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gh-159739: Handle overlapping mmap writes by jelle-openai · Pull Request #160189 · python/cpython · GitHub

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gh-159739: Handle overlapping mmap writes #160189

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gh-159739: Handle overlapping mmap writes #160189

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gh-159739: Handle overlapping mmap writes
jelle-openai Oct 22, 2026
1353f4a
gh-159739: Preserve overlapping mmap extended slice sources
JelleZijlstra Oct 22, 2026
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26 changes: 26 additions & 0 deletions Lib/test/test_mmap.py
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Original file line number Diff line number Diff line change
Expand Up @@ -795,6 +795,32 @@ def test_weakref(self):
gc_collect()
self.assertIs(wr(), None)

def test_write_overlapping_memoryview(self):
data = bytes(range(256)) * 16
expected = data[:1] + data[:-1]
with mmap.mmap(-1, len(data)) as mm, memoryview(mm) as view:
mm[:] = data
mm.seek(1)
self.assertEqual(mm.write(view[:-1]), len(data) - 1)
self.assertEqual(mm[:], expected)
mm[:] = data
mm[1:] = view[:-1]
self.assertEqual(mm[:], expected)

def test_extended_slice_overlapping_memoryview(self):
data = b'abcdefghijkl'
for step in (-3, -2, -1, 2, 3):
target = slice(None, None, step)
size = len(data[target])
for offset in (0, len(data) - size):
with self.subTest(step=step, offset=offset):
expected = bytearray(data)
expected[target] = data[offset:offset + size]
with mmap.mmap(-1, len(data)) as mm, memoryview(mm) as view:
mm[:] = data
mm[target] = view[offset:offset + size]
self.assertEqual(mm[:], expected)

def test_write_returning_the_number_of_bytes_written(self):
mm = mmap.mmap(-1, 16)
self.assertEqual(mm.write(b""), 0)
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2026-10-21-21-26-10.gh-issue-159739.yuOQX5.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Fix overlapping writes and slice assignments to :class:`mmap.mmap` objects.
21 changes: 16 additions & 5 deletions Modules/mmapmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -752,7 +752,7 @@ mmap_mmap_write_impl(mmap_object *self, Py_buffer *data)

CHECK_VALID(NULL);
PyObject *result;
if (safe_memcpy(self->data + self->pos, data->buf, data->len) < 0) {
if (safe_memmove(self->data + self->pos, data->buf, data->len) < 0) {
result = NULL;
}
else {
Expand Down Expand Up @@ -1722,16 +1722,27 @@ mmap_ass_subscript_impl(mmap_object *self, PyObject *item, PyObject *value)
if (slicelen == 0) {
}
else if (step == 1) {
if (safe_memcpy(self->data + start, vbuf.buf, slicelen) < 0) {
if (safe_memmove(self->data + start, vbuf.buf, slicelen) < 0) {
result = -1;
}
}
else {
if (safe_copy_from_slice(self->data, (char *)vbuf.buf, start, step,
slicelen) < 0)
{
/* The source may overlap the destination, so copy it before

JelleZijlstra Oct 22, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Feels like this can hurt performance, let's look for a better solution.
assigning to the extended slice. */
char *source = PyMem_Malloc(slicelen);
if (source == NULL) {
PyErr_NoMemory();
result = -1;
}
else {
if (safe_memcpy(source, vbuf.buf, slicelen) < 0 ||
safe_copy_from_slice(self->data, source, start, step,
slicelen) < 0)
{
result = -1;
}
PyMem_Free(source);
}
}
PyBuffer_Release(&vbuf);
return result;
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