Fix slow watchers event loss after compaction

Signed-off-by: Chao Chen <[email protected]>

server/storage/mvcc/watchable_store.go

@@ -370,6 +370,12 @@ func (s *watchableStore) syncWatchers() int {
 	victims := make(watcherBatch)
 	wb := newWatcherBatch(wg, evs)
 	for w := range wg.watchers {
+		if w.minRev < compactionRev {
+			// skip the watcher that failed to send compacted watch response due to w.ch is full
+			// next retry of syncWatchers would try to resend the compacted watch response to w.ch
+			// TODO prioritize sending compacted watch response over other watch responses with events.
+			continue
+		}
 		w.minRev = curRev + 1
 
 		eb, ok := wb[w]