From 58badb1711e12b6e8b5240ab96cdd01b95012de7 Mon Sep 17 00:00:00 2001
From: "Gregory P. Smith" <68491+gpshead@users.noreply.github.com>
Date: Sat, 22 Nov 2025 10:29:51 -0800
Subject: [PATCH] gh-98552: flush std streams in the multiprocessing forkserver
 before fork (#141849)

* flush std streams in the multiprocessing forkserver before fork

* NEWS
---
 Lib/multiprocessing/forkserver.py                             | 1 +
 .../Library/2025-11-22-18-00-38.gh-issue-98552.d5KNy-.rst     | 4 ++++
 2 files changed, 5 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2025-11-22-18-00-38.gh-issue-98552.d5KNy-.rst

diff --git a/Lib/multiprocessing/forkserver.py b/Lib/multiprocessing/forkserver.py
index cc8947c5e04..8a4e8d835b0 100644
--- a/Lib/multiprocessing/forkserver.py
+++ b/Lib/multiprocessing/forkserver.py
@@ -326,6 +326,7 @@ def sigchld_handler(*_unused):
                                     len(fds)))
                         child_r, child_w, *fds = fds
                         s.close()
+                        util._flush_std_streams()
                         pid = os.fork()
                         if pid == 0:
                             # Child
diff --git a/Misc/NEWS.d/next/Library/2025-11-22-18-00-38.gh-issue-98552.d5KNy-.rst b/Misc/NEWS.d/next/Library/2025-11-22-18-00-38.gh-issue-98552.d5KNy-.rst
new file mode 100644
index 00000000000..37a71ac1fff
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-11-22-18-00-38.gh-issue-98552.d5KNy-.rst
@@ -0,0 +1,4 @@
+The :mod:`multiprocessing` forkserver process now flushes stdout and stderr
+before it forks to avoid the confusion children inheriting any buffered but
+not yet written output data.  Normally there is none, but when using
+:func:`multiprocessing.set_forkserver_preload` there *could* be.