BUG: Fix small reduce bug and test string multiply-reduce

Author: seberg

numpy/_core/src/umath/reduction.c

@@ -21,6 +21,7 @@
 #include "array_coercion.h"
 #include "array_method.h"
 #include "ctors.h"
+#include "refcount.h"
 
 #include "numpy/ufuncobject.h"
 #include "lowlevel_strided_loops.h"
@@ -438,7 +439,7 @@ PyUFunc_ReduceWrapper(PyArrayMethod_Context *context,
     Py_INCREF(result);
 
     if (initial_buf != NULL && PyDataType_REFCHK(PyArray_DESCR(result))) {
-        PyArray_Item_XDECREF(initial_buf, PyArray_DESCR(result));
+        PyArray_ClearBuffer(PyArray_DESCR(result), initial_buf, 0, 1, 1);
     }
     PyMem_FREE(initial_buf);
     NPY_AUXDATA_FREE(auxdata);
@@ -450,7 +451,7 @@ PyUFunc_ReduceWrapper(PyArrayMethod_Context *context,
 
 fail:
     if (initial_buf != NULL && PyDataType_REFCHK(op_dtypes[0])) {
-        PyArray_Item_XDECREF(initial_buf, op_dtypes[0]);
+        PyArray_ClearBuffer(op_dtypes[0], initial_buf, 0, 1, 1);
     }
     PyMem_FREE(initial_buf);
     NPY_AUXDATA_FREE(auxdata);

numpy/_core/tests/test_stringdtype.py

@@ -755,6 +755,16 @@ def test_add_promoter_reduce():
     np.add.reduce(np.array(["a", "b"], dtype="U"), dtype=np.dtypes.StringDType)
 
 
+def test_multiply_reduce():
+    # At the time of writing (NumPy 2.0) this is very limited (and rather
+    # ridiculous anyway).  But it works and actually makes some sense...
+    # (NumPy does not allow non-scalar initial values)
+    repeats = np.array([2, 3, 4])
+    val = "school-🚌"
+    res = np.multiply.reduce(repeats, initial=val, dtype=np.dtypes.StringDType)
+    assert res == val * np.prod(repeats)
+
+
 @pytest.mark.parametrize("use_out", [True, False])
 @pytest.mark.parametrize("other", [2, [2, 1, 3, 4, 1, 3]])
 @pytest.mark.parametrize(