stream.EventSnapshot: reduce the fields on the struct

Many of the fields are only needed in one place, and by using a closure
they can be removed from the struct. This reduces the scope of the variables
making it esier to see how they are used.

agent/consul/stream/event_snapshot.go

@@ -3,30 +3,19 @@ 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 just keeping the Snapshot around. Once the EventSnapshot is dropped
-// from memory, any subscribers still reading from it may do so by following
-// their pointers but eventually the snapshot is garbage collected automatically
-// by Go's runtime, simplifying snapshot and buffer management dramatically.
+// 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 {
-	// Request that this snapshot satisfies.
-	Request *SubscribeRequest
-
 	// Snap is the first item in the buffer containing the snapshot. Once the
-	// snapshot is complete, subsequent update's BufferItems are appended such
-	// that subscribers just need to follow this buffer for the duration of their
-	// subscription stream.
+	// 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
-
-	// topicBufferHead stored the current most-recent published item from before
-	// the snapshot was taken such that anything published during snapshot
-	// publishing can be captured.
-	topicBufferHead *BufferItem
-
-	// SnapFn is the function that will make the snapshot for this request.
-	fn SnapFn
 }
 
 // SnapFn is the type of function needed to generate a snapshot for a topic and
@@ -43,39 +32,32 @@ type SnapFn func(req *SubscribeRequest, buf *EventBuffer) (uint64, error)
 func NewEventSnapshot(req *SubscribeRequest, topicBufferHead *BufferItem, fn SnapFn) *EventSnapshot {
 	buf := NewEventBuffer()
 	s := &EventSnapshot{
-		Request:         req,
-		Snap:            buf.Head(),
-		snapBuffer:      buf,
-		topicBufferHead: topicBufferHead,
-		fn:              fn,
+		Snap:       buf.Head(),
+		snapBuffer: buf,
 	}
-	go s.doSnapshot()
+
+	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) doSnapshot() {
-	// Call snapshot func
-	idx, err := s.fn(s.Request, s.snapBuffer)
-	if err != nil {
-		// Append an error result to signal to subscribers that this snapshot is no
-		// good.
-		s.snapBuffer.AppendErr(err)
-		return
-	}
-
-	// We wrote the snapshot events to the buffer, send the "end of snapshot" event
-	s.snapBuffer.Append([]Event{{
-		Topic: s.Request.Topic,
-		Key:   s.Request.Key,
-		Index: idx,
-		Payload: &agentpb.Event_EndOfSnapshot{
-			EndOfSnapshot: true,
-		},
-	}})
-
+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 := s.topicBufferHead
+	item := topicBufferHead
 	for {
 		// Find the next item that we should include.
 		next, err := item.NextNoBlock()
@@ -108,14 +90,12 @@ func (s *EventSnapshot) doSnapshot() {
 			return
 		}
 
-		if len(next.Events) > 0 {
-			if 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
-			}
+		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