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

* flush std streams in the multiprocessing forkserver before fork

* NEWS

diff --git a/Lib/multiprocessing/forkserver.py b/Lib/multiprocessing/forkserver.py
index cc8947c5e04fb11b0ea3ef39db3a599851e940a3..8a4e8d835b0c91d67d6e2e2825f620db02d6a6d7 100644 (file)
--- 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 (file)
index 0000000..37a71ac
--- /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.