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gh-151814: Fix unbounded memory growth from repeated empty writes to io.TextIOWrapper #151817

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gh-151814: Fix unbounded memory growth from repeated empty writes to io.TextIOWrapper #151817

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c6b5163
Fix unbounded memory growth from repeated empty writes to io.TextIOWr…
StanFromIreland Jun 20, 2026
0d3fe57
Remove test
StanFromIreland Jun 21, 2026
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-06-20-21-15-13.gh-issue-151814.OIbgsO.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix unbounded memory growth in :class:`io.TextIOWrapper` when repeatedly
writing an empty string.
44 changes: 25 additions & 19 deletions Modules/_io/textio.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1820,32 +1820,38 @@ _io_TextIOWrapper_write_impl(textio *self, PyObject *text)
}
}

if (self->pending_bytes == NULL) {
assert(self->pending_bytes_count == 0);
self->pending_bytes = b;
}
else if (!PyList_CheckExact(self->pending_bytes)) {
PyObject *list = PyList_New(2);
if (list == NULL) {
if (bytes_len > 0) {

@StanFromIreland StanFromIreland Jun 20, 2026 •
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Git seems to render the diff poorly, locally I see with -w (--ignore-all-space):

$ git show -w HEAD -- Modules/_io/textio.c
commit c6b5163133619febd0fbe8c327e52399b1a54ffd (HEAD -> textio-acc, origin/textio-acc)
Author: Stan Ulbrych <stan@python.org>
Date:   Sat Jun 20 21:16:54 2026 +0100

    Fix unbounded memory growth from repeated empty writes to io.TextIOWrapper

diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index 24e08cec88f..5b2a20a30c2 100644
--- a/Modules/_io/textio.c
+++ b/Modules/_io/textio.c
@@ -1820,6 +1820,7 @@ _io_TextIOWrapper_write_impl(textio *self, PyObject *text)
         }
     }
 
+    if (bytes_len > 0) {
         if (self->pending_bytes == NULL) {
             assert(self->pending_bytes_count == 0);
             self->pending_bytes = b;
@@ -1846,6 +1847,11 @@ _io_TextIOWrapper_write_impl(textio *self, PyObject *text)
         }
 
         self->pending_bytes_count += bytes_len;
+    }
+    else {
+        Py_DECREF(b);
+    }
+
     if (self->pending_bytes_count >= self->chunk_size || needflush ||
         text_needflush) {
         if (_textiowrapper_writeflush(self) < 0)

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if (self->pending_bytes == NULL) {
assert(self->pending_bytes_count == 0);
self->pending_bytes = b;
}
else if (!PyList_CheckExact(self->pending_bytes)) {
PyObject *list = PyList_New(2);
if (list == NULL) {
Py_DECREF(b);
return NULL;
}
// Since Python 3.12, allocating GC object won't trigger GC and release
// GIL. See https://github.com/python/cpython/issues/97922
assert(!PyList_CheckExact(self->pending_bytes));
PyList_SET_ITEM(list, 0, self->pending_bytes);
PyList_SET_ITEM(list, 1, b);
self->pending_bytes = list;
}
else {
if (PyList_Append(self->pending_bytes, b) < 0) {
Py_DECREF(b);
return NULL;
}
Py_DECREF(b);
return NULL;
}
// Since Python 3.12, allocating GC object won't trigger GC and release
// GIL. See https://github.com/python/cpython/issues/97922
assert(!PyList_CheckExact(self->pending_bytes));
PyList_SET_ITEM(list, 0, self->pending_bytes);
PyList_SET_ITEM(list, 1, b);
self->pending_bytes = list;

self->pending_bytes_count += bytes_len;
}
else {
if (PyList_Append(self->pending_bytes, b) < 0) {
Py_DECREF(b);
return NULL;
}
Py_DECREF(b);
}

self->pending_bytes_count += bytes_len;
if (self->pending_bytes_count >= self->chunk_size || needflush ||
text_needflush) {
if (_textiowrapper_writeflush(self) < 0)
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