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/provider.go

@@ -0,0 +1,92 @@
+package settings
+
+import (
+	"fmt"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings/helpers"
+	"github.com/qdm12/gluetun/internal/constants"
+	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gotree"
+)
+
+// Provider contains settings specific to a VPN provider.
+type Provider struct {
+	// Name is the VPN service provider name.
+	// It cannot be nil in the internal state.
+	Name *string
+	// ServerSelection is the settings to
+	// select the VPN server.
+	ServerSelection ServerSelection
+	// PortForwarding is the settings about port forwarding.
+	PortForwarding PortForwarding
+}
+
+func (p Provider) validate(vpnType string, allServers models.AllServers) (err error) {
+	// Validate Name
+	var validNames []string
+	if vpnType == constants.OpenVPN {
+		validNames = constants.AllProviders()
+		validNames = append(validNames, "pia") // Retro-compatibility
+	} else { // Wireguard
+		validNames = []string{
+			constants.Custom,
+			constants.Ivpn,
+			constants.Mullvad,
+			constants.Windscribe,
+		}
+	}
+	if !helpers.IsOneOf(*p.Name, validNames...) {
+		return fmt.Errorf("%w: %q can only be one of %s",
+			ErrVPNProviderNameNotValid, *p.Name, helpers.ChoicesOrString(validNames))
+	}
+
+	err = p.ServerSelection.validate(*p.Name, allServers)
+	if err != nil {
+		return fmt.Errorf("server selection settings validation failed: %w", err)
+	}
+
+	err = p.PortForwarding.validate(*p.Name)
+	if err != nil {
+		return fmt.Errorf("port forwarding settings validation failed: %w", err)
+	}
+
+	return nil
+}
+
+func (p *Provider) copy() (copied Provider) {
+	return Provider{
+		Name:            helpers.CopyStringPtr(p.Name),
+		ServerSelection: p.ServerSelection.copy(),
+		PortForwarding:  p.PortForwarding.copy(),
+	}
+}
+
+func (p *Provider) mergeWith(other Provider) {
+	p.Name = helpers.MergeWithStringPtr(p.Name, other.Name)
+	p.ServerSelection.mergeWith(other.ServerSelection)
+	p.PortForwarding.mergeWith(other.PortForwarding)
+}
+
+func (p *Provider) overrideWith(other Provider) {
+	p.Name = helpers.OverrideWithStringPtr(p.Name, other.Name)
+	p.ServerSelection.overrideWith(other.ServerSelection)
+	p.PortForwarding.overrideWith(other.PortForwarding)
+}
+
+func (p *Provider) setDefaults() {
+	p.Name = helpers.DefaultStringPtr(p.Name, constants.PrivateInternetAccess)
+	p.ServerSelection.setDefaults(*p.Name)
+	p.PortForwarding.setDefaults()
+}
+
+func (p Provider) String() string {
+	return p.toLinesNode().String()
+}
+
+func (p Provider) toLinesNode() (node *gotree.Node) {
+	node = gotree.New("VPN provider settings:")
+	node.Appendf("Name: %s", *p.Name)
+	node.AppendNode(p.ServerSelection.toLinesNode())
+	node.AppendNode(p.PortForwarding.toLinesNode())
+	return node
+}