BUG: don't create array with invalid memory in where

juliantaylor · mherkazandjian · commit e0b733166d7d · 2017-05-31T01:15:15.000+02:00
When creating the tuple view of np.where, make sure the arrays point to valid memory when they are empty. Numpy assumes that even empty arrays have valid memory, e.g. in the source stride == 0 assignments. Closes numpygh-8922.
diff --git a/numpy/core/src/multiarray/item_selection.c b/numpy/core/src/multiarray/item_selection.c
@@ -2176,6 +2176,7 @@ PyArray_Nonzero(PyArrayObject *self)
     NpyIter_IterNextFunc *iternext;
     NpyIter_GetMultiIndexFunc *get_multi_index;
     char **dataptr;
+    int is_empty = 0;
 
     /*
      * First count the number of non-zeros in 'self'.
@@ -2329,13 +2330,22 @@ PyArray_Nonzero(PyArrayObject *self)
         return NULL;
     }
 
+    for (i = 0; i < ndim; ++i) {
+        if (PyArray_DIMS(ret)[i] == 0) {
+            is_empty = 1;
+            break;
+        }
+    }
+
     /* Create views into ret, one for each dimension */
     for (i = 0; i < ndim; ++i) {
         npy_intp stride = ndim * NPY_SIZEOF_INTP;
+        /* the result is an empty array, the view must point to valid memory */
+        npy_intp data_offset = is_empty ? 0 : i * NPY_SIZEOF_INTP;
 
         PyArrayObject *view = (PyArrayObject *)PyArray_New(Py_TYPE(ret), 1,
                                     &nonzero_count, NPY_INTP, &stride,
-                                    PyArray_BYTES(ret) + i*NPY_SIZEOF_INTP,
+                                    PyArray_BYTES(ret) + data_offset,
                                     0, PyArray_FLAGS(ret), (PyObject *)ret);
         if (view == NULL) {
             Py_DECREF(ret);
diff --git a/numpy/core/tests/test_multiarray.py b/numpy/core/tests/test_multiarray.py
@@ -6633,6 +6633,14 @@ def test_string(self):
         assert_equal(np.where(True, a, b), "abcd")
         assert_equal(np.where(False, b, a), "abcd")
 
+    def test_empty_result(self):
+        # pass empty where result through an assignment which reads the data of
+        # empty arrays, error detectable with valgrind, see gh-8922
+        x = np.zeros((1, 1))
+        ibad = np.vstack(np.where(x == 99.))
+        assert_array_equal(ibad,
+                           np.atleast_2d(np.array([[],[]], dtype=np.intp)))
+
 
 if not IS_PYPY:
     # sys.getsizeof() is not valid on PyPy
