Permission denied when multiple Processes write to the same file

[Trobolt]

Zarr version

3.1.3

Numcodecs version

0.16.3

Python Version

3.13.4

Operating System

Windows 11 24H2

Installation

using pip into virtual environment

Description

I am encountering an issue when writing to a Zarr array using Python multiprocessing. (Or more specifically: Missuse Dask's Multiprocessing, as i write into a zarr array inside a map_block function) The problem occurs when the array chunks contain multiple elements. Writing works correctly when each chunk contains only a single element, but fails when the chunk size is larger.

Example:

Here group is a zarr group and zarr_array is another zarr array.

My code that writes to b_arr in a multprocessed manner works, when this creates the array: b_arr=group.create_array( name=name, shape=zarr_array.cdata_shape, chunks=(1, 1), dtype=np.uint64, fill_value=0, )

and here not: b_arr=group.create_array( name=name, shape=zarr_array.cdata_shape, chunks=zarr_array.cdata_shape, dtype=np.uint64, fill_value=0, )

I believe that zarr should work when mutliple processes concurrenty write into an array, so i consider it a bug.

Steps to reproduce

# /// script
# requires-python = ">=3.11"
# dependencies = [
#   "zarr@git+https://github.com/zarr-developers/zarr-python.git@main",
# ]
# ///
#
# This script automatically imports the development branch of zarr to check for issues

import zarr
# your reproducer code
# zarr.print_debug_info()

Additional output

No response

[FlynnOConnell]

I am getting this error as well

File [~\AppData\Roaming\uv\python\cpython-3.12.9-windows-x86_64-none\Lib\pathlib.py:1376](http://localhost:8888/lab/workspaces/auto-P/tree/dev/~/AppData/Roaming/uv/python/cpython-3.12.9-windows-x86_64-none/Lib/pathlib.py#line=1375), in Path.replace(self, target)
   1366 def replace(self, target):
   1367     """
   1368     Rename this path to the target path, overwriting if that path exists.
   1369 
   (...)   1374     Returns the new Path instance pointing to the target path.
   1375     """
-> 1376     os.replace(self, target)
   1377     return self.with_segments(target)

PermissionError: [WinError 5] Access is denied: 'zarr\\plane01_stitched.zarr\\c\\9\\0\\0.f03a0e3c2a8747b5a96eb94e4a5c04b3.partial' -> 'zarr\\plane01_stitched.zarr\\c\\9\\0\\0'

[melonora]

I can confirm as well

[MaksHess]

Same here