flatcar-update: Replace ncat with socat

The payload server already uses socat, so let's use the same for
response server. We do it, because we are about to replace nmap's ncat
with openbsd netcat due to licensing issues.

bin/flatcar-update

@@ -267,12 +267,36 @@ tee -a /tmp/response > /dev/null <<-EOF
 	</updatecheck><event status="ok"></event></app></response>
 EOF
 
+# Cleanups for local socat servers below
+
 true > /tmp/payload-server-pids
-trap "umount /usr/share/update_engine/update-payload-key.pub.pem 2> /dev/null || true; rm -f /tmp/response /tmp/payload-server ; cat /tmp/payload-server-pids | xargs -r kill ; rm -f /tmp/payload-server-pids" EXIT INT
-ncat --keep-open -c "echo -en 'HTTP/1.1 200 OK\ncontent-type: text/xml\ncontent-length: $(stat --printf='%s\n' /tmp/response)\n\n'; cat /tmp/response" -l "$LISTEN_PORT_1" &
+trap "umount /usr/share/update_engine/update-payload-key.pub.pem 2> /dev/null || true ; cat /tmp/payload-server-pids | xargs -r kill ; rm -f /tmp/response /tmp/payload-server /tmp/response-server /tmp/payload-server-pids" EXIT INT
+
+
+# Setup for XML response server
+
+# Helper script because inline quoting is insane
+tee /tmp/response-server > /dev/null <<'EOF'
+#!/bin/bash
+set -euo pipefail
+read -a WORDS
+if [[ ${#WORDS[@]} -ne 3 ]] || [[ ${WORDS[0]} != POST ]] || [[ ${WORDS[1]} != /update ]] ; then
+  echo -ne "HTTP/1.1 400 Bad request\r\n\r\n"; exit 0
+fi
+echo -ne "HTTP/1.1 200 OK\r\n"
+echo -ne "Content-Type: text/xml\r\n"
+LEN=$(stat --printf='%s\n' /tmp/response)
+echo -ne "Content-Length: ${LEN}\r\n"
+echo -ne "\r\n"
+cat /tmp/response
+EOF
+
+chmod +x /tmp/response-server
+socat TCP-LISTEN:"${LISTEN_PORT_1}",reuseaddr,fork SYSTEM:'/tmp/response-server' &
 CHILDPID="$!"
 echo "${CHILDPID}" >> /tmp/payload-server-pids
 
+# Setup for payload server
 
 # Helper script because inline quoting is insane
 tee /tmp/payload-server > /dev/null <<'EOF'