Fix UTF generation for numpy in property-based tests (zarr-developers#2801)

* Fix UTF generation for numpy in property-based tests

* Add changelog entry

---------

Co-authored-by: Deepak Cherian <[email protected]>

Author: moradology

changes/2801.bugfix.rst

@@ -0,0 +1 @@
+Ensure utf8 compliant strings are used to construct numpy arrays in property-based tests

src/zarr/testing/strategies.py

@@ -51,6 +51,21 @@ def v2_dtypes() -> st.SearchStrategy[np.dtype]:
     )
 
 
+def safe_unicode_for_dtype(dtype: np.dtype[np.str_]) -> st.SearchStrategy[str]:
+    """Generate UTF-8-safe text constrained to max_len of dtype."""
+    # account for utf-32 encoding (i.e. 4 bytes/character)
+    max_len = max(1, dtype.itemsize // 4)
+
+    return st.text(
+        alphabet=st.characters(
+            blacklist_categories=["Cs"],  # Avoid *technically allowed* surrogates
+            min_codepoint=32,
+        ),
+        min_size=1,
+        max_size=max_len,
+    )
+
+
 # From https://zarr-specs.readthedocs.io/en/latest/v3/core/v3.0.html#node-names
 # 1. must not be the empty string ("")
 # 2. must not include the character "/"
@@ -86,7 +101,12 @@ def numpy_arrays(
     Generate numpy arrays that can be saved in the provided Zarr format.
     """
     zarr_format = draw(zarr_formats)
-    return draw(npst.arrays(dtype=v3_dtypes() if zarr_format == 3 else v2_dtypes(), shape=shapes))
+    dtype = draw(v3_dtypes() if zarr_format == 3 else v2_dtypes())
+    if np.issubdtype(dtype, np.str_):
+        safe_unicode_strings = safe_unicode_for_dtype(dtype)
+        return draw(npst.arrays(dtype=dtype, shape=shapes, elements=safe_unicode_strings))
+
+    return draw(npst.arrays(dtype=dtype, shape=shapes))
 
 
 @st.composite  # type: ignore[misc]