bizarre trap behavior while reading a file

[tytus64]

I started execution of the following script and and repeatedly sent a SIGHUP
to it from another shell. If SIGHUP is received within the first while loop
the loop terminates with "wc -l" printing fewer than 4158 lines. If SIGHUP
is received within the second while loop the loop still continues and the
second "wc -l" prints always 4158 lines.

I am really curious why is this happening!!!

This is obviously a test scrip. I spent quite a long time debugging my work
script to get to this point ;).

#!/bin/bash

log_file=<file with 4158 lines>
line_num=0

trap "sig_received=true" HUP

rm out.log

cat $log_file | {
    while read line
    do
        line_num=`expr $line_num + 1`
        echo $line_num: $line >> ./out.log
    done
}


wc -l out.log
rm out.log
line_num=0

while read line
do
    line_num=`expr $line_num + 1`
    echo $line_num: $line >> ./out.log
done < $log_file

wc -l out.log

exit 0

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