https://github.com/python/cpython/commit/58badb1711e12b6e8b5240ab96cdd01b95012de7
commit: 58badb1711e12b6e8b5240ab96cdd01b95012de7
branch: main
author: Gregory P. Smith <[email protected]>
committer: gpshead <[email protected]>
date: 2025-11-22T18:29:51Z
summary:
gh-98552: flush std streams in the multiprocessing forkserver before fork
(#141849)
* flush std streams in the multiprocessing forkserver before fork
* NEWS
files:
A Misc/NEWS.d/next/Library/2025-11-22-18-00-38.gh-issue-98552.d5KNy-.rst
M Lib/multiprocessing/forkserver.py
diff --git a/Lib/multiprocessing/forkserver.py
b/Lib/multiprocessing/forkserver.py
index cc8947c5e04fb1..8a4e8d835b0c91 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 00000000000000..37a71ac1fff3f3
--- /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.
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