Use mmap in ReadAll if possible

Author: tgulacsi

excelize.go

@@ -175,7 +175,7 @@ func (f *File) checkOpenReaderOptions() error {
 // OpenReader read data stream from io.Reader and return a populated
 // spreadsheet file.
 func OpenReader(r io.Reader, opts ...Options) (*File, error) {
-	b, err := io.ReadAll(r)
+	b, err := readAll(r)
 	if err != nil {
 		return nil, err
 	}

lib.go

@@ -118,7 +118,7 @@ func (f *File) readBytes(name string) []byte {
 	if err != nil {
 		return content
 	}
-	content, _ = io.ReadAll(file)
+	content, _ = readAll(file)
 	f.Pkg.Store(name, content)
 	_ = file.Close()
 	return content

lib_nonwindows.go

@@ -0,0 +1,35 @@
+//go:build !windows
+
+// Copyright 2025 The excelize Authors. All rights reserved. Use of
+// this source code is governed by a BSD-style license that can be found in
+// the LICENSE file.
+
+package excelize
+
+import (
+	"io"
+	"os"
+	"runtime"
+	"syscall"
+)
+
+// readAll is like io.ReadAll, but uses mmap if possible.
+func readAll(r io.Reader) ([]byte, error) {
+	if fder, ok := r.(interface {
+		Fd() int
+		Stat() (os.FileInfo, error)
+	}); ok {
+		if fi, err := fder.Stat(); err == nil {
+			if b, err := syscall.Mmap(fder.Fd(), 0, int(fi.Size()),
+				syscall.PROT_READ,
+				syscall.MAP_PRIVATE|syscall.MAP_POPULATE,
+			); err == nil {
+				runtime.SetFinalizer(&b, func(_ any) error {
+					return syscall.Munmap(b)
+				})
+				return b, nil
+			}
+		}
+	}
+	return io.ReadAll(r)
+}

lib_windows.go

@@ -0,0 +1,16 @@
+//go:build windows
+
+// Copyright 2025 The excelize Authors. All rights reserved. Use of
+// this source code is governed by a BSD-style license that can be found in
+// the LICENSE file.
+
+package excelize
+
+import (
+	"io"
+)
+
+// readAll is like io.ReadAll, but uses mmap if possible.
+func readAll(r io.Reader) ([]byte, error) {
+	return io.ReadAll(r)
+}