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

Author: gpshead

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

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.

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;
     }