rbd: fix problems with Image.Discard()

The return value of rbd_discard() is the number of bytes that have been
discarded, or in case of an error, a negative errno value.

It seems Image.Discard() never can have worked as intended, unless the
error was disregarded. Just like Image.Write(), Image.Discard() should
return a tuple of the number of bytes discarded and an error.

Fixes: ceph/go-ceph#123
Signed-off-by: Niels de Vos <ndevos@redhat.com>

rbd/rbd.go

@@ -885,13 +885,17 @@ func (image *Image) Seek(offset int64, whence int) (int64, error) {
 }
 
 // int rbd_discard(rbd_image_t image, uint64_t ofs, uint64_t len);
-func (image *Image) Discard(ofs uint64, length uint64) error {
+func (image *Image) Discard(ofs uint64, length uint64) (int, error) {
 	if err := image.validate(imageIsOpen); err != nil {
-		return err
+		return 0, err
 	}
 
-	return RBDError(C.rbd_discard(image.image, C.uint64_t(ofs),
-		C.uint64_t(length)))
+	ret := C.rbd_discard(image.image, C.uint64_t(ofs), C.uint64_t(length))
+	if ret < 0 {
+		return 0, RBDError(ret)
+	}
+
+	return int(ret), nil
 }
 
 func (image *Image) ReadAt(data []byte, off int64) (n int, err error) {

rbd/rbd_test.go

@@ -462,6 +462,53 @@ func TestImageSeek(t *testing.T) {
 	conn.Shutdown()
 }
 
+func TestImageDiscard(t *testing.T) {
+	conn, _ := rados.NewConn()
+	conn.ReadDefaultConfigFile()
+	conn.Connect()
+
+	poolname := GetUUID()
+	err := conn.MakePool(poolname)
+	assert.NoError(t, err)
+
+	ioctx, err := conn.OpenIOContext(poolname)
+	require.NoError(t, err)
+
+	name := GetUUID()
+	img, err := Create(ioctx, name, 1<<22, 22)
+	assert.NoError(t, err)
+
+	err = img.Open()
+	assert.NoError(t, err)
+
+	n, err := img.Discard(0, 1<<16)
+	assert.NoError(t, err)
+	assert.Equal(t, n, 1<<16)
+
+	err = img.Close()
+	assert.NoError(t, err)
+
+	err = img.Open(true)
+	assert.NoError(t, err)
+
+	// when read-only, discard should fail
+	_, err = img.Discard(0, 1<<16)
+	assert.Error(t, err)
+
+	err = img.Close()
+	assert.NoError(t, err)
+
+	_, err = img.Discard(0, 1<<16)
+	assert.Equal(t, err, ErrImageNotOpen)
+
+	err = img.Remove()
+	assert.NoError(t, err)
+
+	ioctx.Destroy()
+	conn.DeletePool(poolname)
+	conn.Shutdown()
+}
+
 func TestIOReaderWriter(t *testing.T) {
 	conn, _ := rados.NewConn()
 	conn.ReadDefaultConfigFile()