socket-activate printer

Previous to this commit, the printer needs to continuously run in the
background, even if it doesn't do much most of the time, to keep the
socket open and handle incoming connections. This commit hands over
handling the open socket to systemd and the printer-code gets started by
systemd only when there is something incoming on the socket. The printer
itself then takes over the socket from systemd, handles the incoming
data and exits afterwards, returning socket management to systemd again.

This reduces running processes as well as the memory footprint of the
running system.

Makefile

@@ -24,14 +24,15 @@ install: printer.arm
 	ssh -o AddKeysToAgent=yes root@$(host) systemctl stop printer || true
 	scp printer.arm root@$(host):
 	scp printer.service root@$(host):/etc/systemd/system
+	scp printer.socket root@$(host):/etc/systemd/system
 	ssh root@$(host) systemctl daemon-reload
-	ssh root@$(host) systemctl enable printer
-	ssh root@$(host) systemctl restart printer
+	ssh root@$(host) systemctl enable printer.socket
+	ssh root@$(host) systemctl restart printer.socket
 
 .PHONY: release
 release: printer.arm printer.x86
 	rm -f release.zip
-	zip release.zip printer.arm printer.x86 printer.service -r
+	zip release.zip printer.arm printer.x86 printer.service printer.socket -r
 
 .PHONY: install_config
 install_config:

install.sh

@@ -3,6 +3,7 @@ cd /home/root/bin
 wget -O release.zip http://github.com/evidlo/remarkable_printer/releases/latest/download/release.zip
 unzip -o release.zip
 mv printer.service /etc/systemd/system
+mv printer.socket /etc/systemd/system
 systemctl daemon-reload
-systemctl enable --now printer.service
+systemctl enable --now printer.socket
 rm printer.x86 release.zip

main.go

@@ -60,32 +60,30 @@ func main() {
 	// ----- Listen for connections -----
 
 	// Listen for incoming connections.
-	l, err := net.Listen("tcp", *CONN_HOST + ":" + *CONN_PORT)
+	l, err := net.FileListener(os.NewFile(3, "systemd-socket"))
 	check(err)
 	defer l.Close()
 
 	// Close the listener when the application closes.
 	fmt.Println("Listening on " + *CONN_HOST + ":" + *CONN_PORT)
-	for {
-		// Listen for an incoming connection.
-		conn, err := l.Accept()
-		if err != nil {
-			fmt.Println("Error accepting: ", err.Error())
-			os.Exit(1)
-		}
-		handleRequest(conn)
-
-		// Restart xochitl
-		if *restart {
-			stdout, err := exec.Command("systemctl", "restart", "xochitl").CombinedOutput()
-			if err != nil {
-				fmt.Println("xochitl restart failed with message:", string(stdout))
-			}
-		}
 
+	// Listen for an incoming connection.
+	conn, err := l.Accept()
+	if err != nil {
+		fmt.Println("Error accepting: ", err.Error())
+		os.Exit(1)
 	}
-}
+	handleRequest(conn)
 
+	// Restart xochitl
+	if *restart {
+		stdout, err := exec.Command("systemctl", "restart", "xochitl").CombinedOutput()
+		if err != nil {
+			fmt.Println("xochitl restart failed with message:", string(stdout))
+		}
+	}
+
+}
 
 func debug(msg ...string) {
 	if LOG_LEVEL == "debug" {

printer.service

@@ -1,9 +1,6 @@
 [Unit]
 Description=Native printing to reMarkable
+Requires=printer.socket
 
 [Service]
 ExecStart=/home/root/printer.arm -restart -debug
-Restart=always
-
-[Install]
-WantedBy=multi-user.target

printer.socket

@@ -0,0 +1,9 @@
+[Unit]
+Description=Socket for native printing to reMarkable
+After=multi-user.target
+
+[Socket]
+ListenStream=10.11.99.1:9100
+
+[Install]
+WantedBy=multi-user.target