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/updater.go

@@ -0,0 +1,113 @@
+package settings
+
+import (
+	"fmt"
+	"net"
+	"strings"
+	"time"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings/helpers"
+	"github.com/qdm12/gluetun/internal/constants"
+	"github.com/qdm12/gotree"
+)
+
+// Updater contains settings to configure the VPN
+// server information updater.
+type Updater struct {
+	// Period is the period for which the updater
+	// should run. It can be set to 0 to disable the
+	// updater. It cannot be nil in the internal state.
+	// TODO change to value and add Enabled field.
+	Period *time.Duration
+	// DNSAddress is the DNS server address to use
+	// to resolve VPN server hostnames to IP addresses.
+	// It cannot be nil in the internal state.
+	DNSAddress net.IP
+	// Providers is the list of VPN service providers
+	// to update server information for.
+	Providers []string
+	// CLI is to precise the updater is running in CLI
+	// mode. This is set automatically and cannot be set
+	// by settings sources. It cannot be nil in the
+	// internal state.
+	CLI *bool
+}
+
+func (u Updater) Validate() (err error) {
+	const minPeriod = time.Minute
+	if *u.Period > 0 && *u.Period < minPeriod {
+		return fmt.Errorf("%w: %d must be larger than %s",
+			ErrUpdaterPeriodTooSmall, *u.Period, minPeriod)
+	}
+
+	for i, provider := range u.Providers {
+		valid := false
+		for _, validProvider := range constants.AllProviders() {
+			if provider == validProvider {
+				valid = true
+				break
+			}
+		}
+		if !valid {
+			return fmt.Errorf("%w: %s at index %d",
+				ErrVPNProviderNameNotValid, provider, i)
+		}
+	}
+
+	return nil
+}
+
+func (u *Updater) copy() (copied Updater) {
+	return Updater{
+		Period:     helpers.CopyDurationPtr(u.Period),
+		DNSAddress: helpers.CopyIP(u.DNSAddress),
+		Providers:  helpers.CopyStringSlice(u.Providers),
+		CLI:        u.CLI,
+	}
+}
+
+// mergeWith merges the other settings into any
+// unset field of the receiver settings object.
+func (u *Updater) mergeWith(other Updater) {
+	u.Period = helpers.MergeWithDuration(u.Period, other.Period)
+	u.DNSAddress = helpers.MergeWithIP(u.DNSAddress, other.DNSAddress)
+	u.Providers = helpers.MergeStringSlices(u.Providers, other.Providers)
+	u.CLI = helpers.MergeWithBool(u.CLI, other.CLI)
+}
+
+// overrideWith overrides fields of the receiver
+// settings object with any field set in the other
+// settings.
+func (u *Updater) overrideWith(other Updater) {
+	u.Period = helpers.OverrideWithDuration(u.Period, other.Period)
+	u.DNSAddress = helpers.OverrideWithIP(u.DNSAddress, other.DNSAddress)
+	u.Providers = helpers.OverrideWithStringSlice(u.Providers, other.Providers)
+	u.CLI = helpers.MergeWithBool(u.CLI, other.CLI)
+}
+
+func (u *Updater) setDefaults() {
+	u.Period = helpers.DefaultDuration(u.Period, 0)
+	u.DNSAddress = helpers.DefaultIP(u.DNSAddress, net.IPv4(1, 1, 1, 1))
+	u.CLI = helpers.DefaultBool(u.CLI, false)
+}
+
+func (u Updater) String() string {
+	return u.toLinesNode().String()
+}
+
+func (u Updater) toLinesNode() (node *gotree.Node) {
+	if *u.Period == 0 {
+		return nil
+	}
+
+	node = gotree.New("Server data updater settings:")
+	node.Appendf("Update period: %s", *u.Period)
+	node.Appendf("DNS address: %s", u.DNSAddress)
+	node.Appendf("Providers to update: %s", strings.Join(u.Providers, ", "))
+
+	if *u.CLI {
+		node.Appendf("CLI mode: enabled")
+	}
+
+	return node
+}