sysfs: reopening file doesn't update fd

internal/sysfs/file.go

@@ -269,7 +269,7 @@ func (f *fsFile) Readdir(n int) (dirents []experimentalsys.Dirent, errno experim
 
 	if f.reopenDir { // re-open the directory if needed.
 		f.reopenDir = false
-		if errno = adjustReaddirErr(f, f.closed, f.reopen()); errno != 0 {
+		if errno = adjustReaddirErr(f, f.closed, f.rewindDir()); errno != 0 {
 			return
 		}
 	}
@@ -418,19 +418,25 @@ func seek(s io.Seeker, offset int64, whence int) (int64, experimentalsys.Errno)
 	return newOffset, experimentalsys.UnwrapOSError(err)
 }
 
-// reopenFile allows re-opening a file for reasons such as applying flags or
-// directory iteration.
-type reopenFile func() experimentalsys.Errno
-
-// compile-time check to ensure fsFile.reopen implements reopenFile.
-var _ reopenFile = (*fsFile)(nil).reopen
-
-// reopen implements the same method as documented on reopenFile.
-func (f *fsFile) reopen() experimentalsys.Errno {
-	_ = f.close()
-	var err error
-	f.file, err = f.fs.Open(f.name)
-	return experimentalsys.UnwrapOSError(err)
+func (f *fsFile) rewindDir() experimentalsys.Errno {
+	// Reopen the directory to rewind it.
+	file, err := f.fs.Open(f.name)
+	if err != nil {
+		return experimentalsys.UnwrapOSError(err)
+	}
+	fi, err := file.Stat()
+	if err != nil {
+		return experimentalsys.UnwrapOSError(err)
+	}
+	// Can't check if it's still the same file,
+	// but is it still a directory, at least?
+	if !fi.IsDir() {
+		return experimentalsys.ENOTDIR
+	}
+	// Only update f on success.
+	_ = f.file.Close()
+	f.file = file
+	return 0
 }
 
 // readdirFile allows masking the `Readdir` function on os.File.

internal/sysfs/file_test.go

@@ -116,6 +116,50 @@ func TestWriteFdNonblock(t *testing.T) {
 	t.Fatal("writeFd should return EAGAIN at some point")
 }
 
+func TestFileReopenFileUpdatesFD(t *testing.T) {
+	tmpDir := t.TempDir()
+	path := path.Join(tmpDir, "file")
+
+	// Open the file twice. Closing the first file, frees its fd.
+	// Then reopening the second file will take over the first fd.
+	// If reopening the file doesn't update the fd, they won't match.
+	f0 := requireOpenFile(t, path, experimentalsys.O_RDWR|experimentalsys.O_CREAT, 0o600)
+	f1 := requireOpenFile(t, path, experimentalsys.O_RDWR, 0o600)
+	defer f1.Close()
+	f0.Close()
+
+	of, ok := f1.(*osFile)
+	require.True(t, ok)
+
+	require.EqualErrno(t, 0, of.reopen())
+	require.Equal(t, of.file.Fd(), of.fd)
+}
+
+func TestFileReopenFileChecksSameFile(t *testing.T) {
+	tmpDir := t.TempDir()
+	path := path.Join(tmpDir, "file")
+
+	f := requireOpenFile(t, path, experimentalsys.O_RDWR|experimentalsys.O_CREAT, 0o600)
+	defer f.Close()
+
+	require.NoError(t, os.Remove(path))
+
+	of, ok := f.(*osFile)
+	require.True(t, ok)
+
+	// Path does not exist anymore.
+	require.EqualErrno(t, experimentalsys.ENOENT, of.reopen())
+	require.Equal(t, of.file.Fd(), of.fd)
+
+	tmp, err := os.Create(path)
+	require.NoError(t, err)
+	defer tmp.Close()
+
+	// Path exists, but is not the same file.
+	require.EqualErrno(t, experimentalsys.ENOENT, of.reopen())
+	require.Equal(t, of.file.Fd(), of.fd)
+}
+
 func TestFileSetAppend(t *testing.T) {
 	tmpDir := t.TempDir()
 
@@ -124,6 +168,7 @@ func TestFileSetAppend(t *testing.T) {
 
 	// Open without APPEND.
 	f, errno := OpenOSFile(fPath, experimentalsys.O_RDWR, 0o600)
+	defer f.Close()
 	require.EqualErrno(t, 0, errno)
 	require.False(t, f.IsAppend())
 

internal/sysfs/osfile.go

@@ -83,21 +83,12 @@ func (f *osFile) SetAppend(enable bool) (errno experimentalsys.Errno) {
 		f.flag &= ^experimentalsys.O_APPEND
 	}
 
-	// Clear any create or trunc flag, as we are re-opening, not re-creating.
-	f.flag &= ^(experimentalsys.O_CREAT | experimentalsys.O_TRUNC)
-
-	// appendMode (bool) cannot be changed later, so we have to re-open the
-	// file. https://github.com/golang/go/blob/go1.20/src/os/file_unix.go#L60
+	// appendMode cannot be changed later, so we have to re-open the file
+	// https://github.com/golang/go/blob/go1.23/src/os/file_unix.go#L60
 	return fileError(f, f.closed, f.reopen())
 }
 
-// compile-time check to ensure osFile.reopen implements reopenFile.
-var _ reopenFile = (*osFile)(nil).reopen
-
 func (f *osFile) reopen() (errno experimentalsys.Errno) {
-	// Clear any create flag, as we are re-opening, not re-creating.
-	f.flag &= ^experimentalsys.O_CREAT
-
 	var (
 		isDir  bool
 		offset int64
@@ -116,22 +107,47 @@ func (f *osFile) reopen() (errno experimentalsys.Errno) {
 		}
 	}
 
-	_ = f.close()
-	f.file, errno = OpenFile(f.path, f.flag, f.perm)
+	// Clear any create or trunc flag, as we are re-opening, not re-creating.
+	flag := f.flag &^ (experimentalsys.O_CREAT | experimentalsys.O_TRUNC)
+	file, errno := OpenFile(f.path, flag, f.perm)
+	if errno != 0 {
+		return errno
+	}
+	errno = f.checkSameFile(file)
 	if errno != 0 {
 		return errno
 	}
 
 	if !isDir {
-		_, err = f.file.Seek(offset, io.SeekStart)
+		_, err = file.Seek(offset, io.SeekStart)
 		if err != nil {
+			_ = file.Close()
 			return experimentalsys.UnwrapOSError(err)
 		}
 	}
 
+	// Only update f on success.
+	_ = f.file.Close()
+	f.file = file
+	f.fd = file.Fd()
 	return 0
 }
 
+func (f *osFile) checkSameFile(osf *os.File) experimentalsys.Errno {
+	fi1, err := f.file.Stat()
+	if err != nil {
+		return experimentalsys.UnwrapOSError(err)
+	}
+	fi2, err := osf.Stat()
+	if err != nil {
+		return experimentalsys.UnwrapOSError(err)
+	}
+	if os.SameFile(fi1, fi2) {
+		return 0
+	}
+	return experimentalsys.ENOENT
+}
+
 // IsNonblock implements the same method as documented on fsapi.File
 func (f *osFile) IsNonblock() bool {
 	return isNonblock(f)

sys/stat.go

@@ -29,9 +29,7 @@ type EpochNanos = int64
 // # Notes
 //
 //   - This is used for WebAssembly ABI emulating the POSIX `stat` system call.
-//     Fields included are required for WebAssembly ABI including wasip1
-//     (a.k.a. wasix) and wasi-filesystem (a.k.a. wasip2). See
-//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/stat.html
+//     See https://pubs.opengroup.org/onlinepubs/9699919799/functions/stat.html
 //   - Fields here are required for WebAssembly ABI including wasip1
 //     (a.k.a. wasix) and wasi-filesystem (a.k.a. wasip2).
 //   - This isn't the same as syscall.Stat_t because wazero supports Windows,