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/portforward.go

@@ -0,0 +1,89 @@
+package settings
+
+import (
+	"fmt"
+	"path/filepath"
+	"strings"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings/helpers"
+	"github.com/qdm12/gluetun/internal/constants"
+	"github.com/qdm12/gotree"
+)
+
+// PortForwarding contains settings for port forwarding.
+type PortForwarding struct {
+	// Enabled is true if port forwarding should be activated.
+	// It cannot be nil for the internal state.
+	Enabled *bool
+	// Filepath is the port forwarding status file path
+	// to use. It can be the empty string to indicate not
+	// to write to a file. It cannot be nil for the
+	// internal state
+	Filepath *string
+}
+
+func (p PortForwarding) validate(vpnProvider string) (err error) {
+	if !*p.Enabled {
+		return nil
+	}
+
+	// Validate Enabled
+	validProviders := []string{constants.PrivateInternetAccess}
+	if !helpers.IsOneOf(vpnProvider, validProviders...) {
+		return fmt.Errorf("%w: for provider %s, it is only available for %s",
+			ErrPortForwardingEnabled, vpnProvider, strings.Join(validProviders, ", "))
+	}
+
+	// Validate Filepath
+	if *p.Filepath != "" { // optional
+		_, err := filepath.Abs(*p.Filepath)
+		if err != nil {
+			return fmt.Errorf("%w: %s", ErrPortForwardingFilepathNotValid, err)
+		}
+	}
+
+	return nil
+}
+
+func (p *PortForwarding) copy() (copied PortForwarding) {
+	return PortForwarding{
+		Enabled:  helpers.CopyBoolPtr(p.Enabled),
+		Filepath: helpers.CopyStringPtr(p.Filepath),
+	}
+}
+
+func (p *PortForwarding) mergeWith(other PortForwarding) {
+	p.Enabled = helpers.MergeWithBool(p.Enabled, other.Enabled)
+	p.Filepath = helpers.MergeWithStringPtr(p.Filepath, other.Filepath)
+}
+
+func (p *PortForwarding) overrideWith(other PortForwarding) {
+	p.Enabled = helpers.OverrideWithBool(p.Enabled, other.Enabled)
+	p.Filepath = helpers.OverrideWithStringPtr(p.Filepath, other.Filepath)
+}
+
+func (p *PortForwarding) setDefaults() {
+	p.Enabled = helpers.DefaultBool(p.Enabled, false)
+	p.Filepath = helpers.DefaultStringPtr(p.Filepath, "/tmp/gluetun/forwarded_port")
+}
+
+func (p PortForwarding) String() string {
+	return p.toLinesNode().String()
+}
+
+func (p PortForwarding) toLinesNode() (node *gotree.Node) {
+	if !*p.Enabled {
+		return nil
+	}
+
+	node = gotree.New("Automatic port forwarding settings:")
+	node.Appendf("Enabled: yes")
+
+	filepath := *p.Filepath
+	if filepath == "" {
+		filepath = "[not set]"
+	}
+	node.Appendf("Forwarded port file path: %s", filepath)
+
+	return node
+}