Revert Data fields to bytes and add get_data_as_view for zero-copy access (#390)

* get data field with view

* refine tc

* refine based on flake check

* run black again

* rebase upstream master

* add comment to tc

* refine raise exception

capnp/lib/capnp.pyx

@@ -15,8 +15,8 @@ from capnp.includes.schema_cpp cimport (MessageReader,)
 
 from builtins import memoryview as BuiltinsMemoryview
 from cpython cimport array, Py_buffer, PyObject_CheckBuffer
-from cpython.buffer cimport PyBUF_SIMPLE, PyBUF_WRITABLE, PyBUF_WRITE, PyBUF_READ, PyBUF_CONTIG_RO
-from cpython.memoryview cimport PyMemoryView_FromMemory
+from cpython.buffer cimport PyBUF_SIMPLE, PyBUF_WRITABLE, PyBUF_WRITE, PyBUF_READ, PyBUF_CONTIG_RO, PyBuffer_FillInfo
+from cpython.memoryview cimport PyMemoryView_FromMemory, PyMemoryView_FromBuffer
 from cpython.bytes cimport PyBytes_FromStringAndSize
 from cpython.exc cimport PyErr_Clear
 from cython.operator cimport dereference as deref
@@ -669,7 +669,7 @@ cdef to_python_reader(C_DynamicValue.Reader self, object parent):
         return (<char*>temp_text.begin())[:temp_text.size()]
     elif type == capnp.TYPE_DATA:
         temp_data = self.asData()
-        return PyMemoryView_FromMemory(<char *> temp_data.begin(), temp_data.size(), PyBUF_READ)
+        return <bytes>((<char*>temp_data.begin())[:temp_data.size()])
     elif type == capnp.TYPE_LIST:
         return _DynamicListReader()._init(self.asList(), parent)
     elif type == capnp.TYPE_STRUCT:
@@ -703,7 +703,7 @@ cdef to_python_builder(C_DynamicValue.Builder self, object parent):
         return (<char*>temp_text.begin())[:temp_text.size()]
     elif type == capnp.TYPE_DATA:
         temp_data = self.asData()
-        return PyMemoryView_FromMemory(<char *> temp_data.begin(), temp_data.size(), PyBUF_WRITE)
+        return <bytes>((<char*>temp_data.begin())[:temp_data.size()])
     elif type == capnp.TYPE_LIST:
         return _DynamicListBuilder()._init(self.asList(), parent)
     elif type == capnp.TYPE_STRUCT:
@@ -1226,6 +1226,29 @@ cdef class _DynamicStructReader:
     cpdef _has_by_field(self, _StructSchemaField field):
         return self.thisptr.hasByField(field.thisptr)
 
+    cpdef get_data_as_view(self, field):
+        """
+        Efficiently get a read-only memoryview for a DATA field without copying.
+        """
+        cdef C_DynamicValue.Reader val
+        cdef capnp.Data.Reader temp_data
+
+        try:
+            val = self.thisptr.get(field)
+        except KjException as e:
+            raise e._to_python() from None
+
+        if val.getType() != capnp.TYPE_DATA:
+            raise TypeError("Field '{}' is not a DATA field".format(field))
+
+        temp_data = val.asData()
+
+        # Return read-only memoryview
+        cdef Py_buffer buf
+        if PyBuffer_FillInfo(&buf, self, <void*>temp_data.begin(), temp_data.size(), 1, PyBUF_CONTIG_RO) < 0:
+            raise KjException("Failed to create buffer info")
+        return PyMemoryView_FromBuffer(&buf)
+
     cpdef _which_str(self):
         try:
             return <char *>helpers.fixMaybe(self.thisptr.which()).getProto().getName().cStr()
@@ -1628,6 +1651,32 @@ cdef class _DynamicStructBuilder:
         """
         return _DynamicOrphan()._init(self.thisptr.disown(field), self._parent)
 
+    cpdef get_data_as_view(self, field):
+        """
+        Efficiently get a writable memoryview for a DATA field without copying.
+
+        This allows in-place modification of the underlying buffer:
+        msg.get_data_as_view('myField')[0] = 0xFF
+        """
+        cdef C_DynamicValue.Builder val
+        cdef capnp.Data.Builder temp_data
+
+        try:
+            val = self.thisptr.get(field)
+        except KjException as e:
+            raise e._to_python() from None
+
+        if val.getType() != capnp.TYPE_DATA:
+            raise TypeError("Field '{}' is not a DATA field".format(field))
+
+        temp_data = val.asData()
+
+        # Return writable memoryview
+        cdef Py_buffer buf
+        if PyBuffer_FillInfo(&buf, self, <void*>temp_data.begin(), temp_data.size(), 0, PyBUF_WRITABLE) < 0:
+            raise KjException("Failed to create buffer info")
+        return PyMemoryView_FromBuffer(&buf)
+
     cpdef as_reader(self):
         """A method for casting this Builder to a Reader