[Python-Dev] Help preventing SIGPIPE/SIG_DFL anti-pattern.
alfred at freebsd.org
Sat Jun 30 12:31:35 EDT 2018
(sorry for the double post, looks like maybe attachments are dropped,
inlined the attachment this time.)
I'm looking for someone in the python community to help with a problem
of anti-patterns showing up dealing with SIGPIPE.
Specifically I've noticed an anti-pattern developing where folks will
try to suppress broken pipe errors written to stdout by setting
SIGPIPE's disposition to SIG_DFL. This is actually very common, and
also rather broken due to the fact that for all but the most simple text
filters this opens up a problem where the process can exit unexpectedly
due to SIGPIPE being generated from a remote connection the program makes.
I have attached a test program which shows the problem.
to use this program it takes several args.
# 1. Illustrate the 'ugly output to stderr' that folks want to avoid:
% python3 t0.py nocatch | head -1
# 2. Illustrate the anti-pattern, the program exits on about line 47
which most folks to not understand
% python3 t0.py dfl | head -1
# 3. Show a better solution where we catch the pipe error and cleanup to
avoid the message:
% python3 t0.py | head -1
I did a recent audit of a few code bases and saw this pattern pop often
often enough that I am asking if there's a way we can discourage the use
of "signal(SIGPIPE, SIG_DFL)" unless the user really understands what
they are doing.
I do have a pull req here: https://github.com/python/cpython/pull/6773
where I am trying to document this on the signal page, but I can't sort
out how to land this doc change.
=== CUT HERE ===
# Program showing the dangers of setting the SIG_PIPE handler to the
default handler (SIG_DFL).
# To illustrate the problem run with:
# ./foo.py dfl
# The program will exit in do_network_stuff() even though there is a an
# The do_network_stuff() simulates a remote connection that closes
before it can be written to
# which happens often enough to be a hazard in practice.
def sigpipe_handler(sig, frame):
sys.stderr.write("Got sigpipe \n\n\n")
# simulate making a connection to a remote service that closes the
# before we can write to it. (In practice a host rebooting, or
otherwise exiting while we are
# trying to interact with it will be the true source of such behavior.)
s1, s2 = socket.socketpair()
# simulate interacting with a remote service that closes its connection
# before we can write to it. Example: connecting to an http
# issuing a GET request, but the remote server is shutting down between
# when our connection finishes the 3-way handshake and when we are able
# to write our "GET /" request to it.
# In theory this function should be resilient to this, however if
SIGPIPE is set
# to SIGDFL then this code will cause termination of the program.
if 'dfl' in sys.argv[1:]:
for x in range(5):
server_conn = get_server_connection()
sys.stderr.write("about to write to server socket...\n")
except BrokenPipeError as bpe:
sys.stderr.write("caught broken pipe on talking to server,
for x in range(10000):
if 'nocatch' in sys.argv[1:]:
except BrokenPipeError as bpe:
if __name__ == '__main__':
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