Fix buffer_info for ctypes buffers (pybind#2502) (#2503)

* tests: New test for ctypes buffers (pybind#2502)

* fix: fix buffer_info segfault on views with no stride (pybind11#2502)

* Explicit conversions in buffer_info to make clang happy (pybind#2502)

* Another explicit cast in buffer_info constructor for clang (pybind#2502)

* Simpler implementation of buffer_info constructor from Py_buffer.

* Move test_ctypes_buffer into test_buffers

* Comment on why view->strides may be NULL (and fix some whitespace)

* Use c_strides() instead of zero when view->strides is NULL.

c_strides and f_strides are moved from numpy.h (py::array)
to buffer_info.h (py::detail) so they can be used from the
buffer_info Py_buffer constructor.

* Increase ctypes buffer test coverage in test_buffers.

* Split ctypes tests and skip one which is broken in PyPy2.

tests/test_buffers.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 import io
 import struct
+import ctypes
 
 import pytest
 
@@ -107,3 +108,55 @@ def test_selective_readonly_buffer():
         memoryview(buf)[0] = b'\0' if env.PY2 else 0
     with pytest.raises(TypeError):
         io.BytesIO(b'1').readinto(buf)
+
+
+def test_ctypes_array_1d():
+    char1d = (ctypes.c_char * 10)()
+    int1d = (ctypes.c_int * 15)()
+    long1d = (ctypes.c_long * 7)()
+
+    for carray in (char1d, int1d, long1d):
+        info = m.get_buffer_info(carray)
+        assert info.itemsize == ctypes.sizeof(carray._type_)
+        assert info.size == len(carray)
+        assert info.ndim == 1
+        assert info.shape == [info.size]
+        assert info.strides == [info.itemsize]
+        assert not info.readonly
+
+
+def test_ctypes_array_2d():
+    char2d = ((ctypes.c_char * 10) * 4)()
+    int2d = ((ctypes.c_int * 15) * 3)()
+    long2d = ((ctypes.c_long * 7) * 2)()
+
+    for carray in (char2d, int2d, long2d):
+        info = m.get_buffer_info(carray)
+        assert info.itemsize == ctypes.sizeof(carray[0]._type_)
+        assert info.size == len(carray) * len(carray[0])
+        assert info.ndim == 2
+        assert info.shape == [len(carray), len(carray[0])]
+        assert info.strides == [info.itemsize * len(carray[0]), info.itemsize]
+        assert not info.readonly
+
+
+@pytest.mark.skipif(
+    "env.PYPY and env.PY2", reason="PyPy2 bytes buffer not reported as readonly"
+)
+def test_ctypes_from_buffer():
+    test_pystr = b"0123456789"
+    for pyarray in (test_pystr, bytearray(test_pystr)):
+        pyinfo = m.get_buffer_info(pyarray)
+
+        if pyinfo.readonly:
+            cbytes = (ctypes.c_char * len(pyarray)).from_buffer_copy(pyarray)
+            cinfo = m.get_buffer_info(cbytes)
+        else:
+            cbytes = (ctypes.c_char * len(pyarray)).from_buffer(pyarray)
+            cinfo = m.get_buffer_info(cbytes)
+
+        assert cinfo.size == pyinfo.size
+        assert cinfo.ndim == pyinfo.ndim
+        assert cinfo.shape == pyinfo.shape
+        assert cinfo.strides == pyinfo.strides
+        assert not cinfo.readonly