 )
-            _initialize_zarr(
-                path=self._path,
-                vars=data_vars,
-                dim_sizes=dim_sizes,
-                chunks=self._chunks,
-                shards=self._shards,
-                coords=self._coords,
-                dim_names=self._dims,
-                dtype=data_dtype,
-                time_units=self._time_units,
-                time_calendar=self._time_calendar,
-                nondim_coords=self._nondim_coords,
-                array_attributes=self._array_attributes,
-                group_attributes=self._group_attributes,
-                mode=self._mode,
-            )
+            # Initialize zarr store locally in a temporary directory and then
+            # immediately rsync it to the final path. This avoids the need for
+            # users to have delete access on the destination filesystem.
+            with tempfile.TemporaryDirectory() as temp_dir:
+                temp_path = os.path.join(temp_dir, "temp.zarr")
+                _initialize_zarr(
+                    path=temp_path,
+                    vars=data_vars,
+                    dim_sizes=dim_sizes,
+                    chunks=self._chunks,
+                    shards=self._shards,
+                    coords=self._coords,
+                    dim_names=self._dims,
+                    dtype=data_dtype,
+                    time_units=self._time_units,
+                    time_calendar=self._time_calendar,
+                    nondim_coords=self._nondim_coords,
+                    array_attributes=self._array_attributes,
+                    group_attributes=self._group_attributes,
+                    mode=self._mode,
+                )
+                fsspec.generic.rsync(temp_path, self._path)
             self._store_initialized = True
             self._dist.barrier()
         else: