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/firewall.go

@@ -0,0 +1,117 @@
+package settings
+
+import (
+	"fmt"
+	"net"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings/helpers"
+	"github.com/qdm12/gotree"
+)
+
+// Firewall contains settings to customize the firewall operation.
+type Firewall struct {
+	VPNInputPorts   []uint16
+	InputPorts      []uint16
+	OutboundSubnets []net.IPNet
+	Enabled         *bool
+	Debug           *bool
+}
+
+func (f Firewall) validate() (err error) {
+	if hasZeroPort(f.VPNInputPorts) {
+		return fmt.Errorf("VPN input ports: %w", ErrFirewallZeroPort)
+	}
+
+	if hasZeroPort(f.InputPorts) {
+		return fmt.Errorf("input ports: %w", ErrFirewallZeroPort)
+	}
+
+	return nil
+}
+
+func hasZeroPort(ports []uint16) (has bool) {
+	for _, port := range ports {
+		if port == 0 {
+			return true
+		}
+	}
+	return false
+}
+
+func (f *Firewall) copy() (copied Firewall) {
+	return Firewall{
+		VPNInputPorts:   helpers.CopyUint16Slice(f.VPNInputPorts),
+		InputPorts:      helpers.CopyUint16Slice(f.InputPorts),
+		OutboundSubnets: helpers.CopyIPNetSlice(f.OutboundSubnets),
+		Enabled:         helpers.CopyBoolPtr(f.Enabled),
+		Debug:           helpers.CopyBoolPtr(f.Debug),
+	}
+}
+
+// mergeWith merges the other settings into any
+// unset field of the receiver settings object.
+// It merges values of slices together, even if they
+// are set in the receiver settings.
+func (f *Firewall) mergeWith(other Firewall) {
+	f.VPNInputPorts = helpers.MergeUint16Slices(f.VPNInputPorts, other.VPNInputPorts)
+	f.InputPorts = helpers.MergeUint16Slices(f.InputPorts, other.InputPorts)
+	f.OutboundSubnets = helpers.MergeIPNetsSlices(f.OutboundSubnets, other.OutboundSubnets)
+	f.Enabled = helpers.MergeWithBool(f.Enabled, other.Enabled)
+	f.Debug = helpers.MergeWithBool(f.Debug, other.Debug)
+}
+
+// overrideWith overrides fields of the receiver
+// settings object with any field set in the other
+// settings.
+func (f *Firewall) overrideWith(other Firewall) {
+	f.VPNInputPorts = helpers.OverrideWithUint16Slice(f.VPNInputPorts, other.VPNInputPorts)
+	f.InputPorts = helpers.OverrideWithUint16Slice(f.InputPorts, other.InputPorts)
+	f.OutboundSubnets = helpers.OverrideWithIPNetsSlice(f.OutboundSubnets, other.OutboundSubnets)
+	f.Enabled = helpers.OverrideWithBool(f.Enabled, other.Enabled)
+	f.Debug = helpers.OverrideWithBool(f.Debug, other.Debug)
+}
+
+func (f *Firewall) setDefaults() {
+	f.Enabled = helpers.DefaultBool(f.Enabled, true)
+	f.Debug = helpers.DefaultBool(f.Debug, false)
+}
+
+func (f Firewall) String() string {
+	return f.toLinesNode().String()
+}
+
+func (f Firewall) toLinesNode() (node *gotree.Node) {
+	node = gotree.New("Firewall settings:")
+
+	node.Appendf("Enabled: %s", helpers.BoolPtrToYesNo(f.Enabled))
+	if !*f.Enabled {
+		return node
+	}
+
+	if *f.Debug {
+		node.Appendf("Debug mode: on")
+	}
+
+	if len(f.VPNInputPorts) > 0 {
+		vpnInputPortsNode := node.Appendf("VPN input ports:")
+		for _, port := range f.VPNInputPorts {
+			vpnInputPortsNode.Appendf("%d", port)
+		}
+	}
+
+	if len(f.InputPorts) > 0 {
+		inputPortsNode := node.Appendf("Input ports:")
+		for _, port := range f.InputPorts {
+			inputPortsNode.Appendf("%d", port)
+		}
+	}
+
+	if len(f.OutboundSubnets) > 0 {
+		outboundSubnets := node.Appendf("Outbound subnets:")
+		for _, subnet := range f.OutboundSubnets {
+			outboundSubnets.Appendf("%s", subnet)
+		}
+	}
+
+	return node
+}