chore(settings): refactor settings processing (#756)

- Better settings tree structure logged using `qdm12/gotree`
- Read settings from environment variables, then files, then secret files
- Settings methods to default them, merge them and override them
- `DNS_PLAINTEXT_ADDRESS` default changed to `127.0.0.1` to use DoT. Warning added if set to something else.
- `HTTPPROXY_LISTENING_ADDRESS` instead of `HTTPPROXY_PORT` (with retro-compatibility)

internal/configuration/sources/env/serverselection.go

@@ -0,0 +1,115 @@
+package env
+
+import (
+	"errors"
+	"fmt"
+	"net"
+	"os"
+	"strconv"
+	"strings"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+	"github.com/qdm12/gluetun/internal/constants"
+)
+
+var (
+	ErrServerNumberNotValid = errors.New("server number is not valid")
+)
+
+func (r *Reader) readServerSelection(vpnProvider, vpnType string) (
+	ss settings.ServerSelection, err error) {
+	ss.VPN = vpnType
+
+	ss.TargetIP, err = readOpenVPNTargetIP()
+	if err != nil {
+		return ss, err
+	}
+
+	countriesCSV := os.Getenv("COUNTRY")
+	if vpnProvider == constants.Cyberghost && countriesCSV == "" {
+		// Retro-compatibility
+		r.onRetroActive("REGION", "COUNTRY")
+		countriesCSV = os.Getenv("REGION")
+	}
+	if countriesCSV != "" {
+		ss.Countries = lowerAndSplit(countriesCSV)
+	}
+
+	ss.Regions = envToCSV("REGION")
+	ss.Cities = envToCSV("CITY")
+	ss.ISPs = envToCSV("ISP")
+	ss.Hostnames = envToCSV("SERVER_HOSTNAME")
+	ss.Names = envToCSV("SERVER_NAME")
+
+	if csv := os.Getenv("SERVER_NUMBER"); csv != "" {
+		numbersStrings := strings.Split(csv, ",")
+		numbers := make([]uint16, len(numbersStrings))
+		for i, numberString := range numbersStrings {
+			number, err := strconv.Atoi(numberString)
+			if err != nil {
+				return ss, fmt.Errorf("%w: %s",
+					ErrServerNumberNotValid, numberString)
+			} else if number < 0 || number > 2^16 {
+				return ss, fmt.Errorf("%w: %d must be between 0 and 2^16",
+					ErrServerNumberNotValid, number)
+			}
+			numbers[i] = uint16(number)
+		}
+		ss.Numbers = numbers
+	}
+
+	// Mullvad only
+	ss.OwnedOnly, err = envToBoolPtr("OWNED")
+	if err != nil {
+		return ss, fmt.Errorf("environment variable OWNED: %w", err)
+	}
+
+	// VPNUnlimited only
+	ss.FreeOnly, err = envToBoolPtr("FREE_ONLY")
+	if err != nil {
+		return ss, fmt.Errorf("environment variable FREE_ONLY: %w", err)
+	}
+
+	// VPNUnlimited only
+	ss.MultiHopOnly, err = envToBoolPtr("MULTIHOP_ONLY")
+	if err != nil {
+		return ss, fmt.Errorf("environment variable MULTIHOP_ONLY: %w", err)
+	}
+
+	// VPNUnlimited only
+	ss.MultiHopOnly, err = envToBoolPtr("STREAM_ONLY")
+	if err != nil {
+		return ss, fmt.Errorf("environment variable STREAM_ONLY: %w", err)
+	}
+
+	ss.OpenVPN, err = r.readOpenVPNSelection()
+	if err != nil {
+		return ss, err
+	}
+
+	ss.Wireguard, err = r.readWireguardSelection()
+	if err != nil {
+		return ss, err
+	}
+
+	return ss, nil
+}
+
+var (
+	ErrInvalidIP = errors.New("invalid IP address")
+)
+
+func readOpenVPNTargetIP() (ip net.IP, err error) {
+	s := os.Getenv("OPENVPN_TARGET_IP")
+	if s == "" {
+		return nil, nil
+	}
+
+	ip = net.ParseIP(s)
+	if ip == nil {
+		return nil, fmt.Errorf("environment variable OPENVPN_TARGET_IP: %w: %s",
+			ErrInvalidIP, s)
+	}
+
+	return ip, nil
+}