Read fails after tcpreplay with error: 0: Resource temporarily unavailabl - bash

Question

I have a very simple script to run. It calls tcpreplay and then ask the user to type in something. Then the read will fail with read: read error: 0: Resource temporarily unavailable.
Here is the code
#!/bin/bash
tcpreplay -ieth4 SMTP.pcap
echo TEST
read HANDLE
echo $HANDLE
And the output is
[root#vse1 quick_test]# ./test.sh
sending out eth4
processing file: SMTP.pcap
Actual: 28 packets (4380 bytes) sent in 0.53 seconds. Rated: 8264.2 bps, 0.06 Mbps, 52.83 pps
Statistics for network device: eth4
Attempted packets: 28
Successful packets: 28
Failed packets: 0
Retried packets (ENOBUFS): 0
Retried packets (EAGAIN): 0
TEST
./test.sh: line 6: read: read error: 0: Resource temporarily unavailable
[root#vse1 quick_test]#
I am wondering if I need to close or clear up any handles or pipes after I run tcpreplay?

Answer 1

Apparently tcpreplay sets O_NONBLOCK on stdin and then doesn't remove it. I'd say it's a bug in tcpreplay. To work it around you can run tcpreplay with stdin redirected from /dev/null. Like this:
tcpreplay -i eth4 SMTP.pcap </dev/null
Addition: note that this tcpreplay behavior breaks non-interactive shells only.
Another addition: alternatively, if you really need tcpreplay to receive your
input you can write a short program which resets O_NONBLOCK. Like this one
(reset-nonblock.c):
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
int
main()
{
if (fcntl(STDIN_FILENO, F_SETFL,
fcntl(STDIN_FILENO, F_GETFL) & ~O_NONBLOCK) < 0) {
perror(NULL);
return 1;
}
return 0;
}
Make it with "make reset-nonblock", then put it in your PATH and use like this:
tcpreplay -i eth4 SMTP.pcap
reset-nonblock

Answer 2

While the C solution works, you can turn off nonblocking input in one line from the command-line using Python. Personally, I alias it to "setblocking" since it is fairly handy.
$ python3 -c $'import os\nos.set_blocking(0, True)'
You can also have Python print the previous state so that it may be changed only temporarily:
$ o=$(python3 -c $'import os\nprint(os.get_blocking(0))\nos.set_blocking(0, True)')
$ somecommandthatreadsstdin
$ python3 -c $'import os\nos.set_blocking(0, '$o')'

Answer 3

Resource temporarily unavailable is EAGAIN (or EWOULDBLOCK) which is the error code for nonblocking file descriptor when no further data is available (would block if wasn't in nonblocking mode). The previous command (tcpreplay in this case) erroneously left STDIN in nonblocking mode. The shell will not correct it, and the following process isn't meant to work with non- default nonblocking STDIN.
In your script, you can also turn off nonblocking with:
perl -MFcntl -e 'fcntl STDIN, F_SETFL, fcntl(STDIN, F_GETFL, 0) & ~O_NONBLOCK'