[Draft] Reuse events used for syncing watchers

server/storage/mvcc/watchable_store.go

@@ -216,6 +216,7 @@ func (s *watchableStore) syncWatchersLoop() {
 	waitDuration := 100 * time.Millisecond
 	delayTicker := time.NewTicker(waitDuration)
 	defer delayTicker.Stop()
+	var evs []mvccpb.Event
 
 	for {
 		s.mu.RLock()
@@ -225,7 +226,7 @@ func (s *watchableStore) syncWatchersLoop() {
 
 		unsyncedWatchers := 0
 		if lastUnsyncedWatchers > 0 {
-			unsyncedWatchers = s.syncWatchers()
+			unsyncedWatchers, evs = s.syncWatchers(evs)
 		}
 		syncDuration := time.Since(st)
 
@@ -334,12 +335,12 @@ func (s *watchableStore) moveVictims() (moved int) {
 //  2. iterate over the set to get the minimum revision and remove compacted watchers
 //  3. use minimum revision to get all key-value pairs and send those events to watchers
 //  4. remove synced watchers in set from unsynced group and move to synced group
-func (s *watchableStore) syncWatchers() int {
+func (s *watchableStore) syncWatchers(evs []mvccpb.Event) (int, []mvccpb.Event) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 
 	if s.unsynced.size() == 0 {
-		return 0
+		return 0, evs
 	}
 
 	s.store.revMu.RLock()
@@ -352,23 +353,42 @@ func (s *watchableStore) syncWatchers() int {
 	compactionRev := s.store.compactMainRev
 
 	wg, minRev := s.unsynced.choose(maxWatchersPerSync, curRev, compactionRev)
-	minBytes, maxBytes := NewRevBytes(), NewRevBytes()
-	minBytes = RevToBytes(Revision{Main: minRev}, minBytes)
-	maxBytes = RevToBytes(Revision{Main: curRev + 1}, maxBytes)
 
-	// UnsafeRange returns keys and values. And in boltdb, keys are revisions.
-	// values are actual key-value pairs in backend.
-	tx := s.store.b.ReadTx()
-	tx.RLock()
-	revs, vs := tx.UnsafeRange(schema.Key, minBytes, maxBytes, 0)
-	evs := kvsToEvents(s.store.lg, wg, revs, vs)
-	// Must unlock after kvsToEvents, because vs (come from boltdb memory) is not deep copy.
-	// We can only unlock after Unmarshal, which will do deep copy.
-	// Otherwise we will trigger SIGSEGV during boltdb re-mmap.
-	tx.RUnlock()
+	if len(evs) == 0 {
+		evs = s.events(minRev, curRev+1)
+	} else {
+		if evs[0].Kv.ModRevision > minRev {
+			evs = append(s.events(minRev, evs[0].Kv.ModRevision), evs...)
+		}
+
+		if len(evs) == 0 {
+			evs = s.events(minRev, curRev+1)
+		}
+		if len(evs) != 0 && evs[len(evs)-1].Kv.ModRevision < curRev {
+			evs = append(evs, s.events(evs[len(evs)-1].Kv.ModRevision+1, curRev+1)...)
+		}
+	}
+	// TODO: Ensure thread safe access to victims
+	victimsMinRev := minRev
+	for _, vic := range s.victims {
+		for w := range vic {
+			if victimsMinRev > w.minRev {
+				victimsMinRev = w.minRev
+			}
+		}
+	}
+	victimsRevIndex := 0
+	for victimsRevIndex < len(evs) && evs[victimsRevIndex].Kv.ModRevision < compactionRev {
+		victimsRevIndex++
+	}
+	evs = evs[victimsRevIndex:]
+	minRevIndex := 0
+	for minRevIndex < len(evs) && evs[minRevIndex].Kv.ModRevision < minRev {
+		minRevIndex++
+	}
 
 	victims := make(watcherBatch)
-	wb := newWatcherBatch(wg, evs)
+	wb := newWatcherBatch(wg, evs[minRevIndex:])
 	for w := range wg.watchers {
 		w.minRev = curRev + 1
 
@@ -409,21 +429,34 @@ func (s *watchableStore) syncWatchers() int {
 	}
 	slowWatcherGauge.Set(float64(s.unsynced.size() + vsz))
 
-	return s.unsynced.size()
+	return s.unsynced.size(), evs
+}
+
+func (s *watchableStore) events(minRev, maxRev int64) []mvccpb.Event {
+	minBytes, maxBytes := NewRevBytes(), NewRevBytes()
+	minBytes = RevToBytes(Revision{Main: minRev}, minBytes)
+	maxBytes = RevToBytes(Revision{Main: maxRev}, maxBytes)
+	// UnsafeRange returns keys and values. And in boltdb, keys are revisions.
+	// values are actual key-value pairs in backend.
+	tx := s.store.b.ReadTx()
+	tx.RLock()
+	revs, vs := tx.UnsafeRange(schema.Key, minBytes, maxBytes, 0)
+	evs := kvsToEvents(s.store.lg, revs, vs)
+	// Must unlock after kvsToEvents, because vs (come from boltdb memory) is not deep copy.
+	// We can only unlock after Unmarshal, which will do deep copy.
+	// Otherwise we will trigger SIGSEGV during boltdb re-mmap.
+	tx.RUnlock()
+	return evs
 }
 
 // kvsToEvents gets all events for the watchers from all key-value pairs
-func kvsToEvents(lg *zap.Logger, wg *watcherGroup, revs, vals [][]byte) (evs []mvccpb.Event) {
+func kvsToEvents(lg *zap.Logger, revs, vals [][]byte) (evs []mvccpb.Event) {
 	for i, v := range vals {
 		var kv mvccpb.KeyValue
 		if err := kv.Unmarshal(v); err != nil {
 			lg.Panic("failed to unmarshal mvccpb.KeyValue", zap.Error(err))
 		}
 
-		if !wg.contains(string(kv.Key)) {
-			continue
-		}
-
 		ty := mvccpb.PUT
 		if isTombstone(revs[i]) {
 			ty = mvccpb.DELETE