pythongh-135862: fix asyncio socket partial writes of non-1d-binary arrays

The :mod:`asyncio` code is writing binary data to a :class:`socket.socket`, and
advancing through the buffer after a partial write by doing
``memoryview(data)[n:]``, where ``n`` is the number of bytes written.

This assumes the :class:`memoryview` is a one dimensional buffer of bytes,
which is not the case e.g. for a :class:`memoryview` of an :class:`array.array`
of non-bytes, or an ``ndarray`` with more than one dimension. Partial writes of
such :class:`memoryview`s will corrupt the stream, repeating or omitting some
data.

When creating a :class:`memoryview` from a :class:`memoryview`, first convert
it to a one-dimensional array of bytes before taking the remaining slice, thus
avoiding these issues.

Add an assertion that the remaining data is bytes, to guard against possible
regressions if additional types of data are allowed in the future.

Author: duaneg

Lib/asyncio/selector_events.py

@@ -1077,7 +1077,11 @@ def write(self, data):
                 self._fatal_error(exc, 'Fatal write error on socket transport')
                 return
             else:
-                data = memoryview(data)[n:]
+                if isinstance(data, memoryview):
+                    data = data.cast('c')[n:]
+                else:
+                    data = memoryview(data)[n:]
+                assert(data.itemsize == 1)
                 if not data:
                     return
             # Not all was written; register write handler.

Lib/test/test_asyncio/test_selector_events.py

@@ -1,5 +1,6 @@
 """Tests for selector_events.py"""
 
+import array
 import collections
 import selectors
 import socket
@@ -13,6 +14,11 @@
 except ImportError:
     ssl = None
 
+try:
+    from _testbuffer import *
+except ImportError:
+    ndarray = None
+
 import asyncio
 from asyncio.selector_events import (BaseSelectorEventLoop,
                                      _SelectorDatagramTransport,
@@ -768,7 +774,39 @@ def test_write_partial_memoryview(self):
         transport.write(data)
 
         self.loop.assert_writer(7, transport._write_ready)
-        self.assertEqual(list_to_buffer([b'ta']), transport._buffer)
+        self.assertEqualBufferContents(b'ta', transport._buffer)
+
+    def test_write_partial_nonbyte_array_memview_write(self):
+        arr = array.array('l', [-1, 1])
+        data = memoryview(arr)
+
+        self.sock.send.return_value = 8
+
+        transport = self.socket_transport()
+        transport.write(data)
+
+        self.loop.assert_writer(7, transport._write_ready)
+        remainder = memoryview(array.array('l', [1]))
+        self.assertEqualBufferContents(remainder.tobytes(), transport._buffer)
+
+    @unittest.skipUnless(ndarray, 'ndarray object required for this test')
+    def test_write_partial_ndarray_memview_write(self):
+        items = (-2, -1, 1, 2)
+        arr = ndarray(items, format='l', shape=(1, 2, 2))
+        data = memoryview(arr)
+
+        self.sock.send.return_value = 16
+
+        transport = self.socket_transport()
+        transport.write(data)
+
+        self.loop.assert_writer(7, transport._write_ready)
+        remainder = memoryview(array.array('l', (1, 2)))
+        self.assertEqualBufferContents(remainder.tobytes(), transport._buffer)
+
+    def assertEqualBufferContents(self, expected, buffer):
+        self.assertEqual(len(buffer), 1)
+        self.assertEqual(expected, buffer[0].tobytes())
 
     def test_write_partial_none(self):
         data = b'data'

Misc/NEWS.d/next/Library/2025-06-26-22-12-13.gh-issue-135862.wYKSKx.rst

@@ -0,0 +1,2 @@
+Fix bug where partial writes of :class:`memoryview` s of non-byte or 2+
+dimensional arrays would write remaining binary data incorrectly.