test: metadata operations on unlinked open files and removed open directories (#11075)

The POSIX suites skirt this: pjdfstest's unlink/14.t covers only fstat and
pread on an unlinked descriptor — its driver has no fchmod at all and never
opens a directory. Pin the full rule in the FUSE integration suite:
ftruncate, fchmod, futimes, fstat, and the f*xattr calls keep working
between the removal of the last name and the final close, for a file after
unlink and a directory after rmdir, with nlink 0 and the changes visible to
a following fstat.

Claude-Session: https://claude.ai/code/session_01GYqLENjZzbV5hgt4L8cSAK

test/fuse_integration/unlinked_open_test.go

@@ -0,0 +1,94 @@
+//go:build linux
+
+package fuse_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"golang.org/x/sys/unix"
+)
+
+// TestMetadataOnUnlinkedOpen covers the POSIX rule that an inode outlives its
+// last name while a descriptor holds it open: metadata operations through the
+// descriptor keep working between the removal and the final close, for a file
+// after unlink and for a directory after rmdir.
+func TestMetadataOnUnlinkedOpen(t *testing.T) {
+	framework := NewFuseTestFramework(t, DefaultTestConfig())
+	defer framework.Cleanup()
+
+	require.NoError(t, framework.Setup(DefaultTestConfig()))
+
+	t.Run("UnlinkedOpenFile", func(t *testing.T) {
+		testUnlinkedOpenFile(t, framework)
+	})
+
+	t.Run("RemovedOpenDirectory", func(t *testing.T) {
+		testRemovedOpenDirectory(t, framework)
+	})
+}
+
+func testUnlinkedOpenFile(t *testing.T, framework *FuseTestFramework) {
+	path := filepath.Join(framework.GetMountPoint(), "unlinked_open_file")
+	fd, err := unix.Open(path, unix.O_CREAT|unix.O_RDWR, 0644)
+	require.NoError(t, err)
+	defer unix.Close(fd)
+	_, err = unix.Write(fd, []byte("hello"))
+	require.NoError(t, err)
+
+	require.NoError(t, os.Remove(path))
+
+	require.NoError(t, unix.Ftruncate(fd, 2), "ftruncate on an unlinked open file")
+	require.NoError(t, unix.Fchmod(fd, 0600), "fchmod on an unlinked open file")
+	require.NoError(t, unix.Futimes(fd, []unix.Timeval{{Sec: 1234567890}, {Sec: 987654321}}),
+		"futimes on an unlinked open file")
+
+	var st unix.Stat_t
+	require.NoError(t, unix.Fstat(fd, &st), "fstat on an unlinked open file")
+	assert.EqualValues(t, 0, st.Nlink, "nlink after unlink")
+	assert.EqualValues(t, 0600, st.Mode&0777, "mode after fchmod")
+	assert.EqualValues(t, 2, st.Size, "size after ftruncate")
+	assert.EqualValues(t, 1234567890, st.Atim.Sec, "atime after futimes")
+	assert.EqualValues(t, 987654321, st.Mtim.Sec, "mtime after futimes")
+
+	testFdXattrRoundTrip(t, fd)
+}
+
+func testRemovedOpenDirectory(t *testing.T, framework *FuseTestFramework) {
+	path := filepath.Join(framework.GetMountPoint(), "removed_open_dir")
+	require.NoError(t, os.Mkdir(path, 0755))
+	fd, err := unix.Open(path, unix.O_RDONLY|unix.O_DIRECTORY, 0)
+	require.NoError(t, err)
+	defer unix.Close(fd)
+
+	require.NoError(t, os.Remove(path))
+
+	require.NoError(t, unix.Fchmod(fd, 0770), "fchmod on a removed open directory")
+	require.NoError(t, unix.Futimes(fd, []unix.Timeval{{Sec: 1234567890}, {Sec: 987654321}}),
+		"futimes on a removed open directory")
+
+	var st unix.Stat_t
+	require.NoError(t, unix.Fstat(fd, &st), "fstat on a removed open directory")
+	assert.EqualValues(t, unix.S_IFDIR, st.Mode&unix.S_IFMT, "type after rmdir")
+	assert.EqualValues(t, 0770, st.Mode&0777, "mode after fchmod")
+	assert.EqualValues(t, 0, st.Nlink, "nlink after rmdir")
+	assert.EqualValues(t, 987654321, st.Mtim.Sec, "mtime after futimes")
+
+	testFdXattrRoundTrip(t, fd)
+}
+
+func testFdXattrRoundTrip(t *testing.T, fd int) {
+	require.NoError(t, unix.Fsetxattr(fd, "user.k", []byte("v"), 0), "fsetxattr on a removed open inode")
+
+	buf := make([]byte, 8)
+	n, err := unix.Fgetxattr(fd, "user.k", buf)
+	require.NoError(t, err, "fgetxattr on a removed open inode")
+	assert.Equal(t, "v", string(buf[:n]))
+
+	require.NoError(t, unix.Fremovexattr(fd, "user.k"), "fremovexattr on a removed open inode")
+	_, err = unix.Fgetxattr(fd, "user.k", buf)
+	assert.ErrorIs(t, err, unix.ENODATA, "fgetxattr after fremovexattr")
+}