Core: fix index throttling to notice a change to max_merge_count on the next merge start/finish

Previously such changes were never noticed, and index throttling kept
kicking in at the original setting from startup.

Closes #8136

Closes #8132

src/main/java/org/elasticsearch/index/engine/internal/InternalEngine.java

@@ -272,7 +272,7 @@ public class InternalEngine extends AbstractIndexShardComponent implements Engin
             try {
                 this.indexWriter = createWriter();
                 mergeScheduler.removeListener(this.throttle);
-                this.throttle = new IndexThrottle(mergeScheduler.getMaxMerges(), logger);
+                this.throttle = new IndexThrottle(mergeScheduler, logger);
                 mergeScheduler.addListener(throttle);
             } catch (IOException e) {
                 maybeFailEngine(e, "start");
@@ -844,7 +844,7 @@ public class InternalEngine extends AbstractIndexShardComponent implements Engin
                         currentIndexWriter().close(false);
                         indexWriter = createWriter();
                         mergeScheduler.removeListener(this.throttle);
-                        this.throttle = new IndexThrottle(mergeScheduler.getMaxMerges(), this.logger);
+                        this.throttle = new IndexThrottle(mergeScheduler, this.logger);
                         mergeScheduler.addListener(throttle);
                         // commit on a just opened writer will commit even if there are no changes done to it
                         // we rely on that for the commit data translog id key
@@ -1722,13 +1722,13 @@ public class InternalEngine extends AbstractIndexShardComponent implements Engin
         private final InternalLock lockReference = new InternalLock(new ReentrantLock());
         private final AtomicInteger numMergesInFlight = new AtomicInteger(0);
         private final AtomicBoolean isThrottling = new AtomicBoolean();
-        private final int maxNumMerges;
+        private final MergeSchedulerProvider mergeScheduler;
         private final ESLogger logger;
 
         private volatile InternalLock lock = NOOP_LOCK;
 
-        public IndexThrottle(int maxNumMerges, ESLogger logger) {
-            this.maxNumMerges = maxNumMerges;
+        public IndexThrottle(MergeSchedulerProvider mergeScheduler, ESLogger logger) {
+            this.mergeScheduler = mergeScheduler;
             this.logger = logger;
         }
 
@@ -1738,6 +1738,7 @@ public class InternalEngine extends AbstractIndexShardComponent implements Engin
 
         @Override
         public synchronized void beforeMerge(OnGoingMerge merge) {
+            int maxNumMerges = mergeScheduler.getMaxMerges();
             if (numMergesInFlight.incrementAndGet() > maxNumMerges) {
                 if (isThrottling.getAndSet(true) == false) {
                     logger.info("now throttling indexing: numMergesInFlight={}, maxNumMerges={}", numMergesInFlight, maxNumMerges);
@@ -1748,6 +1749,7 @@ public class InternalEngine extends AbstractIndexShardComponent implements Engin
 
         @Override
         public synchronized void afterMerge(OnGoingMerge merge) {
+            int maxNumMerges = mergeScheduler.getMaxMerges();
             if (numMergesInFlight.decrementAndGet() < maxNumMerges) {
                 if (isThrottling.getAndSet(false)) {
                     logger.info("stop throttling indexing: numMergesInFlight={}, maxNumMerges={}", numMergesInFlight, maxNumMerges);