https://github.com/python/cpython/commit/83b047ef80836c24672cf3e2e807e68efed7d495
commit: 83b047ef80836c24672cf3e2e807e68efed7d495
branch: main
author: Gregory P. Smith <[email protected]>
committer: gpshead <[email protected]>
date: 2026-10-31T16:09:23-07:00
summary:
gh-160324: Keep `io.StringIO` state after `MemoryError` (GH-160325)
Allocate before consuming the accumulated writes in realize() and
make_intermediate(), and reset the size when the contents are lost,
so that later calls do not index outside the internal buffer.
Shrink the buffer of the accumulating writer before finishing it, so
that getvalue(), read() and the other methods leave the contents
unchanged when they fail with MemoryError instead of dropping them.
ANT-2026-MPHGX2FL
files:
A Misc/NEWS.d/next/Library/2026-10-22-08-15-56.gh-issue-160324.sIoRlz.rst
M Lib/test/test_io/test_memoryio.py
M Modules/_io/stringio.c
diff --git a/Lib/test/test_io/test_memoryio.py
b/Lib/test/test_io/test_memoryio.py
index e8b47951b403c11..89b0e4c9463ac0a 100644
--- a/Lib/test/test_io/test_memoryio.py
+++ b/Lib/test/test_io/test_memoryio.py
@@ -1237,6 +1237,39 @@ def __str__(self):
memio2.write(MyStr("world"))
self.assertEqual(memio2.getvalue(), "hello world")
+ @support.nomemtest
+ def test_memory_error_consistent_state(self):
+ # A MemoryError while copying the accumulated writes into the
+ # internal buffer must leave the size of the stream consistent with
+ # the buffer, so that using the object again does not read or write
+ # outside it.
+ piece = "0123456789" * 10
+ text = piece * 50
+ calls = [("readline",), ("read", 5), ("read",), ("getvalue",),
+ ("truncate", 3), ("write", "ab"), ("__next__",)]
+ for name, *args in calls:
+ for n in range(3):
+ with self.subTest(method=name, n=n):
+ memio = self.ioclass()
+ for _ in range(50):
+ memio.write(piece)
+ memio.seek(0)
+ method = getattr(memio, name)
+ try:
+ with support.inject_memory_error_cm(n, n + 1):
+ method(*args)
+ except MemoryError:
+ pass
+ else:
+ continue
+ # The contents are unchanged.
+ self.assertEqual(memio.getvalue(), text)
+ self.assertEqual(memio.seek(0, 2), len(text))
+ memio.seek(0)
+ self.assertEqual(memio.readline(), text)
+ memio.write("!")
+ self.assertEqual(memio.getvalue(), text + "!")
+
class CStringIOPickleTest(PyStringIOPickleTest):
UnsupportedOperation = io.UnsupportedOperation
diff --git
a/Misc/NEWS.d/next/Library/2026-10-22-08-15-56.gh-issue-160324.sIoRlz.rst
b/Misc/NEWS.d/next/Library/2026-10-22-08-15-56.gh-issue-160324.sIoRlz.rst
new file mode 100644
index 000000000000000..d54069128e2c0eb
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-22-08-15-56.gh-issue-160324.sIoRlz.rst
@@ -0,0 +1,3 @@
+Fix a crash or garbage data when an :class:`io.StringIO` object is used again
+after one of its methods failed with :exc:`MemoryError`. The object now keeps
+its contents.
diff --git a/Modules/_io/stringio.c b/Modules/_io/stringio.c
index 7800a4fcfca4dd8..25ebe8909278487 100644
--- a/Modules/_io/stringio.c
+++ b/Modules/_io/stringio.c
@@ -1,6 +1,7 @@
#include "Python.h"
#include <stddef.h> // offsetof()
#include "pycore_object.h"
+#include "pycore_unicodeobject.h" // _PyUnicode_ResizeCompact()
#include "pycore_weakref.h" // FT_CLEAR_WEAKREFS()
#include "_iomodule.h"
@@ -143,25 +144,52 @@ resize_buffer(stringio *self, size_t size)
return -1;
}
+/* Shrink the buffer of the writer to its contents, so that the following
+ PyUnicodeWriter_Finish() has nothing to allocate and cannot fail.
+ If this fails, the writer is unchanged. */
+static int
+shrink_writer(stringio *self)
+{
+ _PyUnicodeWriter *writer = (_PyUnicodeWriter *)self->writer;
+ if (writer->readonly || writer->buffer == NULL
+ || writer->pos == 0 || writer->pos == writer->size)
+ {
+ return 0;
+ }
+ PyObject *buffer = _PyUnicode_ResizeCompact(writer->buffer, writer->pos);
+ if (buffer == NULL) {
+ return -1;
+ }
+ writer->buffer = NULL;
+ _PyUnicodeWriter_SetBuffer(writer, buffer);
+ return 0;
+}
+
static PyObject *
make_intermediate(stringio *self)
{
- PyObject *intermediate = PyUnicodeWriter_Finish(self->writer);
- self->writer = NULL;
- self->state = STATE_REALIZED;
- if (intermediate == NULL)
+ /* Create the new writer and shrink the old one first: if this fails,
+ the object keeps its contents. */
+ PyUnicodeWriter *writer = PyUnicodeWriter_Create(0);
+ if (writer == NULL)
return NULL;
+ if (shrink_writer(self) < 0) {
+ PyUnicodeWriter_Discard(writer);
+ return NULL;
+ }
- self->writer = PyUnicodeWriter_Create(0);
- if (self->writer == NULL) {
- Py_DECREF(intermediate);
+ PyObject *intermediate = PyUnicodeWriter_Finish(self->writer);
+ self->writer = writer;
+ if (intermediate == NULL) {
+ /* The contents are lost, the new writer is empty. */
+ self->string_size = 0;
return NULL;
}
if (PyUnicodeWriter_WriteStr(self->writer, intermediate)) {
Py_DECREF(intermediate);
+ self->string_size = 0;
return NULL;
}
- self->state = STATE_ACCUMULATING;
return intermediate;
}
@@ -174,23 +202,30 @@ realize(stringio *self)
if (self->state == STATE_REALIZED)
return 0;
assert(self->state == STATE_ACCUMULATING);
+
+ /* Allocate the internal buffer and shrink the writer first: if this
+ fails, the object stays in the accumulating state and keeps its
+ contents. */
+ if (resize_buffer(self, self->string_size) < 0)
+ return -1;
+ if (shrink_writer(self) < 0)
+ return -1;
self->state = STATE_REALIZED;
intermediate = PyUnicodeWriter_Finish(self->writer);
self->writer = NULL;
- if (intermediate == NULL)
+ if (intermediate == NULL) {
+ /* The contents are lost, the internal buffer contains nothing. */
+ self->string_size = 0;
return -1;
+ }
- /* Append the intermediate string to the internal buffer.
- The length should be equal to the current cursor position.
- */
+ /* Copy the intermediate string to the internal buffer. */
len = PyUnicode_GET_LENGTH(intermediate);
- if (resize_buffer(self, len) < 0) {
- Py_DECREF(intermediate);
- return -1;
- }
+ assert(len == self->string_size);
if (!PyUnicode_AsUCS4(intermediate, self->buf, len, 0)) {
Py_DECREF(intermediate);
+ self->string_size = 0;
return -1;
}
@@ -358,7 +393,9 @@ _io_StringIO_read_impl(stringio *self, Py_ssize_t size)
/* Optimization for seek(0); read() */
if (self->state == STATE_ACCUMULATING && self->pos == 0 && size == n) {
PyObject *result = make_intermediate(self);
- self->pos = self->string_size;
+ if (result != NULL) {
+ self->pos = self->string_size;
+ }
return result;
}
_______________________________________________
Python-checkins mailing list -- [email protected]
To unsubscribe send an email to [email protected]
https://mail.python.org/mailman3//lists/python-checkins.python.org
Member address: [email protected]
