feat(GODT-2538): implement smart picking of default IMAP/SMTP ports

utils/port-blocker/port-blocker.go

@@ -0,0 +1,83 @@
+// Copyright (c) 2023 Proton AG
+//
+// This file is part of Proton Mail Bridge.
+//
+// Proton Mail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Proton Mail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Proton Mail Bridge. If not, see <https://www.gnu.org/licenses/>.
+
+// port-blocker is a command-line that ensure a port or range of ports is occupied by creating listeners.
+package main
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"strconv"
+)
+
+func main() {
+	argCount := len(os.Args)
+	if (len(os.Args) < 2) || (argCount > 3) {
+		exitWithUsage("Invalid number of arguments.")
+	}
+
+	startPort := parsePort(os.Args[1])
+	endPort := startPort
+	if argCount == 3 {
+		endPort = parsePort(os.Args[2])
+	}
+
+	runBlocker(startPort, endPort)
+}
+
+func parsePort(portString string) int {
+	result, err := strconv.Atoi(portString)
+	if err != nil {
+		exitWithUsage(fmt.Sprintf("Invalid port '%v'.", portString))
+	}
+
+	if (result < 1024) || (result > 65535) { // ports below 1024 are reserved.
+		exitWithUsage("Ports must be in the range [1024-65535].")
+	}
+
+	return result
+}
+
+func exitWithUsage(message string) {
+	fmt.Printf("Usage: port-blocker <startPort> [<endPort>]\n")
+	if len(message) > 0 {
+		fmt.Println(message)
+	}
+	os.Exit(1)
+}
+
+func runBlocker(startPort, endPort int) {
+	if endPort < startPort {
+		exitWithUsage("startPort must be less than or equal to endPort.")
+	}
+
+	for port := startPort; port <= endPort; port++ {
+		listener, err := net.Listen("tcp", "127.0.0.1:"+strconv.Itoa(port))
+		if err != nil {
+			fmt.Printf("Port %v is already blocked. Skipping.\n", port)
+		} else {
+			//goland:noinspection GoDeferInLoop
+			defer func() {
+				_ = listener.Close()
+			}()
+		}
+	}
+
+	fmt.Println("Blocking requested ports. Press enter to exit.")
+	_, _ = fmt.Scanln()
+}