[feature] Added SIGUSR2 support to force config reload #181

Closes #181

---------

Co-authored-by: Federico Capoano <f.capoano@openwisp.io>

Author: Viscous106

docs/user/debugging.rst

@@ -31,3 +31,32 @@ You can inspect the version of openwisp-config currently installed with:
 .. code-block:: shell
 
     openwisp-config --version
+
+Forcing Configuration Update
+----------------------------
+
+You can force openwisp-config to immediately download and apply the latest
+configuration from the controller using the ``--force-update`` option:
+
+.. code-block:: shell
+
+    openwisp-config --force-update
+
+This command checks if the openwisp-config agent is running and sends a
+SIGUSR2 signal to trigger an immediate configuration update. If the agent
+is not running, the command will exit with an error.
+
+Alternatively, you can manually send the SIGUSR2 signal to the agent
+process:
+
+.. code-block:: shell
+
+    kill -USR2 "$(pgrep -P 1 -f openwisp-config)"
+
+This is useful when you need to:
+
+- Force the device to fetch the latest configuration without waiting for
+  the next polling interval
+- Apply configuration changes immediately after making updates in OpenWISP
+  Controller
+- Troubleshoot configuration synchronization issues

openwisp-config/files/openwisp.agent

@@ -9,6 +9,17 @@ while [ -n "$1" ]; do
 			export VERSION=1
 			break
 			;;
+		--force-update)
+			# Check if agent is running and send SIGUSR2 to force configuration reload
+			AGENT_PID=$(pgrep -P 1 -f openwisp-config)
+			if [ -z "$AGENT_PID" ]; then
+				echo "Error: openwisp-config agent is not running" >&2
+				exit 1
+			fi
+			kill -USR2 "$AGENT_PID"
+			echo "Sent SIGUSR2 to openwisp-config agent (PID: $AGENT_PID) to force configuration update"
+			exit 0
+			;;
 		--url)
 			export URL="${2%/}"
 			shift
@@ -915,6 +926,15 @@ handle_sigusr1() {
 		-p daemon.info
 }
 
+handle_sigusr2() {
+	logger -s "Received SIGUSR2: forcing configuration update..." \
+		-t openwisp \
+		-p daemon.info
+	# Remove checksum files to ensure configuration_changed returns 1
+	# which will trigger update_configuration on the next loop iteration
+	rm -f "$CONFIGURATION_CHECKSUM" "$PERSISTENT_CHECKSUM"
+}
+
 # ensure both UUID and KEY are defined
 # otherwise perform registration
 if [ -z "$UUID" ] || [ -z "$KEY" ]; then
@@ -972,8 +992,11 @@ while true; do
 
 	# handle SIGUSR1 to interrupt sleep
 	trap handle_sigusr1 USR1
+	# handle SIGUSR2 to force downloading and applying config again
+	trap handle_sigusr2 USR2
 	sleep "$INTERVAL" &
 	wait $!
-	# ignore SIGUSR1 signals again
+	# ignore SIGUSR1 and SIGUSR2 signals again
 	trap "" USR1
+	trap "" USR2
 done