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/health.go

@@ -0,0 +1,83 @@
+package settings
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings/helpers"
+	"github.com/qdm12/gotree"
+	"github.com/qdm12/govalid/address"
+)
+
+// Health contains settings for the healthcheck and health server.
+type Health struct {
+	// ServerAddress is the listening address
+	// for the health check server.
+	// It cannot be the empty string in the internal state.
+	ServerAddress string
+	// AddressToPing is the IP address or domain name to
+	// ping periodically for the health check.
+	// It cannot be the empty string in the internal state.
+	AddressToPing string
+	VPN           HealthyWait
+}
+
+func (h Health) Validate() (err error) {
+	uid := os.Getuid()
+	_, err = address.Validate(h.ServerAddress,
+		address.OptionListening(uid))
+	if err != nil {
+		return fmt.Errorf("%w: %s",
+			ErrServerAddressNotValid, err)
+	}
+
+	err = h.VPN.validate()
+	if err != nil {
+		return fmt.Errorf("health VPN settings validation failed: %w", err)
+	}
+
+	return nil
+}
+
+func (h *Health) copy() (copied Health) {
+	return Health{
+		ServerAddress: h.ServerAddress,
+		AddressToPing: h.AddressToPing,
+		VPN:           h.VPN.copy(),
+	}
+}
+
+// MergeWith merges the other settings into any
+// unset field of the receiver settings object.
+func (h *Health) MergeWith(other Health) {
+	h.ServerAddress = helpers.MergeWithString(h.ServerAddress, other.ServerAddress)
+	h.AddressToPing = helpers.MergeWithString(h.AddressToPing, other.AddressToPing)
+	h.VPN.mergeWith(other.VPN)
+}
+
+// OverrideWith overrides fields of the receiver
+// settings object with any field set in the other
+// settings.
+func (h *Health) OverrideWith(other Health) {
+	h.ServerAddress = helpers.OverrideWithString(h.ServerAddress, other.ServerAddress)
+	h.AddressToPing = helpers.OverrideWithString(h.AddressToPing, other.AddressToPing)
+	h.VPN.overrideWith(other.VPN)
+}
+
+func (h *Health) SetDefaults() {
+	h.ServerAddress = helpers.DefaultString(h.ServerAddress, "127.0.0.1:9999")
+	h.AddressToPing = helpers.DefaultString(h.AddressToPing, "github.com")
+	h.VPN.setDefaults()
+}
+
+func (h Health) String() string {
+	return h.toLinesNode().String()
+}
+
+func (h Health) toLinesNode() (node *gotree.Node) {
+	node = gotree.New("Health settings:")
+	node.Appendf("Server listening address: %s", h.ServerAddress)
+	node.Appendf("Address to ping: %s", h.AddressToPing)
+	node.AppendNode(h.VPN.toLinesNode("VPN"))
+	return node
+}