vfs: fix open files disappearing from directory listings

In this commit

aaadb48d48 vfs: keep virtual directory status accurate and reduce deadlock potential

We reworked the virtual directory detection to use an atomic bool so
that we could run part of the cache forgetting only with a read lock.

Unfortunately this had a bug which meant that directories with virtual
items could be forgotten.

This commit changes the boolean into a count of virtual entries which
should be more accurate.

Fixes #8082

vfs/dir_test.go

@@ -607,3 +607,51 @@ func TestDirRename(t *testing.T) {
 func TestDirStructSize(t *testing.T) {
 	t.Logf("Dir struct has size %d bytes", unsafe.Sizeof(Dir{}))
 }
+
+// Check that open files appear in the directory listing properly after a forget
+func TestDirFileOpen(t *testing.T) {
+	_, vfs, dir, _ := dirCreate(t)
+
+	assert.False(t, dir.hasVirtual())
+	assert.False(t, dir.parent.hasVirtual())
+
+	_, err := dir.Mkdir("sub")
+	require.NoError(t, err)
+
+	assert.True(t, dir.hasVirtual())
+	assert.True(t, dir.parent.hasVirtual())
+
+	fd0, err := vfs.Create("dir/sub/file0")
+	require.NoError(t, err)
+	_, err = fd0.Write([]byte("hello"))
+	require.NoError(t, err)
+	defer func() {
+		require.NoError(t, fd0.Close())
+	}()
+
+	fd2, err := vfs.Create("dir/sub/file2")
+	require.NoError(t, err)
+	_, err = fd2.Write([]byte("hello world!"))
+	require.NoError(t, err)
+	require.NoError(t, fd2.Close())
+	assert.True(t, dir.hasVirtual())
+
+	assert.True(t, dir.hasVirtual())
+	assert.True(t, dir.parent.hasVirtual())
+
+	// Now forget the directory
+	hasVirtual := dir.parent.ForgetAll()
+	assert.True(t, hasVirtual)
+
+	assert.True(t, dir.hasVirtual())
+	assert.True(t, dir.parent.hasVirtual())
+
+	// Check the files can still be found
+	fi, err := vfs.Stat("dir/sub/file0")
+	require.NoError(t, err)
+	assert.Equal(t, int64(5), fi.Size())
+
+	fi, err = vfs.Stat("dir/sub/file2")
+	require.NoError(t, err)
+	assert.Equal(t, int64(12), fi.Size())
+}