Avoid selecting an outdated merger from cache

We cache a merger for partial input as well, because it is automatically
invalidated as soon as the new data comes in.

However, there was a race condition where a cached merger for a partial
input is used even after the input stream was complete. This commit
fixes the problem.

Fix #4034

src/matcher.go

@@ -102,7 +102,7 @@ func (m *Matcher) Loop() {
 		if !cacheCleared {
 			if count == prevCount {
 				// Look up mergerCache
-				if cached, found := m.mergerCache[patternString]; found {
+				if cached, found := m.mergerCache[patternString]; found && cached.final == request.final {
 					merger = cached
 				}
 			} else {