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/helpers/merge.go

@@ -0,0 +1,226 @@
+package helpers
+
+import (
+	"net"
+	"time"
+
+	"github.com/qdm12/golibs/logging"
+	"inet.af/netaddr"
+)
+
+func MergeWithBool(existing, other *bool) (result *bool) {
+	if existing != nil {
+		return existing
+	} else if other == nil {
+		return nil
+	}
+	result = new(bool)
+	*result = *other
+	return result
+}
+
+func MergeWithString(existing, other string) (result string) {
+	if existing != "" {
+		return existing
+	}
+	return other
+}
+
+func MergeWithStringPtr(existing, other *string) (result *string) {
+	if existing != nil {
+		return existing
+	} else if other == nil {
+		return nil
+	}
+	result = new(string)
+	*result = *other
+	return result
+}
+
+func MergeWithInt(existing, other *int) (result *int) {
+	if existing != nil {
+		return existing
+	} else if other == nil {
+		return nil
+	}
+	result = new(int)
+	*result = *other
+	return result
+}
+
+func MergeWithUint8(existing, other *uint8) (result *uint8) {
+	if existing != nil {
+		return existing
+	} else if other == nil {
+		return nil
+	}
+	result = new(uint8)
+	*result = *other
+	return result
+}
+
+func MergeWithUint16(existing, other *uint16) (result *uint16) {
+	if existing != nil {
+		return existing
+	} else if other == nil {
+		return nil
+	}
+	result = new(uint16)
+	*result = *other
+	return result
+}
+
+func MergeWithIP(existing, other net.IP) (result net.IP) {
+	if existing != nil {
+		return existing
+	} else if other == nil {
+		return nil
+	}
+	result = make(net.IP, len(other))
+	copy(result, other)
+	return result
+}
+
+func MergeWithDuration(existing, other *time.Duration) (result *time.Duration) {
+	if existing != nil {
+		return existing
+	}
+	return other
+}
+
+func MergeWithLogLevel(existing, other *logging.Level) (result *logging.Level) {
+	if existing != nil {
+		return existing
+	} else if other == nil {
+		return nil
+	}
+	result = new(logging.Level)
+	*result = *other
+	return result
+}
+
+func MergeStringSlices(a, b []string) (result []string) {
+	if a == nil && b == nil {
+		return nil
+	}
+
+	seen := make(map[string]struct{}, len(a)+len(b))
+	result = make([]string, 0, len(a)+len(b))
+	for _, s := range a {
+		if _, ok := seen[s]; ok {
+			continue // duplicate
+		}
+		result = append(result, s)
+		seen[s] = struct{}{}
+	}
+	for _, s := range b {
+		if _, ok := seen[s]; ok {
+			continue // duplicate
+		}
+		result = append(result, s)
+		seen[s] = struct{}{}
+	}
+	return result
+}
+
+func MergeUint16Slices(a, b []uint16) (result []uint16) {
+	if a == nil && b == nil {
+		return nil
+	}
+
+	seen := make(map[uint16]struct{}, len(a)+len(b))
+	result = make([]uint16, 0, len(a)+len(b))
+	for _, n := range a {
+		if _, ok := seen[n]; ok {
+			continue // duplicate
+		}
+		result = append(result, n)
+		seen[n] = struct{}{}
+	}
+	for _, n := range b {
+		if _, ok := seen[n]; ok {
+			continue // duplicate
+		}
+		result = append(result, n)
+		seen[n] = struct{}{}
+	}
+	return result
+}
+
+func MergeIPNetsSlices(a, b []net.IPNet) (result []net.IPNet) {
+	if a == nil && b == nil {
+		return nil
+	}
+
+	seen := make(map[string]struct{}, len(a)+len(b))
+	result = make([]net.IPNet, 0, len(a)+len(b))
+	for _, ipNet := range a {
+		key := ipNet.String()
+		if _, ok := seen[key]; ok {
+			continue // duplicate
+		}
+		result = append(result, ipNet)
+		seen[key] = struct{}{}
+	}
+	for _, ipNet := range b {
+		key := ipNet.String()
+		if _, ok := seen[key]; ok {
+			continue // duplicate
+		}
+		result = append(result, ipNet)
+		seen[key] = struct{}{}
+	}
+	return result
+}
+
+func MergeNetaddrIPsSlices(a, b []netaddr.IP) (result []netaddr.IP) {
+	if a == nil && b == nil {
+		return nil
+	}
+
+	seen := make(map[string]struct{}, len(a)+len(b))
+	result = make([]netaddr.IP, 0, len(a)+len(b))
+	for _, ip := range a {
+		key := ip.String()
+		if _, ok := seen[key]; ok {
+			continue // duplicate
+		}
+		result = append(result, ip)
+		seen[key] = struct{}{}
+	}
+	for _, ip := range b {
+		key := ip.String()
+		if _, ok := seen[key]; ok {
+			continue // duplicate
+		}
+		result = append(result, ip)
+		seen[key] = struct{}{}
+	}
+	return result
+}
+
+func MergeIPPrefixesSlices(a, b []netaddr.IPPrefix) (result []netaddr.IPPrefix) {
+	if a == nil && b == nil {
+		return nil
+	}
+
+	seen := make(map[string]struct{}, len(a)+len(b))
+	result = make([]netaddr.IPPrefix, 0, len(a)+len(b))
+	for _, ipPrefix := range a {
+		key := ipPrefix.String()
+		if _, ok := seen[key]; ok {
+			continue // duplicate
+		}
+		result = append(result, ipPrefix)
+		seen[key] = struct{}{}
+	}
+	for _, ipPrefix := range b {
+		key := ipPrefix.String()
+		if _, ok := seen[key]; ok {
+			continue // duplicate
+		}
+		result = append(result, ipPrefix)
+		seen[key] = struct{}{}
+	}
+	return result
+}