From 7b0ed43af55c1e2844aa0ccd5e088b2ddd38dbdb Mon Sep 17 00:00:00 2001 From: "Miss Islington (bot)" <31488909+miss-islington@users.noreply.github.com> Date: Fri, 24 Aug 2018 22:28:58 -0400 Subject: [PATCH] bpo-33550: Warn not to set SIGPIPE to SIG_DFL (GH-6773) (cherry picked from commit a2510732da4c75789cc1750a034bdc9d9fdffb5b) Co-authored-by: Alfred Perlstein --- Doc/library/signal.rst | 34 ++++++++++++++++++++++++++++++++++ 1 file changed, 34 insertions(+) diff --git a/Doc/library/signal.rst b/Doc/library/signal.rst index 67eaa2c6381..fa90dff0d9f 100644 --- a/Doc/library/signal.rst +++ b/Doc/library/signal.rst @@ -485,3 +485,37 @@ be sent, and the handler raises an exception. :: signal.alarm(0) # Disable the alarm +Note on SIGPIPE +--------------- + +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() + 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.