os: Stat and ReadDir on Windows use the file handle without holding a reference #82293

// Command winstat shows that (*os.File).Stat and ReadDir on Windows use
// the file's handle without holding a reference to it:
//
//   - after Close they fail with ERROR_INVALID_HANDLE rather than an
//     error matching os.ErrClosed;
//   - while Close runs concurrently, the handle value may be closed and
//     reused by another open, so Stat can describe a different file.
package main

import (
	"errors"
	"flag"
	"fmt"
	"os"
	"path/filepath"
	"runtime"
	"sync"
	"sync/atomic"
)

const otherSize = 12345 // size of the regular file whose handle may be reused

func main() {
	trials := flag.Int("n", 20000, "trials for the concurrent part")
	flag.Parse()
	fmt.Println(runtime.Version(), runtime.GOOS+"/"+runtime.GOARCH)

	dir, err := os.MkdirTemp("", "winstat")
	if err != nil {
		panic(err)
	}
	defer os.RemoveAll(dir)
	other := filepath.Join(dir, "other")
	if err := os.WriteFile(other, make([]byte, otherSize), 0o666); err != nil {
		panic(err)
	}

	// Part A: methods on a closed directory.
	f, err := os.Open(dir)
	if err != nil {
		panic(err)
	}
	if err := f.Close(); err != nil {
		panic(err)
	}
	_, statErr := f.Stat()
	_, readDirErr := f.ReadDir(-1)
	_, namesErr := f.Readdirnames(-1)
	for _, c := range []struct {
		name string
		err  error
	}{{"Stat", statErr}, {"ReadDir", readDirErr}, {"Readdirnames", namesErr}} {
		fmt.Printf("closed dir %-12s err=%q Is(os.ErrClosed)=%v\n", c.name, c.err, errors.Is(c.err, os.ErrClosed))
	}

	// Part B: Stat racing Close while the handle value is reused.
	var wrong, wrongSize atomic.Int64
	var example atomic.Value
	for i := 0; i < *trials; i++ {
		f, err := os.Open(dir)
		if err != nil {
			panic(err)
		}
		start := make(chan struct{})
		var wg sync.WaitGroup
		wg.Add(1)
		go func() {
			defer wg.Done()
			<-start
			for {
				fi, err := f.Stat()
				if err != nil {
					return // the File is closed
				}
				if !fi.IsDir() {
					wrong.Add(1)
					if fi.Size() == otherSize {
						wrongSize.Add(1)
					}
					example.CompareAndSwap(nil, fmt.Sprintf("Stat of directory %q returned IsDir=false Size=%d Mode=%v", fi.Name(), fi.Size(), fi.Mode()))
				}
			}
		}()
		close(start)
		runtime.Gosched()
		f.Close()
		// Reopen other files so that the freed handle value is reused.
		var opened []*os.File
		for j := 0; j < 4; j++ {
			g, err := os.Open(other)
			if err != nil {
				panic(err)
			}
			opened = append(opened, g)
		}
		wg.Wait()
		for _, g := range opened {
			g.Close()
		}
	}
	fmt.Printf("concurrent part: %d trials, Stat of the directory described a non-directory %d times (%d with the other file's size)\n",
		*trials, wrong.Load(), wrongSize.Load())
	if v := example.Load(); v != nil {
		fmt.Println("example:", v)
	}
}