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Possible race condition in bash

From: Nikolay Borisov
Subject: Possible race condition in bash
Date: Sat, 21 Nov 2020 10:06:59 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0


It's possible to have the following script never terminate:


subfun() {

    trap "echo received term for $*;exit 1" SIGTERM

    echo my pid $BASHPID


    while true; do

        echo $BASHPID subfun $* $i


        sleep 5



trap 'echo main; exit 1' 0 1 2 3 15

subfun xxx &


subfun yyy &


echo funpid=$funpid twopid=$twopid mypid=$$

echo killing $funpid $twopid

kill $funpid $twopid

echo waiting on everything


echo done

The output is:

my pid 12186
12186 subfun xxx 0
funpid=12186 twopid=12187 mypid=12185
killing 12186 12187
waiting on everything
my pid 12187
12187 subfun yyy 0
received term for xxx
12187 subfun yyy 1
12187 subfun yyy 2
12187 subfun yyy 3

and 12187 keeps printing. It would seem there $! can return a pid of the
subshell before it's in a state that it can be killed. This has to do
with setting of the trap handler, because if the trap handler of the
main shell is changed to : trap 'echo main; exit 1' EXIT - Then it never
hangs. I.e the more trap handlers that subshells have to reset to their
defaults the higher the chances this race condition is hit. Also if I
change the sleep in the subshell function to anything below 5 I also
seem to be unable to hit it. Also sometimes when it doesn't hang I get
the following output:

my pid 12283
12283 subfun xxx 0
funpid=12283 twopid=12284 mypid=12282
killing 12283 12284
waiting on everything
received term for xxx

It misses a "Received term for yyy" line.

I reproduced this behavior both with version 4.4.20(1)-release (that
comes with ubuntu 18.04) as well as 5.0.18(1)-release built from source.
Any ideas what might be causing it?

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