all: Remove lib/util package (#9049)

Grab-bag packages are nasty, this cleans it up a little by splitting it
into topical packages sempahore, netutil, stringutil, structutil.

lib/structutil/structutil.go

@@ -0,0 +1,170 @@
+// Copyright (C) 2023 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package structutil
+
+import (
+	"reflect"
+	"strconv"
+	"strings"
+)
+
+type defaultParser interface {
+	ParseDefault(string) error
+}
+
+// SetDefaults sets default values on a struct, based on the default annotation.
+func SetDefaults(data any) {
+	s := reflect.ValueOf(data).Elem()
+	t := s.Type()
+
+	for i := 0; i < s.NumField(); i++ {
+		f := s.Field(i)
+		tag := t.Field(i).Tag
+
+		v := tag.Get("default")
+		if len(v) > 0 {
+			if f.CanInterface() {
+				if parser, ok := f.Interface().(defaultParser); ok {
+					if err := parser.ParseDefault(v); err != nil {
+						panic(err)
+					}
+					continue
+				}
+			}
+
+			if f.CanAddr() && f.Addr().CanInterface() {
+				if parser, ok := f.Addr().Interface().(defaultParser); ok {
+					if err := parser.ParseDefault(v); err != nil {
+						panic(err)
+					}
+					continue
+				}
+			}
+
+			switch f.Interface().(type) {
+			case string:
+				f.SetString(v)
+
+			case int, uint32, int32, int64, uint64:
+				i, err := strconv.ParseInt(v, 10, 64)
+				if err != nil {
+					panic(err)
+				}
+				f.SetInt(i)
+
+			case float64, float32:
+				i, err := strconv.ParseFloat(v, 64)
+				if err != nil {
+					panic(err)
+				}
+				f.SetFloat(i)
+
+			case bool:
+				f.SetBool(v == "true")
+
+			case []string:
+				// We don't do anything with string slices here. Any default
+				// we set will be appended to by the XML decoder, so we fill
+				// those after decoding.
+
+			default:
+				panic(f.Type())
+			}
+		} else if f.CanSet() && f.Kind() == reflect.Struct && f.CanAddr() {
+			if addr := f.Addr(); addr.CanInterface() {
+				SetDefaults(addr.Interface())
+			}
+		}
+	}
+}
+
+func FillNilExceptDeprecated(data any) {
+	fillNil(data, true)
+}
+
+func FillNil(data any) {
+	fillNil(data, false)
+}
+
+func fillNil(data any, skipDeprecated bool) {
+	s := reflect.ValueOf(data).Elem()
+	t := s.Type()
+	for i := 0; i < s.NumField(); i++ {
+		if skipDeprecated && strings.HasPrefix(t.Field(i).Name, "Deprecated") {
+			continue
+		}
+
+		f := s.Field(i)
+
+		for f.Kind() == reflect.Ptr && f.IsZero() && f.CanSet() {
+			newValue := reflect.New(f.Type().Elem())
+			f.Set(newValue)
+			f = f.Elem()
+		}
+
+		if f.CanSet() {
+			if f.IsZero() {
+				switch f.Kind() {
+				case reflect.Map:
+					f.Set(reflect.MakeMap(f.Type()))
+				case reflect.Slice:
+					f.Set(reflect.MakeSlice(f.Type(), 0, 0))
+				case reflect.Chan:
+					f.Set(reflect.MakeChan(f.Type(), 0))
+				}
+			}
+
+			switch f.Kind() {
+			case reflect.Slice:
+				if f.Type().Elem().Kind() != reflect.Struct {
+					continue
+				}
+				for i := 0; i < f.Len(); i++ {
+					fillNil(f.Index(i).Addr().Interface(), skipDeprecated)
+				}
+			case reflect.Struct:
+				if f.CanAddr() {
+					if addr := f.Addr(); addr.CanInterface() {
+						fillNil(addr.Interface(), skipDeprecated)
+					}
+				}
+			}
+		}
+	}
+}
+
+// FillNilSlices sets default value on slices that are still nil.
+func FillNilSlices(data any) error {
+	s := reflect.ValueOf(data).Elem()
+	t := s.Type()
+
+	for i := 0; i < s.NumField(); i++ {
+		f := s.Field(i)
+		tag := t.Field(i).Tag
+
+		v := tag.Get("default")
+		if len(v) > 0 {
+			switch f.Interface().(type) {
+			case []string:
+				if f.IsNil() {
+					// Treat the default as a comma separated slice
+					vs := strings.Split(v, ",")
+					for i := range vs {
+						vs[i] = strings.TrimSpace(vs[i])
+					}
+
+					rv := reflect.MakeSlice(reflect.TypeOf([]string{}), len(vs), len(vs))
+					for i, v := range vs {
+						rv.Index(i).SetString(v)
+					}
+					f.Set(rv)
+				}
+			}
+		}
+	}
+	return nil
+}