support for readonly buffers (#863) (#1466)

include/pybind11/buffer_info.h

@@ -22,13 +22,14 @@ struct buffer_info {
     ssize_t ndim = 0;             // Number of dimensions
     std::vector<ssize_t> shape;   // Shape of the tensor (1 entry per dimension)
     std::vector<ssize_t> strides; // Number of bytes between adjacent entries (for each per dimension)
+    bool readonly = false;        // flag to indicate if the underlying storage may be written to
 
     buffer_info() { }
 
     buffer_info(void *ptr, ssize_t itemsize, const std::string &format, ssize_t ndim,
-                detail::any_container<ssize_t> shape_in, detail::any_container<ssize_t> strides_in)
+                detail::any_container<ssize_t> shape_in, detail::any_container<ssize_t> strides_in, bool readonly=false)
     : ptr(ptr), itemsize(itemsize), size(1), format(format), ndim(ndim),
-      shape(std::move(shape_in)), strides(std::move(strides_in)) {
+      shape(std::move(shape_in)), strides(std::move(strides_in)), readonly(readonly) {
         if (ndim != (ssize_t) shape.size() || ndim != (ssize_t) strides.size())
             pybind11_fail("buffer_info: ndim doesn't match shape and/or strides length");
         for (size_t i = 0; i < (size_t) ndim; ++i)
@@ -36,19 +37,23 @@ struct buffer_info {
     }
 
     template <typename T>
-    buffer_info(T *ptr, detail::any_container<ssize_t> shape_in, detail::any_container<ssize_t> strides_in)
+    buffer_info(T *ptr, detail::any_container<ssize_t> shape_in, detail::any_container<ssize_t> strides_in, bool readonly=false)
-    : buffer_info(private_ctr_tag(), ptr, sizeof(T), format_descriptor<T>::format(), static_cast<ssize_t>(shape_in->size()), std::move(shape_in), std::move(strides_in)) { }
+    : buffer_info(private_ctr_tag(), ptr, sizeof(T), format_descriptor<T>::format(), static_cast<ssize_t>(shape_in->size()), std::move(shape_in), std::move(strides_in), readonly) { }
 
-    buffer_info(void *ptr, ssize_t itemsize, const std::string &format, ssize_t size)
+    buffer_info(void *ptr, ssize_t itemsize, const std::string &format, ssize_t size, bool readonly=false)
-    : buffer_info(ptr, itemsize, format, 1, {size}, {itemsize}) { }
+    : buffer_info(ptr, itemsize, format, 1, {size}, {itemsize}, readonly) { }
 
     template <typename T>
-    buffer_info(T *ptr, ssize_t size)
+    buffer_info(T *ptr, ssize_t size, bool readonly=false)
-    : buffer_info(ptr, sizeof(T), format_descriptor<T>::format(), size) { }
+    : buffer_info(ptr, sizeof(T), format_descriptor<T>::format(), size, readonly) { }
+
+    template <typename T>
+    buffer_info(const T *ptr, ssize_t size, bool readonly=true)
+    : buffer_info(const_cast<T*>(ptr), sizeof(T), format_descriptor<T>::format(), size, readonly) { }
 
     explicit buffer_info(Py_buffer *view, bool ownview = true)
     : buffer_info(view->buf, view->itemsize, view->format, view->ndim,
-            {view->shape, view->shape + view->ndim}, {view->strides, view->strides + view->ndim}) {
+            {view->shape, view->shape + view->ndim}, {view->strides, view->strides + view->ndim}, view->readonly) {
         this->view = view;
         this->ownview = ownview;
     }
@@ -70,6 +75,7 @@ struct buffer_info {
         strides = std::move(rhs.strides);
         std::swap(view, rhs.view);
         std::swap(ownview, rhs.ownview);
+        readonly = rhs.readonly;
         return *this;
     }
 
@@ -81,8 +87,8 @@ private:
     struct private_ctr_tag { };
 
     buffer_info(private_ctr_tag, void *ptr, ssize_t itemsize, const std::string &format, ssize_t ndim,
-                detail::any_container<ssize_t> &&shape_in, detail::any_container<ssize_t> &&strides_in)
+                detail::any_container<ssize_t> &&shape_in, detail::any_container<ssize_t> &&strides_in, bool readonly)
-    : buffer_info(ptr, itemsize, format, ndim, std::move(shape_in), std::move(strides_in)) { }
+    : buffer_info(ptr, itemsize, format, ndim, std::move(shape_in), std::move(strides_in), readonly) { }
 
     Py_buffer *view = nullptr;
     bool ownview = false;

include/pybind11/detail/class.h

@@ -491,6 +491,13 @@ extern "C" inline int pybind11_getbuffer(PyObject *obj, Py_buffer *view, int fla
     view->len = view->itemsize;
     for (auto s : info->shape)
         view->len *= s;
+    view->readonly = info->readonly;
+    if ((flags & PyBUF_WRITABLE) == PyBUF_WRITABLE && info->readonly) {
+        if (view)
+            view->obj = nullptr;
+        PyErr_SetString(PyExc_BufferError, "Writable buffer requested for readonly storage");
+        return -1;
+    }
     if ((flags & PyBUF_FORMAT) == PyBUF_FORMAT)
         view->format = const_cast<char *>(info->format.c_str());
     if ((flags & PyBUF_STRIDES) == PyBUF_STRIDES) {

include/pybind11/pytypes.h

@@ -1345,7 +1345,7 @@ public:
         buf.strides = py_strides.data();
         buf.shape = py_shape.data();
         buf.suboffsets = nullptr;
-        buf.readonly = false;
+        buf.readonly = info.readonly;
         buf.internal = nullptr;
 
         m_ptr = PyMemoryView_FromBuffer(&buf);

tests/test_buffers.cpp

@@ -166,4 +166,30 @@ TEST_SUBMODULE(buffers, m) {
         .def_readwrite("value", (int32_t DerivedBuffer::*) &DerivedBuffer::value)
         .def_buffer(&DerivedBuffer::get_buffer_info);
 
+    struct BufferReadOnly {
+        const uint8_t value = 0;
+        BufferReadOnly(uint8_t value): value(value) {}
+
+        py::buffer_info get_buffer_info() {
+            return py::buffer_info(&value, 1);
+        }
+    };
+    py::class_<BufferReadOnly>(m, "BufferReadOnly", py::buffer_protocol())
+        .def(py::init<uint8_t>())
+        .def_buffer(&BufferReadOnly::get_buffer_info);
+
+    struct BufferReadOnlySelect {
+        uint8_t value = 0;
+        bool readonly = false;
+
+        py::buffer_info get_buffer_info() {
+            return py::buffer_info(&value, 1, readonly);
+        }
+    };
+    py::class_<BufferReadOnlySelect>(m, "BufferReadOnlySelect", py::buffer_protocol())
+        .def(py::init<>())
+        .def_readwrite("value", &BufferReadOnlySelect::value)
+        .def_readwrite("readonly", &BufferReadOnlySelect::readonly)
+        .def_buffer(&BufferReadOnlySelect::get_buffer_info);
+
 }

tests/test_buffers.py

@@ -1,8 +1,14 @@
+import io
 import struct
+import sys
+
 import pytest
+
 from pybind11_tests import buffers as m
 from pybind11_tests import ConstructorStats
 
+PY3 = sys.version_info[0] >= 3
+
 pytestmark = pytest.requires_numpy
 
 with pytest.suppress(ImportError):
@@ -85,3 +91,28 @@ def test_pointer_to_member_fn():
         buf.value = 0x12345678
         value = struct.unpack('i', bytearray(buf))[0]
         assert value == 0x12345678
+
+
+@pytest.unsupported_on_pypy
+def test_readonly_buffer():
+    buf = m.BufferReadOnly(0x64)
+    view = memoryview(buf)
+    assert view[0] == 0x64 if PY3 else b'd'
+    assert view.readonly
+
+
+@pytest.unsupported_on_pypy
+def test_selective_readonly_buffer():
+    buf = m.BufferReadOnlySelect()
+
+    memoryview(buf)[0] = 0x64 if PY3 else b'd'
+    assert buf.value == 0x64
+
+    io.BytesIO(b'A').readinto(buf)
+    assert buf.value == ord(b'A')
+
+    buf.readonly = True
+    with pytest.raises(TypeError):
+        memoryview(buf)[0] = 0 if PY3 else b'\0'
+    with pytest.raises(TypeError):
+        io.BytesIO(b'1').readinto(buf)