Fix buffer protocol inheritance

Fixes #878.

include/pybind11/class_support.h

@@ -447,11 +447,17 @@ inline void enable_dynamic_attributes(PyHeapTypeObject *heap_type) {
 
 /// buffer_protocol: Fill in the view as specified by flags.
 extern "C" inline int pybind11_getbuffer(PyObject *obj, Py_buffer *view, int flags) {
-    auto tinfo = get_type_info(Py_TYPE(obj));
+    // Look for a `get_buffer` implementation in this type's info or any bases (following MRO).
+    type_info *tinfo = nullptr;
+    for (auto type : reinterpret_borrow<tuple>(Py_TYPE(obj)->tp_mro)) {
+        tinfo = get_type_info((PyTypeObject *) type.ptr());
+        if (tinfo && tinfo->get_buffer)
+            break;
+    }
     if (view == nullptr || obj == nullptr || !tinfo || !tinfo->get_buffer) {
         if (view)
             view->obj = nullptr;
-        PyErr_SetString(PyExc_BufferError, "generic_type::getbuffer(): Internal error");
+        PyErr_SetString(PyExc_BufferError, "pybind11_getbuffer(): Internal error");
         return -1;
     }
     memset(view, 0, sizeof(Py_buffer));

tests/test_buffers.cpp

@@ -74,6 +74,11 @@ private:
     float *m_data;
 };
 
+class SquareMatrix : public Matrix {
+public:
+    SquareMatrix(ssize_t n) : Matrix(n, n) { }
+};
+
 struct PTMFBuffer {
     int32_t value = 0;
 
@@ -141,6 +146,10 @@ test_initializer buffers([](py::module &m) {
         })
         ;
 
+    // Derived classes inherit the buffer protocol and the buffer access function
+    py::class_<SquareMatrix, Matrix>(m, "SquareMatrix")
+        .def(py::init<ssize_t>());
+
     py::class_<PTMFBuffer>(m, "PTMFBuffer", py::buffer_protocol())
         .def(py::init<>())
         .def_readwrite("value", &PTMFBuffer::value)

tests/test_buffers.py

@@ -36,6 +36,7 @@ def test_from_python():
 @pytest.unsupported_on_pypy
 def test_to_python():
     m = Matrix(5, 5)
+    assert memoryview(m).shape == (5, 5)
 
     assert m[2, 3] == 0
     m[2, 3] = 4
@@ -63,6 +64,16 @@ def test_to_python():
     assert cstats.move_assignments == 0
 
 
+@pytest.unsupported_on_pypy
+def test_inherited_protocol():
+    """SquareMatrix is derived from Matrix and inherits the buffer protocol"""
+    from pybind11_tests import SquareMatrix
+
+    matrix = SquareMatrix(5)
+    assert memoryview(matrix).shape == (5, 5)
+    assert np.asarray(matrix).shape == (5, 5)
+
+
 @pytest.unsupported_on_pypy
 def test_ptmf():
     for cls in [PTMFBuffer, ConstPTMFBuffer, DerivedPTMFBuffer]: