feat(是否存在): 新增分区和配置项存在性判断

internal.go

@@ -1,6 +1,8 @@
 package goini
 
 import (
+	"errors"
+	"os"
 	"reflect"
 	"regexp"
 	"strconv"
@@ -143,6 +145,42 @@ func hasSurroundingQuotes(value string) bool {
 	return first == last && (first == '"' || first == '\'' || first == '`')
 }
 
+func (ini *GoINI) save(data string) error {
+	if ini.filename == "" {
+		return errFilenameRequired
+	}
+	info, err := os.Stat(ini.filename)
+	if err != nil {
+		return err
+	}
+	return ini.saveFile(ini.filename, data, info.Mode().Perm())
+}
+
+func (ini *GoINI) saveFile(filename string, data string, perm os.FileMode) error {
+	if filename == "" {
+		return errFilenameRequired
+	}
+	perm = perm.Perm()
+	writePerm := perm | 0o200
+	if info, err := os.Stat(filename); err == nil {
+		if info.Mode().IsRegular() {
+			if err := os.Chmod(filename, writePerm); err != nil {
+				return err
+			}
+		}
+	} else if !errors.Is(err, os.ErrNotExist) {
+		return err
+	}
+	if err := os.WriteFile(filename, []byte(data), writePerm); err != nil {
+		return err
+	}
+	if err := os.Chmod(filename, perm); err != nil {
+		return err
+	}
+	ini.filename = filename
+	return nil
+}
+
 func (ini *GoINI) rebuildIndexes() {
 	ini.sectionKeys = make(map[string][]string)
 	ini.sectionOrder = nil