Pull readonly in from the C level

Tests/test_arrow.py

@@ -147,3 +147,11 @@ def test_release_array():
     array, schema = img.__arrow_c_array__()
     del(array)
     del(schema)
+
+
+def test_readonly():
+    img = hopper('L')
+    reloaded = Image.fromarrow(img, img.mode, img.size)
+    assert reloaded.readonly == 1
+    reloaded._readonly = 0
+    assert reloaded.readonly == 1

src/PIL/Image.py

@@ -584,6 +584,14 @@ class Image:
     def mode(self) -> str:
         return self._mode
 
+    @property
+    def readonly(self) -> int:
+        return (self._im and self._im.readonly) or self._readonly
+
+    @readonly.setter
+    def readonly(self, readonly: int) -> None:
+        self._readonly = readonly
+
     def _new(self, im: core.ImagingCore) -> Image:
         new = Image()
         new.im = im

src/_imaging.c

@@ -3821,6 +3821,11 @@ _getattr_unsafe_ptrs(ImagingObject *self, void *closure) {
     );
 }
 
+static PyObject *
+_getattr_readonly(ImagingObject *self, void *closure) {
+    return PyLong_FromLong(self->image->read_only);
+}
+
 static struct PyGetSetDef getsetters[] = {
     {"mode", (getter)_getattr_mode},
     {"size", (getter)_getattr_size},
@@ -3828,6 +3833,7 @@ static struct PyGetSetDef getsetters[] = {
     {"id", (getter)_getattr_id},
     {"ptr", (getter)_getattr_ptr},
     {"unsafe_ptrs", (getter)_getattr_unsafe_ptrs},
+    {"readonly", (getter)_getattr_readonly},
     {NULL}
 };