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gh-151814: Fix unbounded memory growth from repeated empty writes to `io.TextIOWrapper` by StanFromIreland · Pull Request #151817 · python/cpython · GitHub

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

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StanFromIreland merged 2 commits into
python:mainfrom
StanFromIreland:textio-acc
Jun 24, 2026
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gh-151814: Fix unbounded memory growth from repeated empty writes to io.TextIOWrapper#151817
StanFromIreland merged 2 commits into
python:mainfrom
StanFromIreland:textio-acc

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StanFromIreland commented Jun 20, 2026 •
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StanFromIreland requested a review from cmaloney June 20, 2026 20:18
bedevere-app Bot mentioned this pull request Jun 20, 2026
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StanFromIreland commented Jun 20, 2026
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Comment thread Modules/_io/textio.c
else if (!PyList_CheckExact(self->pending_bytes)) {
PyObject *list = PyList_New(2);
if (list == NULL) {
if (bytes_len > 0) {

StanFromIreland Jun 20, 2026 •
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

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)

asottile Jun 23, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

you can use ?w=1 on GitHub fwiw

StanFromIreland Jun 23, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Cool, thanks! But, it seems to be limited and unfortunately doesn’t support all options, ?ignore-all-space=1 didn't work for me.

StanFromIreland changed the title gh-151814: Fix unbounded memory growth from repeated empty writes to io.TextIOWr… gh-151814: Fix unbounded memory growth from repeated empty writes to io.TextIOWrapper Jun 20, 2026
cmaloney reviewed Jun 20, 2026
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Comment thread Modules/_io/textio.c
Comment thread Lib/test/test_io/test_textio.py Outdated
StanFromIreland requested a review from cmaloney June 21, 2026 09:29
StanFromIreland added needs backport to 3.13 only security fixes needs backport to 3.14 bugs and security fixes needs backport to 3.15 bugs and security fixes labels Jun 22, 2026
StanFromIreland merged commit c613072 into python:main Jun 24, 2026
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StanFromIreland deleted the textio-acc branch June 24, 2026 11:47

miss-islington-app Bot commented Jun 24, 2026

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Thanks @StanFromIreland for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14, 3.15.
🐍🍒⛏🤖

StanFromIreland commented Jun 24, 2026

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Thanks for the review!

bedevere-app Bot commented Jun 24, 2026

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GH-152071 is a backport of this pull request to the 3.15 branch.

bedevere-app Bot removed the needs backport to 3.15 bugs and security fixes label Jun 24, 2026

bedevere-app Bot commented Jun 24, 2026

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GH-152072 is a backport of this pull request to the 3.14 branch.

bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Jun 24, 2026

bedevere-app Bot commented Jun 24, 2026

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GH-152073 is a backport of this pull request to the 3.13 branch.

bedevere-app Bot removed the needs backport to 3.13 only security fixes label Jun 24, 2026
StanFromIreland added a commit that referenced this pull request Jun 24, 2026
[3.15] gh-151814: Fix unbounded memory growth from repeated empty wri…
6bad84d 
…tes to `io.TextIOWrapper` (GH-151817)

(cherry picked from commit c613072)

Co-authored-by: Stan Ulbrych <stan@python.org>
StanFromIreland added a commit that referenced this pull request Jun 24, 2026
[3.14] gh-151814: Fix unbounded memory growth from repeated empty wri…
a88629a 
…tes to `io.TextIOWrapper` (GH-151817)

(cherry picked from commit c613072)

Co-authored-by: Stan Ulbrych <stan@python.org>
StanFromIreland added a commit that referenced this pull request Jun 24, 2026
[3.13] gh-151814: Fix unbounded memory growth from repeated empty wri…
36fd455 
…tes to `io.TextIOWrapper` (GH-151817) (#152073)

(cherry picked from commit c613072)

Co-authored-by: Stan Ulbrych <stan@python.org>
cmaloney mentioned this pull request Jun 26, 2026
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