ttcn3-tcpdump-start.sh: Wait for tcpdump to start recording

We generate some fake transit and we wait until we catch tcpdump already
saved some packet into the pcap file, this way making sure it is already
recording before starting the test.

The -U flag (--packet-buffered) is added to increase the chances to
sleep less time waiting for stuff being saved into the pcap file.
According to tcpdump manual:
"""
If  the  -w option is specified, make the saved raw packet output ``packet-buffered''; i.e., as each packet is saved, it will
              be written to the output file, rather than being written only when the output buffer fills.
"""

Change-Id: I91cfd84ff5356857a13af1901abfe2204a93f76d

ttcn3-tcpdump-start.sh

@@ -1,6 +1,7 @@
 #!/bin/sh
 
 PIDFILE=/tmp/tcpdump.pid
+TCPDUMP=/usr/sbin/tcpdump
 TESTCASE=$1
 
 if [ "z$TTCN3_PCAP_PATH" = "z" ]; then
@@ -15,20 +16,28 @@ fi
 # NOTE: This requires you to be root or something like
 # "laforge ALL=NOPASSWD: /usr/sbin/tcpdump, /bin/kill" in your sudoers file
 if [ "$(id -u)" = "0" ]; then
-	CMD=/usr/sbin/tcpdump
+	CMD=$TCPDUMP
 else
-	CMD="sudo /usr/sbin/tcpdump"
+	CMD="sudo $TCPDUMP"
 fi
-$CMD -s 0 -n -i any -w "$TTCN3_PCAP_PATH/$TESTCASE.pcap" >$TTCN3_PCAP_PATH/$TESTCASE.pcap.log 2>&1 &
+$CMD -U -s 0 -n -i any -w "$TTCN3_PCAP_PATH/$TESTCASE.pcap" >$TTCN3_PCAP_PATH/$TESTCASE.pcap.log 2>&1 &
 PID=$!
 echo $PID > $PIDFILE
 
-# Wait until tcpdump creates the pcap file to give it some time to start listenting.
+# Wait until tcpdump creates the pcap file and starts recording.
+# We generate some traffic until we see tcpdump catches it.
 # Timeout is 10 seconds.
+ping 127.0.0.1 >/dev/null 2>&1 &
+PID=$!
 i=0
-while [ ! -f "$TTCN3_PCAP_PATH/$TESTCASE.pcap" ] && [ $i -lt 10 ]
+while [ ! -f "$TTCN3_PCAP_PATH/$TESTCASE.pcap" ] ||
+      [ "$($TCPDUMP -r "$TTCN3_PCAP_PATH/$TESTCASE.pcap" 2>/dev/null | wc -l)" -eq 0 ]
 do
 	echo "Waiting for tcpdump to start... $i"
 	sleep 1
 	i=$((i+1))
+	if [ $i -eq 10 ]; then
+		break
+	fi
 done
+kill $PID