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gh-135335: flush stdout/stderr in forkserver after preloading modules (#135338)

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If a preloaded module writes to stdout or stderr, and the stream is buffered,
child processes will inherit the buffered data after forking. Attempt to
prevent this by flushing the streams after preload.

Co-authored-by: Mikhail Efimov <efimov.mikhail@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>
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Duane Griffin 2025-06-19 00:17:02 +12:00 • committed by GitHub
parent 5f6ab92465
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Lib/multiprocessing/forkserver.py
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@ -222,6 +222,10 @@ def main(listener_fd, alive_r, preload, main_path=None, sys_path=None,
except ImportError:
pass
# gh-135335: flush stdout/stderr in case any of the preloaded modules
# wrote to them, otherwise children might inherit buffered data
util._flush_std_streams()
util._close_stdin()
sig_r, sig_w = os.pipe()