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bpo-33550: Warn not to set SIGPIPE to SIG_DFL #6773

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bpo-33550: Warn not to set SIGPIPE to SIG_DFL #6773

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34 changes: 34 additions & 0 deletions Doc/library/signal.rst
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Expand Up @@ -503,3 +503,37 @@ be sent, and the handler raises an exception. ::

signal.alarm(0) # Disable the alarm

Note on SIGPIPE
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By the way, I don't think this is suitable for the signal module documentation. Maybe we can create a HOWTO document for signals? A HOWTO document would also allow us adding more best practices in the future.

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Agreed. Until we have such document, I'm fine putting this in the documentation as is so it's searchable.

---------------

Piping output of your program to tools like :manpage:`head(1)` will
cause a :const:`SIGPIPE` signal to be sent to your process when the receiver
of its standard output closes early. This results in an exception
like :code:`BrokenPipeError: [Errno 32] Broken pipe`. To handle this
case, wrap your entry point to catch this exception as follows::

import os
import sys

def main():
try:
# simulate large output (your code replaces this loop)
for x in range(10000):
print("y")
# flush output here to force SIGPIPE to be triggered
# while inside this try block.
sys.stdout.flush()
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Perhaps it would be clearer if you made it an indefinite while True loop; then you wouldn’t need the flush

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The goal here is to give a recipe which covers common uses. Showing that it's important to use sys.stdout.flush() because not all programs are an infinite loop, a significant portion do a single task outputting some output and that flush is required to avoid the pipe error.

except BrokenPipeError:
# Python flushes standard streams on exit; redirect remaining output
# to devnull to avoid another BrokenPipeError at shutdown
devnull = os.open(os.devnull, os.O_WRONLY)
os.dup2(devnull, sys.stdout.fileno())
sys.exit(1) # Python exits with error code 1 on EPIPE

if __name__ == '__main__':
main()

Do not set :const:`SIGPIPE`'s disposition to :const:`SIG_DFL`
in order to avoid :exc:`BrokenPipeError`. Doing that would cause
your program to exit unexpectedly also whenever any socket connection
is interrupted while your program is still writing to it.