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gh-121849: Fix PyUnicodeWriter_WriteSubstring() crash if len=0 #121896

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Jul 17, 2024
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gh-121849: Fix PyUnicodeWriter_WriteSubstring() crash if len=0 #121896

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6 changes: 5 additions & 1 deletion Lib/test/test_capi/test_unicode.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1736,7 +1736,7 @@ def test_basic(self):
writer.write_char('=')

# test PyUnicodeWriter_WriteSubstring()
writer.write_substring("[long]", 1, 5);
writer.write_substring("[long]", 1, 5)

# test PyUnicodeWriter_WriteStr()
writer.write_str(" value ")
Expand Down Expand Up @@ -1862,6 +1862,10 @@ def test_ucs4(self):
with self.assertRaises(ValueError):
writer.write_ucs4("text", -1)

def test_substring_empty(self):
writer = self.create_writer(0)
writer.write_substring("abc", 1, 1)
self.assertEqual(writer.finish(), '')


@unittest.skipIf(ctypes is None, 'need ctypes')
Expand Down
23 changes: 12 additions & 11 deletions Objects/unicodeobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -13637,27 +13637,28 @@ int
_PyUnicodeWriter_WriteSubstring(_PyUnicodeWriter *writer, PyObject *str,
Py_ssize_t start, Py_ssize_t end)
{
Py_UCS4 maxchar;
Py_ssize_t len;

assert(0 <= start);
assert(end <= PyUnicode_GET_LENGTH(str));
assert(start <= end);

if (end == 0)
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return 0;

if (start == 0 && end == PyUnicode_GET_LENGTH(str))
return _PyUnicodeWriter_WriteStr(writer, str);

if (PyUnicode_MAX_CHAR_VALUE(str) > writer->maxchar)
Py_ssize_t len = end - start;
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if (len == 0) {
return 0;
}

Py_UCS4 maxchar;
if (PyUnicode_MAX_CHAR_VALUE(str) > writer->maxchar) {
maxchar = _PyUnicode_FindMaxChar(str, start, end);
else
}
else {
maxchar = writer->maxchar;
len = end - start;

if (_PyUnicodeWriter_Prepare(writer, len, maxchar) < 0)
}
if (_PyUnicodeWriter_Prepare(writer, len, maxchar) < 0) {
return -1;
}

_PyUnicode_FastCopyCharacters(writer->buffer, writer->pos,
str, start, len);
Expand Down
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