Move nc backend (#9030)

* move_nc_backend: Move script

This PR moves the script from the old directory to  new directory.

* move_nc_backend: Update path

This commit updates the path inside backend documentation

* move_nc_backend: Missing line

THis commit fixed the missing line inside a Makefile

* move_nc_backend: Return file

* move_nc_backend: Fix Makefile.am

* move_nc_backend: Fix shellckeck warning

* move_nc_backend: Rename script

THis commit renames the script to nc-exporting.sh

* move_nc_backend: Missing Makefile change

* move_nc_backend: Remove unecessary line

exporting/Makefile.am

@@ -23,3 +23,7 @@ dist_noinst_DATA = \
     TIMESCALE.md \
     WALKTHROUGH.md \
     $(NULL)
+
+dist_noinst_SCRIPTS = \
+    nc-exporting.sh \
+    $(NULL)

exporting/nc-exporting.sh

@@ -0,0 +1,158 @@
+#!/usr/bin/env bash
+
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+# This is a simple backend database proxy, written in BASH, using the nc command.
+# Run the script without any parameters for help.
+
+MODE="${1}"
+MY_PORT="${2}"
+BACKEND_HOST="${3}"
+BACKEND_PORT="${4}"
+FILE="${NETDATA_NC_BACKEND_DIR-/tmp}/netdata-nc-backend-${MY_PORT}"
+
+log() {
+	logger --stderr --id=$$ --tag "netdata-nc-backend" "${*}"
+}
+
+mync() {
+	local ret
+
+	log "Running: nc ${*}"
+	nc "${@}"
+	ret=$?
+
+	log "nc stopped with return code ${ret}."
+
+	return ${ret}
+}
+
+listen_save_replay_forever() {
+	local file="${1}" port="${2}" real_backend_host="${3}" real_backend_port="${4}" ret delay=1 started ended
+
+	while true
+	do
+		log "Starting nc to listen on port ${port} and save metrics to ${file}"
+		
+		started=$(date +%s)
+		mync -l -p "${port}" | tee -a -p --output-error=exit "${file}"
+		ended=$(date +%s)
+		
+		if [ -s "${file}" ]
+			then
+			if [ -n "${real_backend_host}" ] && [ -n "${real_backend_port}" ]
+				then
+				log "Attempting to send the metrics to the real backend at ${real_backend_host}:${real_backend_port}"
+				
+				mync "${real_backend_host}" "${real_backend_port}" <"${file}"
+				ret=$?
+
+				if [ ${ret} -eq 0 ]
+					then
+					log "Successfuly sent the metrics to ${real_backend_host}:${real_backend_port}"
+					mv "${file}" "${file}.old"
+					touch "${file}"
+				else
+					log "Failed to send the metrics to ${real_backend_host}:${real_backend_port} (nc returned ${ret}) - appending more data to ${file}"
+				fi
+			else
+				log "No backend configured - appending more data to ${file}"
+			fi
+		fi
+
+		# prevent a CPU hungry infinite loop
+		# if nc cannot listen to port
+		if [ $((ended - started)) -lt 5 ]
+			then
+			log "nc has been stopped too fast."
+			delay=30
+		else
+			delay=1
+		fi
+
+		log "Waiting ${delay} seconds before listening again for data."
+		sleep ${delay}
+	done
+}
+
+if [ "${MODE}" = "start" ]
+	then
+
+	# start the listener, in exclusive mode
+	# only one can use the same file/port at a time
+	{
+		flock -n 9
+		# shellcheck disable=SC2181
+		if [ $? -ne 0 ]
+			then
+			log "Cannot get exclusive lock on file ${FILE}.lock - Am I running multiple times?"
+			exit 2
+		fi
+
+		# save our PID to the lock file
+		echo "$$" >"${FILE}.lock"
+
+		listen_save_replay_forever "${FILE}" "${MY_PORT}" "${BACKEND_HOST}" "${BACKEND_PORT}"
+		ret=$?
+
+		log "listener exited."
+		exit ${ret}
+
+	} 9>>"${FILE}.lock"
+
+	# we can only get here if ${FILE}.lock cannot be created
+	log "Cannot create file ${FILE}."
+	exit 3
+
+elif [ "${MODE}" = "stop" ]
+	then
+
+	{
+		flock -n 9
+		# shellcheck disable=SC2181
+		if [ $? -ne 0 ]
+			then
+			pid=$(<"${FILE}".lock)
+			log "Killing process ${pid}..."
+			kill -TERM "-${pid}"
+			exit 0
+		fi
+
+		log "File ${FILE}.lock has been locked by me but it shouldn't. Is a collector running?"
+		exit 4
+
+	} 9<"${FILE}.lock"
+
+	log "File ${FILE}.lock does not exist. Is a collector running?"
+	exit 5
+
+else
+
+	cat <<EOF
+Usage:
+
+    "${0}" start|stop PORT [BACKEND_HOST BACKEND_PORT]
+
+    PORT          The port this script will listen
+                  (configure netdata to use this as a second backend)
+
+    BACKEND_HOST  The real backend host
+    BACKEND_PORT  The real backend port
+
+    This script can act as fallback backend for netdata.
+    It will receive metrics from netdata, save them to
+    ${FILE}
+    and once netdata reconnects to the real-backend, this script
+    will push all metrics collected to the real-backend too and
+    wait for a failure to happen again.
+
+    Only one netdata can connect to this script at a time.
+    If you need fallback for multiple netdata, run this script
+    multiple times with different ports.
+
+    You can run me in the background with this:
+
+    screen -d -m "${0}" start PORT [BACKEND_HOST BACKEND_PORT]
+EOF
+	exit 1
+fi