open-consul/agent/consul/stream/event_snapshot.go

package stream

// eventSnapshot represents the state of memdb for a given topic and key at some
// point in time. It is modelled as a buffer of events so that snapshots can be
// streamed to possibly multiple subscribers concurrently, and can be trivially
// cached by retaining a reference to a Snapshot. Once the reference to eventSnapshot
// is dropped from memory, any subscribers still reading from it may do so by following
// their pointers. When the last subscribe unsubscribes the snapshot is garbage
// collected automatically by Go's runtime. This simplifies snapshot and buffer
// management dramatically.
type eventSnapshot struct {
	// Snap is the first item in the buffer containing the snapshot. Once the
	// snapshot is complete, subsequent BufferItems are appended to snapBuffer,
	// so that subscribers receive all the events from the same buffer.
	Snap *bufferItem

	// snapBuffer is the Head of the snapshot buffer the fn should write to.
	snapBuffer *eventBuffer
}

type snapFunc func(req *SubscribeRequest, buf SnapshotAppender) (uint64, error)

// newEventSnapshot creates a snapshot buffer based on the subscription request.
// The current buffer head for the topic in question is passed so that once the
// snapshot is complete and has been delivered into the buffer, any events
// published during snapshotting can be immediately appended and won't be
// missed. Once the snapshot is delivered the topic buffer is spliced onto the
// snapshot buffer so that subscribers will naturally follow from the snapshot
// to wait for any subsequent updates.
func newEventSnapshot(req *SubscribeRequest, topicBufferHead *bufferItem, fn snapFunc) *eventSnapshot {
	buf := newEventBuffer()
	s := &eventSnapshot{
		Snap:       buf.Head(),
		snapBuffer: buf,
	}

	go func() {
		idx, err := fn(req, s.snapBuffer)
		if err != nil {
			s.snapBuffer.AppendErr(err)
			return
		}
		// We wrote the snapshot events to the buffer, send the "end of snapshot" event
		s.snapBuffer.Append([]Event{{
			Topic:   req.Topic,
			Key:     req.Key,
			Index:   idx,
			Payload: endOfSnapshot{},
		}})
		s.spliceFromTopicBuffer(topicBufferHead, idx)
	}()
	return s
}

func (s *eventSnapshot) spliceFromTopicBuffer(topicBufferHead *bufferItem, idx uint64) {
	// Now splice on the topic buffer. We need to iterate through the buffer to
	// find the first event after the current snapshot.
	item := topicBufferHead
	for {
		// Find the next item that we should include.
		next, err := item.NextNoBlock()
		if err != nil {
			// Append an error result to signal to subscribers that this snapshot is
			// no good.
			s.snapBuffer.AppendErr(err)
			return
		}

		if next == nil {
			// This is the head of the topic buffer (or was just now which is after
			// the snapshot completed). We don't want any of the events (if any) in
			// the snapshot buffer as they came before the snapshot but we do need to
			// wait for the next update.
			follow, err := item.FollowAfter()
			if err != nil {
				s.snapBuffer.AppendErr(err)
				return
			}

			s.snapBuffer.AppendBuffer(follow)
			// We are done, subscribers will now follow future updates to the topic
			// after reading the snapshot events.
			return
		}

		if next.Err != nil {
			s.snapBuffer.AppendErr(next.Err)
			return
		}

		if len(next.Events) > 0 && next.Events[0].Index > idx {
			// We've found an update in the topic buffer that happened after our
			// snapshot was taken, splice it into the snapshot buffer so subscribers
			// can continue to read this and others after it.
			s.snapBuffer.AppendBuffer(next)
			return
		}
		// We don't need this item, continue to next
		item = next
	}
}

// err returns an error if the snapshot func has failed with an error or nil
// otherwise. Nil doesn't necessarily mean there won't be an error but there
// hasn't been one yet.
func (s *eventSnapshot) err() error {
	// Fetch the head of the buffer, this is atomic. If the snapshot func errored
	// then the last event will be an error.
	head := s.snapBuffer.Head()
	return head.Err
}