rbd: add minimal support for listing watchers on an RBD image

Checking if there are other clients interested in changes to an RBD
image is important for certain actions. For example, the Ceph-CSI
project would not want to remove an image in case it is still mounted
somewhere.

Signed-off-by: Niels de Vos <ndevos@redhat.com>

rbd/watchers_mimic.go

@@ -0,0 +1,57 @@
+// +build !luminous
+//
+// Ceph Mimic is the first version that supports watchers through librbd.
+
+package rbd
+
+// #cgo LDFLAGS: -lrbd
+// #include <errno.h>
+// #include <rbd/librbd.h>
+import "C"
+
+// ImageWatcher is a representation of the rbd_image_watcher_t from librbd.h
+type ImageWatcher struct {
+	Addr   string
+	Id     int64
+	Cookie uint64
+}
+
+// ListWatchers returns the watchers on an RBD image. In case of an error, nil
+// and an error are returned.
+//
+// Note:
+//   Only supported in Ceph Mimic and newer.
+//
+// Implements:
+//   int rbd_watchers_list(rbd_image_t image,
+//                         rbd_image_watcher_t *watchers, size_t *max_watchers)
+func (image *Image) ListWatchers() ([]ImageWatcher, error) {
+	if err := image.validate(imageIsOpen); err != nil {
+		return nil, err
+	}
+
+	count := C.ulong(0)
+	ret := C.rbd_watchers_list(image.image, nil, &count)
+	if ret != 0 && ret != -C.ERANGE {
+		return nil, getError(ret)
+	}
+	if ret == 0 && count == 0 {
+		return nil, nil
+	}
+
+	watchers := make([]C.rbd_image_watcher_t, count)
+	ret = C.rbd_watchers_list(image.image, &watchers[0], &count)
+	if ret != 0 && ret != -C.ERANGE {
+		return nil, getError(ret)
+	}
+	defer C.rbd_watchers_list_cleanup(&watchers[0], count)
+
+	imageWatchers := make([]ImageWatcher, len(watchers))
+	for i, watcher := range watchers {
+		imageWatchers[i].Addr = C.GoString(watcher.addr)
+		imageWatchers[i].Id = int64(watcher.id)
+		imageWatchers[i].Cookie = uint64(watcher.cookie)
+	}
+
+	return imageWatchers, nil
+}

rbd/watchers_mimic_test.go

@@ -0,0 +1,117 @@
+// +build !luminous
+//
+// Ceph Mimic is the first version that supports watchers through librbd.
+
+package rbd
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestListWatchers(t *testing.T) {
+	conn := radosConnect(t)
+	require.NotNil(t, conn)
+	defer conn.Shutdown()
+
+	poolname := GetUUID()
+	err := conn.MakePool(poolname)
+	require.NoError(t, err)
+	defer conn.DeletePool(poolname)
+
+	ioctx, err := conn.OpenIOContext(poolname)
+	require.NoError(t, err)
+	defer ioctx.Destroy()
+
+	name := GetUUID()
+	options := NewRbdImageOptions()
+	err = CreateImage(ioctx, name, 1<<22, options)
+	require.NoError(t, err)
+	defer func() { assert.NoError(t, RemoveImage(ioctx, name)) }()
+
+	t.Run("imageNotOpen", func(t *testing.T) {
+		image, err := OpenImageReadOnly(ioctx, name, NoSnapshot)
+		require.NoError(t, err)
+		require.NotNil(t, image)
+
+		err = image.Close()
+		require.NoError(t, err)
+
+		_, err = image.ListWatchers()
+		assert.Equal(t, ErrImageNotOpen, err)
+	})
+
+	t.Run("noWatchers", func(t *testing.T) {
+		// open image read-only, as OpenImage() automatically adds a watcher
+		image, err := OpenImageReadOnly(ioctx, name, NoSnapshot)
+		require.NoError(t, err)
+		require.NotNil(t, image)
+		defer func() { assert.NoError(t, image.Close()) }()
+
+		watchers, err := image.ListWatchers()
+		assert.NoError(t, err)
+		assert.Equal(t, 0, len(watchers))
+	})
+
+	t.Run("addWatchers", func(t *testing.T) {
+		// open image read-only, as OpenImage() automatically adds a watcher
+		image, err := OpenImageReadOnly(ioctx, name, NoSnapshot)
+		require.NoError(t, err)
+		require.NotNil(t, image)
+		defer func() { assert.NoError(t, image.Close()) }()
+
+		watchers, err := image.ListWatchers()
+		assert.NoError(t, err)
+		assert.Equal(t, 0, len(watchers))
+
+		// opening an image writable adds a watcher automatically
+		image2, err := OpenImage(ioctx, name, NoSnapshot)
+		require.NoError(t, err)
+		require.NotNil(t, image2)
+
+		watchers, err = image.ListWatchers()
+		assert.NoError(t, err)
+		assert.Equal(t, 1, len(watchers))
+
+		watchers, err = image2.ListWatchers()
+		assert.NoError(t, err)
+		assert.Equal(t, 1, len(watchers))
+
+		image3, err := OpenImage(ioctx, name, NoSnapshot)
+		require.NoError(t, err)
+		require.NotNil(t, image3)
+
+		watchers, err = image.ListWatchers()
+		assert.NoError(t, err)
+		assert.Equal(t, 2, len(watchers))
+
+		watchers, err = image2.ListWatchers()
+		assert.NoError(t, err)
+		assert.Equal(t, 2, len(watchers))
+
+		watchers, err = image3.ListWatchers()
+		assert.NoError(t, err)
+		assert.Equal(t, 2, len(watchers))
+
+		// closing an image removes the watchers
+		err = image3.Close()
+		require.NoError(t, err)
+
+		watchers, err = image.ListWatchers()
+		assert.NoError(t, err)
+		assert.Equal(t, 1, len(watchers))
+
+		watchers, err = image2.ListWatchers()
+		assert.NoError(t, err)
+		assert.Equal(t, 1, len(watchers))
+
+		err = image2.Close()
+		require.NoError(t, err)
+
+		watchers, err = image.ListWatchers()
+		assert.NoError(t, err)
+		assert.Equal(t, 0, len(watchers))
+	})
+}