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/dns.go

@@ -0,0 +1,82 @@
+package settings
+
+import (
+	"fmt"
+	"net"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings/helpers"
+	"github.com/qdm12/gotree"
+)
+
+// DNS contains settings to configure DNS.
+type DNS struct {
+	// ServerAddress is the DNS server to use inside
+	// the Go program and for the system.
+	// It defaults to '127.0.0.1' to be used with the
+	// DoT server. It cannot be nil in the internal
+	// state.
+	ServerAddress net.IP
+	// KeepNameserver is true if the Docker DNS server
+	// found in /etc/resolv.conf should be kept.
+	// Note settings this to true will go around the
+	// DoT server blocking.
+	// It defaults to false and cannot be nil in the
+	// internal state.
+	KeepNameserver *bool
+	// DOT contains settings to configure the DoT
+	// server.
+	DoT DoT
+}
+
+func (d DNS) validate() (err error) {
+	err = d.DoT.validate()
+	if err != nil {
+		return fmt.Errorf("failed validating DoT settings: %w", err)
+	}
+
+	return nil
+}
+
+func (d *DNS) Copy() (copied DNS) {
+	return DNS{
+		ServerAddress:  helpers.CopyIP(d.ServerAddress),
+		KeepNameserver: helpers.CopyBoolPtr(d.KeepNameserver),
+		DoT:            d.DoT.copy(),
+	}
+}
+
+// mergeWith merges the other settings into any
+// unset field of the receiver settings object.
+func (d *DNS) mergeWith(other DNS) {
+	d.ServerAddress = helpers.MergeWithIP(d.ServerAddress, other.ServerAddress)
+	d.KeepNameserver = helpers.MergeWithBool(d.KeepNameserver, other.KeepNameserver)
+	d.DoT.mergeWith(other.DoT)
+}
+
+// overrideWith overrides fields of the receiver
+// settings object with any field set in the other
+// settings.
+func (d *DNS) overrideWith(other DNS) {
+	d.ServerAddress = helpers.OverrideWithIP(d.ServerAddress, other.ServerAddress)
+	d.KeepNameserver = helpers.OverrideWithBool(d.KeepNameserver, other.KeepNameserver)
+	d.DoT.overrideWith(other.DoT)
+}
+
+func (d *DNS) setDefaults() {
+	localhost := net.IPv4(127, 0, 0, 1) //nolint:gomnd
+	d.ServerAddress = helpers.DefaultIP(d.ServerAddress, localhost)
+	d.KeepNameserver = helpers.DefaultBool(d.KeepNameserver, false)
+	d.DoT.setDefaults()
+}
+
+func (d DNS) String() string {
+	return d.toLinesNode().String()
+}
+
+func (d DNS) toLinesNode() (node *gotree.Node) {
+	node = gotree.New("DNS settings:")
+	node.Appendf("DNS server address to use: %s", d.ServerAddress)
+	node.Appendf("Keep existing nameserver(s): %s", helpers.BoolPtrToYesNo(d.KeepNameserver))
+	node.AppendNode(d.DoT.toLinesNode())
+	return node
+}