From b7d2ebdd4b9f4b757bdeda921e122a7b430131b6 Mon Sep 17 00:00:00 2001
From: AZero13 <gfunni234@gmail.com>
Date: Sat, 20 Dec 2025 01:34:57 -0500
Subject: [PATCH 1/3] gh-143010: Prevent a TOCTOU issue by gh-143010: Prevent a
 TOCTOU issue by only calling open once

We can literally just use open(path, 'xb+') for _create_carefully.
---
 Lib/mailbox.py                                              | 6 +-----
 .../Library/2025-12-20-01-49-02.gh-issue-143010._-SWX0.rst  | 1 +
 2 files changed, 2 insertions(+), 5 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2025-12-20-01-49-02.gh-issue-143010._-SWX0.rst

diff --git a/Lib/mailbox.py b/Lib/mailbox.py
index 4a44642765cc9a..65923e9c5de324 100644
--- a/Lib/mailbox.py
+++ b/Lib/mailbox.py
@@ -2181,11 +2181,7 @@ def _unlock_file(f):
 
 def _create_carefully(path):
     """Create a file if it doesn't exist and open for reading and writing."""
-    fd = os.open(path, os.O_CREAT | os.O_EXCL | os.O_RDWR, 0o666)
-    try:
-        return open(path, 'rb+')
-    finally:
-        os.close(fd)
+    return open(path, 'xb+')
 
 def _create_temporary(path):
     """Create a temp file based on path and open for reading and writing."""
diff --git a/Misc/NEWS.d/next/Library/2025-12-20-01-49-02.gh-issue-143010._-SWX0.rst b/Misc/NEWS.d/next/Library/2025-12-20-01-49-02.gh-issue-143010._-SWX0.rst
new file mode 100644
index 00000000000000..cdc554b41c988d
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-12-20-01-49-02.gh-issue-143010._-SWX0.rst
@@ -0,0 +1 @@
+_create_carefully: Prevent a TOCTOU by simplifying.

From 879f581ec1efae3bbfa4efe74143249238883cde Mon Sep 17 00:00:00 2001
From: AZero13 <gfunni234@gmail.com>
Date: Sun, 21 Dec 2025 14:21:57 -0500
Subject: [PATCH 2/3] Fix News

---
 .../next/Library/2025-12-20-01-49-02.gh-issue-143010._-SWX0.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Misc/NEWS.d/next/Library/2025-12-20-01-49-02.gh-issue-143010._-SWX0.rst b/Misc/NEWS.d/next/Library/2025-12-20-01-49-02.gh-issue-143010._-SWX0.rst
index cdc554b41c988d..f57bc28cd9ac24 100644
--- a/Misc/NEWS.d/next/Library/2025-12-20-01-49-02.gh-issue-143010._-SWX0.rst
+++ b/Misc/NEWS.d/next/Library/2025-12-20-01-49-02.gh-issue-143010._-SWX0.rst
@@ -1 +1 @@
-_create_carefully: Prevent a TOCTOU by simplifying.
+mailbox: Fixed a bug in :mod:mailbox where the precise timing of an external event could result in the library opening an existing file instead of a file it expected to create.

From 7b1b9f2c04bc25cee83e9fc2a6d5c3d32867266e Mon Sep 17 00:00:00 2001
From: AZero13 <gfunni234@gmail.com>
Date: Sun, 21 Dec 2025 18:21:47 -0500
Subject: [PATCH 3/3] Update 2025-12-20-01-49-02.gh-issue-143010._-SWX0.rst

Co-authored-by: sobolevn <mail@sobolevn.me>
---
 .../next/Library/2025-12-20-01-49-02.gh-issue-143010._-SWX0.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Misc/NEWS.d/next/Library/2025-12-20-01-49-02.gh-issue-143010._-SWX0.rst b/Misc/NEWS.d/next/Library/2025-12-20-01-49-02.gh-issue-143010._-SWX0.rst
index f57bc28cd9ac24..4914d0b7be727b 100644
--- a/Misc/NEWS.d/next/Library/2025-12-20-01-49-02.gh-issue-143010._-SWX0.rst
+++ b/Misc/NEWS.d/next/Library/2025-12-20-01-49-02.gh-issue-143010._-SWX0.rst
@@ -1 +1 @@
-mailbox: Fixed a bug in :mod:mailbox where the precise timing of an external event could result in the library opening an existing file instead of a file it expected to create.
+Fixed a bug in :mod:`mailbox` where the precise timing of an external event could result in the library opening an existing file instead of a file it expected to create.