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/sources/env/helpers.go

@@ -0,0 +1,134 @@
+package env
+
+import (
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/qdm12/govalid/binary"
+	"github.com/qdm12/govalid/integer"
+)
+
+func envToCSV(envKey string) (values []string) {
+	csv := os.Getenv(envKey)
+	if csv == "" {
+		return nil
+	}
+	return lowerAndSplit(csv)
+}
+
+func envToStringPtr(envKey string) (stringPtr *string) {
+	s := os.Getenv(envKey)
+	if s == "" {
+		return nil
+	}
+	return &s
+}
+
+func envToBoolPtr(envKey string) (boolPtr *bool, err error) {
+	s := os.Getenv(envKey)
+	if s == "" {
+		return nil, nil //nolint:nilnil
+	}
+	value, err := binary.Validate(s)
+	if err != nil {
+		return nil, err
+	}
+	return &value, nil
+}
+
+func envToIntPtr(envKey string) (intPtr *int, err error) {
+	s := os.Getenv(envKey)
+	if s == "" {
+		return nil, nil //nolint:nilnil
+	}
+	value, err := strconv.Atoi(s)
+	if err != nil {
+		return nil, err
+	}
+	return &value, nil
+}
+
+func envToUint8Ptr(envKey string) (uint8Ptr *uint8, err error) {
+	s := os.Getenv(envKey)
+	if s == "" {
+		return nil, nil //nolint:nilnil
+	}
+
+	const min, max = 0, 255
+	value, err := integer.Validate(s, integer.OptionRange(min, max))
+	if err != nil {
+		return nil, err
+	}
+
+	uint8Ptr = new(uint8)
+	*uint8Ptr = uint8(value)
+	return uint8Ptr, nil
+}
+
+func envToUint16Ptr(envKey string) (uint16Ptr *uint16, err error) {
+	s := os.Getenv(envKey)
+	if s == "" {
+		return nil, nil //nolint:nilnil
+	}
+
+	const min, max = 0, 65535
+	value, err := integer.Validate(s, integer.OptionRange(min, max))
+	if err != nil {
+		return nil, err
+	}
+
+	uint16Ptr = new(uint16)
+	*uint16Ptr = uint16(value)
+	return uint16Ptr, nil
+}
+
+func envToDurationPtr(envKey string) (durationPtr *time.Duration, err error) {
+	s := os.Getenv(envKey)
+	if s == "" {
+		return nil, nil //nolint:nilnil
+	}
+
+	durationPtr = new(time.Duration)
+	*durationPtr, err = time.ParseDuration(s)
+	if err != nil {
+		return nil, err
+	}
+
+	return durationPtr, nil
+}
+
+func lowerAndSplit(csv string) (values []string) {
+	csv = strings.ToLower(csv)
+	return strings.Split(csv, ",")
+}
+
+var ErrDecodeBase64 = errors.New("cannot decode base64 string")
+
+func decodeBase64(b64String string) (decoded string, err error) {
+	b, err := base64.StdEncoding.DecodeString(b64String)
+	if err != nil {
+		return "", fmt.Errorf("%w: %s: %s",
+			ErrDecodeBase64, b64String, err)
+	}
+	return string(b), nil
+}
+
+func unsetEnvKeys(envKeys []string, err error) (newErr error) {
+	newErr = err
+	for _, envKey := range envKeys {
+		unsetErr := os.Unsetenv(envKey)
+		if unsetErr != nil && newErr == nil {
+			newErr = fmt.Errorf("cannot unset environment variable %s: %w", envKey, unsetErr)
+		}
+	}
+	return newErr
+}
+
+func stringPtr(s string) *string { return &s }
+func uint16Ptr(n uint16) *uint16 { return &n }
+func boolPtr(b bool) *bool       { return &b }