Geofiles: Implement mmap in filesystem to reduce ram usage (#5480)

For https://github.com/XTLS/Xray-core/issues/4422

common/platform/filesystem/file.go

@@ -1,9 +1,12 @@
+//go:build !windows && !wasm
+
 package filesystem
 
 import (
 	"io"
 	"os"
 	"path/filepath"
+	"syscall"
 
 	"github.com/xtls/xray-core/common/buf"
 	"github.com/xtls/xray-core/common/platform"
@@ -16,6 +19,29 @@ var NewFileReader FileReaderFunc = func(path string) (io.ReadCloser, error) {
 }
 
 func ReadFile(path string) ([]byte, error) {
+	file, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+
+	stat, err := file.Stat()
+	if err != nil {
+		return nil, err
+	}
+
+	size := stat.Size()
+	if size == 0 {
+		return []byte{}, nil
+	}
+
+	// use mmap to save RAM
+	bs, err := syscall.Mmap(int(file.Fd()), 0, int(size), syscall.PROT_READ, syscall.MAP_SHARED)
+	if err == nil {
+		return bs, nil
+	}
+
+	// fallback
 	reader, err := NewFileReader(path)
 	if err != nil {
 		return nil, err