rados: add wrapper for rados_ioctx_snap_set_read() function

Implemented SetReadSnap() which sets the snapshot from which reads
would be performed.

Signed-off-by: Mudit Agarwal <muagarwa@redhat.com>

rados/snapshot.go

@@ -168,3 +168,22 @@ func (ioctx *IOContext) RollbackSnap(oid, snapName string) error {
 	ret := C.rados_ioctx_snap_rollback(ioctx.ioctx, coid, cSnapName)
 	return getError(ret)
 }
+
+// SnapHead is the representation of LIBRADOS_SNAP_HEAD from librados.
+// SnapHead can be used to reset the IOContext to stop reading from a snapshot.
+const SnapHead = SnapID(C.LIBRADOS_SNAP_HEAD)
+
+// SetReadSnap sets the snapshot from which reads are performed.
+// Subsequent reads will return data as it was at the time of that snapshot.
+// Pass SnapHead for no snapshot (i.e. normal operation).
+//
+// Implements:
+//  void rados_ioctx_snap_set_read(rados_ioctx_t io, rados_snap_t snap);
+func (ioctx *IOContext) SetReadSnap(snapID SnapID) error {
+	if err := ioctx.validate(); err != nil {
+		return err
+	}
+
+	C.rados_ioctx_snap_set_read(ioctx.ioctx, (C.rados_snap_t)(snapID))
+	return nil
+}

rados/snapshot_test.go

@@ -254,3 +254,59 @@ func (suite *RadosTestSuite) TestRollbackSnapshot() {
 		assert.Equal(t, bytesOut, bytesIn)
 	})
 }
+
+func (suite *RadosTestSuite) TestSetReadSnapshot() {
+	suite.SetupConnection()
+	ioctx, err := suite.conn.OpenIOContext(suite.pool)
+	require.NoError(suite.T(), err)
+
+	bytesIn := []byte("The Order of the Phoenix")
+	err = ioctx.Write("obj", bytesIn, 0)
+	assert.NoError(suite.T(), err)
+
+	// Take snap.
+	err = ioctx.CreateSnap("mySnap")
+	assert.NoError(suite.T(), err)
+	defer func() {
+		assert.NoError(suite.T(), ioctx.RemoveSnap("mySnap"))
+	}()
+
+	// Get Snap ID.
+	snapID, err := ioctx.LookupSnap("mySnap")
+	assert.NoError(suite.T(), err)
+	assert.NotNil(suite.T(), snapID)
+
+	// Overwrite the object.
+	bytesOver := []byte("The Half blood Prince")
+	err = ioctx.Write("obj", bytesOver, 0)
+	assert.NoError(suite.T(), err)
+
+	// Set read to mySnap.
+	err = ioctx.SetReadSnap(snapID)
+	assert.NoError(suite.T(), err)
+
+	// Read the object.
+	bytesOut := make([]byte, len(bytesIn))
+	nOut, err := ioctx.Read("obj", bytesOut, 0)
+	assert.NoError(suite.T(), err)
+	assert.Equal(suite.T(), nOut, len(bytesIn))
+	assert.Equal(suite.T(), bytesOut, bytesIn)
+
+	// Set read to SnapHead (back to normal).
+	err = ioctx.SetReadSnap(SnapHead)
+	assert.NoError(suite.T(), err)
+
+	// Read the same object.
+	bytesOut = make([]byte, len(bytesOver))
+	nOut, err = ioctx.Read("obj", bytesOut, 0)
+	assert.NoError(suite.T(), err)
+	assert.Equal(suite.T(), nOut, len(bytesOver))
+	assert.Equal(suite.T(), bytesOut, bytesOver)
+
+	suite.T().Run("invalidIOContext", func(t *testing.T) {
+		ioctx := &IOContext{}
+		err := ioctx.SetReadSnap(SnapHead)
+		assert.Error(t, err)
+		assert.Equal(t, err, ErrInvalidIOContext)
+	})
+}