From: Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com>
Date: Sat, 22 Nov 2025 20:23:22 +0000 (+0100)
Subject: [3.14] gh-98552: flush std streams in the multiprocessing forkserver before fork... 
X-Git-Tag: v3.14.1~64
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=061965c584810b97458f229a6ac5b3b79286e743;p=thirdparty%2FPython%2Fcpython.git

[3.14] gh-98552: flush std streams in the multiprocessing forkserver before fork (GH-141849) (#141851)

gh-98552: flush std streams in the multiprocessing forkserver before fork (GH-141849)

* flush std streams in the multiprocessing forkserver before fork

* NEWS
(cherry picked from commit 58badb1711e12b6e8b5240ab96cdd01b95012de7)

Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>
---

diff --git a/Lib/multiprocessing/forkserver.py b/Lib/multiprocessing/forkserver.py
index cc8947c5e04f..8a4e8d835b0c 100644
--- a/Lib/multiprocessing/forkserver.py
+++ b/Lib/multiprocessing/forkserver.py
@@ -326,6 +326,7 @@ def main(listener_fd, alive_r, preload, main_path=None, sys_path=None,
                                     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 000000000000..37a71ac1fff3
--- /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.