Ensure shard is refreshed once it's inactive

Once a shard goes inactive we want the shard to be refreshed if
the refresh interval is default since we might hold on to unnecessary
segments and in the inactive case we stopped indexing and can release
old segments.

Relates to elastic#27500

core/src/main/java/org/elasticsearch/index/shard/IndexShard.java

@@ -872,7 +872,7 @@ public DocsStats docStats() {
         long numDeletedDocs = 0;
         long sizeInBytes = 0;
         try (Engine.Searcher searcher = acquireSearcher("docStats", Engine.SearcherScope.INTERNAL)) {
-            // we don't wait for a pending refreshes here since it's a stats call instead we mark it as accesssed only which will cause
+            // we don't wait for a pending refreshes here since it's a stats call instead we mark it as accessed only which will cause
             // the next scheduled refresh to go through and refresh the stats as well
             markSearcherAccessed();
             for (LeafReaderContext reader : searcher.reader().leaves()) {
@@ -971,7 +971,7 @@ public TranslogStats translogStats() {
     public CompletionStats completionStats(String... fields) {
         CompletionStats completionStats = new CompletionStats();
         try (Engine.Searcher currentSearcher = acquireSearcher("completion_stats")) {
-            // we don't wait for a pending refreshes here since it's a stats call instead we mark it as accesssed only which will cause
+            // we don't wait for a pending refreshes here since it's a stats call instead we mark it as accessed only which will cause
             // the next scheduled refresh to go through and refresh the stats as well
             markSearcherAccessed();
             completionStats.add(CompletionFieldStats.completionStats(currentSearcher.reader(), fields));
@@ -2456,7 +2456,9 @@ public boolean scheduledRefresh() {
         boolean listenerNeedsRefresh = refreshListeners.refreshNeeded();
         if (isReadAllowed() && (listenerNeedsRefresh || getEngine().refreshNeeded())) {
             if (listenerNeedsRefresh == false // if we have a listener that is waiting for a refresh we need to force it
-                && isSearchIdle() && indexSettings.isExplicitRefresh() == false) {
+                && isSearchIdle()
+                && indexSettings.isExplicitRefresh() == false
+                && active.get()) { // it must be active otherwise we might not free up segment memory once the shard became inactive
                 // lets skip this refresh since we are search idle and
                 // don't necessarily need to refresh. the next searcher access will register a refreshListener and that will
                 // cause the next schedule to refresh.

core/src/test/java/org/elasticsearch/index/shard/IndexShardTests.java

@@ -2684,6 +2684,15 @@ public void testScheduledRefresh() throws IOException, InterruptedException {
 
         });
         latch1.await();
+
+        indexDoc(primary, "test", "2", "{\"foo\" : \"bar\"}");
+        assertFalse(primary.scheduledRefresh());
+        assertTrue(primary.isSearchIdle());
+        primary.checkIdle(0);
+        assertTrue(primary.scheduledRefresh()); // make sure we refresh once the shard is inactive
+        try (Engine.Searcher searcher = primary.acquireSearcher("test")) {
+            assertEquals(3, searcher.reader().numDocs());
+        }
         closeShards(primary);
     }