bash: match PATTERN in tail -f and kill background process

x.sh

# execute it with source

unalias tail
echo 1 > /tmp/TEST_LOG;

sleep 100 &
pid_bg=$!

i=0
while
        IFS= read -r
do 
        ((i++, pid_tail = i == 1 ? $REPLY : pid_tail))
        [[ $REPLY == PATTERN ]] && kill $pid_bg $pid_tail;
done < <(
        tail -f /tmp/TEST_LOG &
        echo $!
)