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/settings/settings.go

@@ -0,0 +1,144 @@
+package settings
+
+import (
+	"fmt"
+
+	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gotree"
+)
+
+type Settings struct {
+	ControlServer ControlServer
+	DNS           DNS
+	Firewall      Firewall
+	Health        Health
+	HTTPProxy     HTTPProxy
+	Log           Log
+	PublicIP      PublicIP
+	Shadowsocks   Shadowsocks
+	System        System
+	Updater       Updater
+	Version       Version
+	VPN           VPN
+}
+
+func (s Settings) Validate(allServers models.AllServers) (err error) {
+	nameToValidation := map[string]func() error{
+		"control server":  s.ControlServer.validate,
+		"dns":             s.DNS.validate,
+		"firewall":        s.Firewall.validate,
+		"health":          s.Health.Validate,
+		"http proxy":      s.HTTPProxy.validate,
+		"log":             s.Log.validate,
+		"public ip check": s.PublicIP.validate,
+		"shadowsocks":     s.Shadowsocks.validate,
+		"system":          s.System.validate,
+		"updater":         s.Updater.Validate,
+		"version":         s.Version.validate,
+		"VPN": func() error {
+			return s.VPN.validate(allServers)
+		},
+	}
+
+	for name, validation := range nameToValidation {
+		err = validation()
+		if err != nil {
+			return fmt.Errorf("failed validating %s settings: %w", name, err)
+		}
+	}
+
+	return nil
+}
+
+func (s *Settings) copy() (copied Settings) {
+	return Settings{
+		ControlServer: s.ControlServer.copy(),
+		DNS:           s.DNS.Copy(),
+		Firewall:      s.Firewall.copy(),
+		Health:        s.Health.copy(),
+		HTTPProxy:     s.HTTPProxy.copy(),
+		Log:           s.Log.copy(),
+		PublicIP:      s.PublicIP.copy(),
+		Shadowsocks:   s.Shadowsocks.copy(),
+		System:        s.System.copy(),
+		Updater:       s.Updater.copy(),
+		Version:       s.Version.copy(),
+		VPN:           s.VPN.copy(),
+	}
+}
+
+func (s *Settings) MergeWith(other Settings) {
+	s.ControlServer.mergeWith(other.ControlServer)
+	s.DNS.mergeWith(other.DNS)
+	s.Firewall.mergeWith(other.Firewall)
+	s.Health.MergeWith(other.Health)
+	s.HTTPProxy.mergeWith(other.HTTPProxy)
+	s.Log.mergeWith(other.Log)
+	s.PublicIP.mergeWith(other.PublicIP)
+	s.Shadowsocks.mergeWith(other.Shadowsocks)
+	s.System.mergeWith(other.System)
+	s.Updater.mergeWith(other.Updater)
+	s.Version.mergeWith(other.Version)
+	s.VPN.mergeWith(other.VPN)
+}
+
+func (s *Settings) OverrideWith(other Settings,
+	allServers models.AllServers) (err error) {
+	patchedSettings := s.copy()
+	patchedSettings.ControlServer.overrideWith(other.ControlServer)
+	patchedSettings.DNS.overrideWith(other.DNS)
+	patchedSettings.Firewall.overrideWith(other.Firewall)
+	patchedSettings.Health.OverrideWith(other.Health)
+	patchedSettings.HTTPProxy.overrideWith(other.HTTPProxy)
+	patchedSettings.Log.overrideWith(other.Log)
+	patchedSettings.PublicIP.overrideWith(other.PublicIP)
+	patchedSettings.Shadowsocks.overrideWith(other.Shadowsocks)
+	patchedSettings.System.overrideWith(other.System)
+	patchedSettings.Updater.overrideWith(other.Updater)
+	patchedSettings.Version.overrideWith(other.Version)
+	patchedSettings.VPN.overrideWith(other.VPN)
+	err = patchedSettings.Validate(allServers)
+	if err != nil {
+		return err
+	}
+	*s = patchedSettings
+	return nil
+}
+
+func (s *Settings) SetDefaults() {
+	s.ControlServer.setDefaults()
+	s.DNS.setDefaults()
+	s.Firewall.setDefaults()
+	s.Health.SetDefaults()
+	s.HTTPProxy.setDefaults()
+	s.Log.setDefaults()
+	s.PublicIP.setDefaults()
+	s.Shadowsocks.setDefaults()
+	s.System.setDefaults()
+	s.Updater.setDefaults()
+	s.Version.setDefaults()
+	s.VPN.setDefaults()
+}
+
+func (s Settings) String() string {
+	return s.toLinesNode().String()
+}
+
+func (s Settings) toLinesNode() (node *gotree.Node) {
+	node = gotree.New("Settings summary:")
+
+	node.AppendNode(s.VPN.toLinesNode())
+	node.AppendNode(s.DNS.toLinesNode())
+	node.AppendNode(s.Firewall.toLinesNode())
+	node.AppendNode(s.Log.toLinesNode())
+	node.AppendNode(s.Health.toLinesNode())
+	node.AppendNode(s.Shadowsocks.toLinesNode())
+	node.AppendNode(s.HTTPProxy.toLinesNode())
+	node.AppendNode(s.ControlServer.toLinesNode())
+	node.AppendNode(s.System.toLinesNode())
+	node.AppendNode(s.PublicIP.toLinesNode())
+	node.AppendNode(s.Updater.toLinesNode())
+	node.AppendNode(s.Version.toLinesNode())
+
+	return node
+}