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gh-145743: Fix inconsistency after calling Struct.__init__() with invalid format #145744

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Merged
serhiy-storchaka merged 2 commits into from
Mar 10, 2026
+33 −16
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gh-145743: Fix inconsistency after calling Struct.__init__() with invalid format #145744

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gh-145743: Fix inconsistancy after calling Struct.__init__() with inv…
serhiy-storchaka Mar 10, 2026
a23f9bb
Update Modules/_struct.c
serhiy-storchaka Mar 10, 2026
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20 changes: 18 additions & 2 deletions Lib/test/test_struct.py
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Original file line number Diff line number Diff line change
Expand Up @@ -584,8 +584,24 @@ def test_Struct_reinitialization(self):
# Issue 9422: there was a memory leak when reinitializing a
# Struct instance. This test can be used to detect the leak
# when running with regrtest -L.
s = struct.Struct('i')
s.__init__('ii')
s = struct.Struct('>h')
s.__init__('>hh')
self.assertEqual(s.format, '>hh')
packed = b'\x00\x01\x00\x02'
self.assertEqual(s.pack(1, 2), packed)
self.assertEqual(s.unpack(packed), (1, 2))

with self.assertRaises(UnicodeEncodeError):
s.__init__('\udc00')
self.assertEqual(s.format, '>hh')
self.assertEqual(s.pack(1, 2), packed)
self.assertEqual(s.unpack(packed), (1, 2))

with self.assertRaises(struct.error):
s.__init__('$')
self.assertEqual(s.format, '>hh')
self.assertEqual(s.pack(1, 2), packed)
self.assertEqual(s.unpack(packed), (1, 2))

def check_sizeof(self, format_str, number_of_codes):
# The size of 'PyStructObject'
Expand Down
29 changes: 15 additions & 14 deletions Modules/_struct.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1620,11 +1620,11 @@ align(Py_ssize_t size, char c, const formatdef *e)
/* calculate the size of a format string */

static int
prepare_s(PyStructObject *self)
prepare_s(PyStructObject *self, PyObject *format)
{
const formatdef *f;
const formatdef *e;
formatcode *codes;
formatcode *codes, *codes0;

const char *s;
const char *fmt;
Expand All @@ -1634,8 +1634,8 @@ prepare_s(PyStructObject *self)

_structmodulestate *state = get_struct_state_structinst(self);

fmt = PyBytes_AS_STRING(self->s_format);
if (strlen(fmt) != (size_t)PyBytes_GET_SIZE(self->s_format)) {
fmt = PyBytes_AS_STRING(format);
if (strlen(fmt) != (size_t)PyBytes_GET_SIZE(format)) {
PyErr_SetString(state->StructError,
"embedded null character");
return -1;
Expand Down Expand Up @@ -1711,13 +1711,7 @@ prepare_s(PyStructObject *self)
PyErr_NoMemory();
return -1;
}
/* Free any s_codes value left over from a previous initialization. */
if (self->s_codes != NULL)
PyMem_Free(self->s_codes);
self->s_codes = codes;
self->s_size = size;
self->s_len = len;

codes0 = codes;
s = fmt;
size = 0;
while ((c = *s++) != '\0') {
Expand Down Expand Up @@ -1757,6 +1751,14 @@ prepare_s(PyStructObject *self)
codes->size = 0;
codes->repeat = 0;

/* Free any s_codes value left over from a previous initialization. */
if (self->s_codes != NULL)
PyMem_Free(self->s_codes);
self->s_codes = codes0;
self->s_size = size;
self->s_len = len;
Py_XSETREF(self->s_format, Py_NewRef(format));

return 0;

overflow:
Expand Down Expand Up @@ -1820,9 +1822,8 @@ Struct___init___impl(PyStructObject *self, PyObject *format)
return -1;
}

Py_SETREF(self->s_format, format);

ret = prepare_s(self);
ret = prepare_s(self, format);
Py_DECREF(format);
return ret;
}

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