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 58badb1)

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

Author: gpshead

Lib/multiprocessing/forkserver.py

@@ -270,6 +270,7 @@ 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

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.