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bash trap ignore signal - race condition

From: Roman Rakus
Subject: bash trap ignore signal - race condition
Date: Tue, 12 Apr 2011 14:33:50 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20101209 Fedora/3.1.7-0.35.b3pre.fc14 Lightning/1.0b3pre Thunderbird/3.1.7

There's a race condition when you can hit following:
      set_signal_handler (sig, SIG_IGN);
      change_signal (sig, savestring (string));
      set_signal_handler (sig, trap_handler);
in trap.c file, set_signal() function.
So bash set signal to be ignored and then set it to be handled by trap_handler. However how should bash behave when the signal is received when the trap_handler has been set before. Check the following reproducer:


cat>  traploop.sh<<EOF
while true; do
    trap exit SIGUSR1;

chmod a+x traploop.sh

while true; do
    sleep 1
    echo "Sending SIGUSR1 to $!"
    kill -SIGUSR1 $!
    wait $!
    echo "No. $i: OK"

You can send SIGUSR1 to bash running traploop.sh in the state, when the signal is ignored, but shouldn't be. Is there any reason why is the signal for short time ignored?


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