vfs: make objects of unknown size readable through the VFS

These objects (eg Google Docs) appear with 0 length in the VFS.

Before this change, these only read 0 bytes.

After this change, even though the size appears to be 0, the objects
can be read to the end.  If the objects are read to the end then the
size on the handle will be updated.

vfs/file_test.go

@@ -7,6 +7,8 @@ import (
 	"testing"
 
 	"github.com/rclone/rclone/fstest"
+	"github.com/rclone/rclone/fstest/mockfs"
+	"github.com/rclone/rclone/fstest/mockobject"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -110,6 +112,51 @@ func TestFileOpenRead(t *testing.T) {
 	require.NoError(t, fd.Close())
 }
 
+func TestFileOpenReadUnknownSize(t *testing.T) {
+	var (
+		contents = []byte("file contents")
+		remote   = "file.txt"
+		ctx      = context.Background()
+	)
+
+	// create a mock object which returns size -1
+	o := mockobject.New(remote).WithContent(contents, mockobject.SeekModeNone)
+	o.SetUnknownSize(true)
+	assert.Equal(t, int64(-1), o.Size())
+
+	// add it to a mock fs
+	f := mockfs.NewFs("test", "root")
+	f.AddObject(o)
+	testObj, err := f.NewObject(ctx, remote)
+	require.NoError(t, err)
+	assert.Equal(t, int64(-1), testObj.Size())
+
+	// create a VFS from that mockfs
+	vfs := New(f, nil)
+
+	// find the file
+	node, err := vfs.Stat(remote)
+	require.NoError(t, err)
+	require.True(t, node.IsFile())
+	file := node.(*File)
+
+	// open it
+	fd, err := file.openRead()
+	require.NoError(t, err)
+	assert.Equal(t, int64(0), fd.Size())
+
+	// check the contents are not empty even though size is empty
+	gotContents, err := ioutil.ReadAll(fd)
+	require.NoError(t, err)
+	assert.Equal(t, contents, gotContents)
+	t.Logf("gotContents = %q", gotContents)
+
+	// check that file size has been updated
+	assert.Equal(t, int64(len(contents)), fd.Size())
+
+	require.NoError(t, fd.Close())
+}
+
 func TestFileOpenWrite(t *testing.T) {
 	r := fstest.NewRun(t)
 	defer r.Finalise()