[3.15] gh-151814: Fix unbounded memory growth from repeated empty writes to `io.TextI...

(cherry picked from commit c61307222e18bfa06691d53e3ad336c46f432de0)

Co-authored-by: Stan Ulbrych <stan@python.org>

diff --git a/Misc/NEWS.d/next/Library/2026-06-20-21-15-13.gh-issue-151814.OIbgsO.rst b/Misc/NEWS.d/next/Library/2026-06-20-21-15-13.gh-issue-151814.OIbgsO.rst
new file mode 100644 (file)
index 0000000..1365fb4
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-06-20-21-15-13.gh-issue-151814.OIbgsO.rst
@@ -0,0 +1,2 @@
+Fix unbounded memory growth in :class:`io.TextIOWrapper` when repeatedly
+writing an empty string.

diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index 1547c04cdf06afa13ad5440b037c137df592928e..aea21e391dd96e843883d5ce0161e717e91c6524 100644 (file)
--- a/Modules/_io/textio.c
+++ b/Modules/_io/textio.c
@@ -1755,32 +1755,38 @@ _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) {
+        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)