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gh-156939: Clear newly allocated bytes in PyBytesWriter_Resize() by vstinner · Pull Request #157455 · python/cpython · GitHub

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gh-156939: Clear newly allocated bytes in PyBytesWriter_Resize() #157455

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Merged
vstinner merged 1 commit into from
Sep 15, 2026
Conversation 0 Commits 1 Checks 53 Files changed
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gh-156939: Clear newly allocated bytes in PyBytesWriter_Resize() #157455

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5 changes: 3 additions & 2 deletions Lib/test/test_capi/test_bytes.py
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Original file line number Diff line number Diff line change
Expand Up @@ -366,7 +366,7 @@ def test_get_data(self):
writer.write(0, b's' * small)
self.assertEqual(writer.get_data(), b's' * small)
writer.resize(large)
self.assertEqual(writer.get_data(), b's' * small + CANARY_BYTE + NEW_BYTE * (large - small - 1))
self.assertEqual(writer.get_data(), b's' * small + NEW_BYTE * (large - small))
writer.write(small, b'L' * (large - small))
self.assertEqual(writer.get_data(), b's' * small + b'L' * (large - small))

Expand Down Expand Up @@ -455,6 +455,7 @@ def test_resize(self):
@unittest.skipUnless(support.Py_DEBUG, 'need debug build')
def test_resize_canary(self):
CANARY_BYTE = self.CANARY_BYTE

for size in (self.SMALL_BUFFER, self.LARGE_BUFFER):
with self.subTest(size=size):
# Truncate the last byte
Expand All @@ -470,7 +471,7 @@ def test_resize_canary(self):
writer = self.create_writer(size)
writer.write(0, data)
writer.resize(0)
self.assertEqual(writer.get_data(), b'')
self.assertEqual(get_data_canary(writer), CANARY_BYTE)
self.assertEqual(writer.finish(), b'')

@support.nomemtest
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64 changes: 34 additions & 30 deletions Objects/bytesobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3677,25 +3677,26 @@ byteswriter_write_canary_byte(PyBytesWriter *writer)
#endif

static inline int
byteswriter_resize(PyBytesWriter *writer, Py_ssize_t size, int resize)
byteswriter_resize(PyBytesWriter *writer, Py_ssize_t new_size, int resize)
{
assert(size >= 0);
assert(new_size >= 0);

Py_ssize_t old_allocated = byteswriter_allocated(writer);
if (size <= old_allocated) {
if (new_size <= old_allocated) {
// Do not shrink the buffer before PyBytesWriter_FinishWithSize()
return 0;
}

Py_ssize_t alloc = new_size;
if (resize && writer->overallocate) {
if (size <= (PY_SSIZE_T_MAX - size / OVERALLOCATE_FACTOR)) {
size += size / OVERALLOCATE_FACTOR;
if (alloc <= (PY_SSIZE_T_MAX - alloc / OVERALLOCATE_FACTOR)) {
alloc += alloc / OVERALLOCATE_FACTOR;
}
}

if (writer->obj != NULL) {
if (writer->use_bytearray) {
if (PyByteArray_Resize(writer->obj, size)) {
if (PyByteArray_Resize(writer->obj, alloc)) {
#ifdef Py_DEBUG
// bytearray can override the canary byte on error
byteswriter_write_canary_byte(writer);
Expand All @@ -3705,45 +3706,48 @@ byteswriter_resize(PyBytesWriter *writer, Py_ssize_t size, int resize)
}
else {
// Can raise MemoryError or OverflowError
if (_PyBytes_ResizeKeepOnError(&writer->obj, size)) {
if (_PyBytes_ResizeKeepOnError(&writer->obj, alloc)) {
assert(writer->obj != NULL);
return -1;
}
assert(_PyBytes_IsMutable(writer->obj));
}
assert(writer->obj != NULL);
}
else if (writer->use_bytearray) {
writer->obj = PyByteArray_FromStringAndSize(NULL, size);
if (writer->obj == NULL) {
return -1;
}
if (resize) {
assert((size_t)size > sizeof(writer->small_buffer));
memcpy(PyByteArray_AS_STRING(writer->obj),
writer->small_buffer,
sizeof(writer->small_buffer));
}
}
else {
writer->obj = PyBytes_FromStringAndSize(NULL, size);
if (writer->obj == NULL) {
return -1;
char *data;
if (writer->use_bytearray) {
writer->obj = PyByteArray_FromStringAndSize(NULL, alloc);
if (writer->obj == NULL) {
return -1;
}
data = PyByteArray_AS_STRING(writer->obj);
}
else {
writer->obj = PyBytes_FromStringAndSize(NULL, alloc);
if (writer->obj == NULL) {
return -1;
}
assert(_PyBytes_IsMutable(writer->obj));
data = PyBytes_AS_STRING(writer->obj);
}

if (resize) {
assert((size_t)size > sizeof(writer->small_buffer));
memcpy(PyBytes_AS_STRING(writer->obj),
writer->small_buffer,
sizeof(writer->small_buffer));
// Copy data from the small buffer
Py_ssize_t old_size = writer->size;
assert((size_t)old_size <= sizeof(writer->small_buffer));
assert(old_size <= alloc);
memcpy(data, writer->small_buffer, old_size);
}
assert(_PyBytes_IsMutable(writer->obj));
}

#ifdef Py_DEBUG
Py_ssize_t allocated = byteswriter_allocated(writer);
if (resize && allocated > old_allocated) {
memset(byteswriter_data(writer) + old_allocated, PyBytesWrite_NEW_BYTE,
allocated - old_allocated);
if (resize) {
Py_ssize_t old_size = writer->size;
assert(allocated > old_size);
memset(byteswriter_data(writer) + old_size, PyBytesWrite_NEW_BYTE,
allocated - old_size);
}
#endif

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