We build too many walls and not enough bridges

pkg/ports/ports.go

@@ -0,0 +1,65 @@
+// Copyright (c) 2020 Proton Technologies AG
+//
+// This file is part of ProtonMail Bridge.
+//
+// ProtonMail 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.
+//
+// ProtonMail 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 ProtonMail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package ports
+
+import (
+	"net"
+	"strconv"
+)
+
+const (
+	maxPortNumber = 65535
+)
+
+// IsPortFree checks if the port is free to use.
+func IsPortFree(port int) bool {
+	if !(0 < port && port < maxPortNumber) {
+		return false
+	}
+	stringPort := ":" + strconv.Itoa(port)
+	isFree := !isOccupied(stringPort)
+	return isFree
+}
+
+func isOccupied(port string) bool {
+	// Try to create server at port.
+	dummyserver, err := net.Listen("tcp", port)
+	if err != nil {
+		return true
+	}
+	_ = dummyserver.Close()
+	return false
+}
+
+// FindFreePortFrom finds first empty port, starting with `startPort`.
+func FindFreePortFrom(startPort int) int {
+	loopedOnce := false
+	freePort := startPort
+	for !IsPortFree(freePort) {
+		freePort++
+		if freePort >= maxPortNumber {
+			freePort = 1
+			if loopedOnce {
+				freePort = startPort
+				break
+			}
+			loopedOnce = true
+		}
+	}
+	return freePort
+}