Allow flush/force_merge/upgrade on shard marked as relocated (#22078)

A shard that is locally marked as relocated, but where the relocation target shard has not been activated yet by the master, can still receive index operations, which in return can lead to flushes being triggered. Flushing is currently (wrongly) prohibited on shards marked as relocated, which makes the flushing process go into an endless retry loop and log warnings until the shard is closed. This commit fixes this situation by allowing flush, force_merge and upgrade operations to run on shards that are marked as relocated.

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

@@ -759,7 +759,7 @@ public class IndexShard extends AbstractIndexShardComponent implements IndicesCl
     }
 
     public Engine.SyncedFlushResult syncFlush(String syncId, Engine.CommitId expectedCommitId) {
-        verifyStartedOrRecovering();
+        verifyNotClosed();
         logger.trace("trying to sync flush. sync id [{}]. expected commit id [{}]]", syncId, expectedCommitId);
         Engine engine = getEngine();
         if (engine.isRecovering()) {
@@ -779,7 +779,7 @@ public class IndexShard extends AbstractIndexShardComponent implements IndicesCl
         // while recovering, and we want to keep the translog at bay (up to deletes, which
         // we don't gc). Yet, we don't use flush internally to clear deletes and flush the indexwriter since
         // we use #writeIndexingBuffer for this now.
-        verifyStartedOrRecovering();
+        verifyNotClosed();
         Engine engine = getEngine();
         if (engine.isRecovering()) {
             throw new IllegalIndexShardStateException(shardId(), state, "flush is only allowed if the engine is not recovery" +
@@ -793,7 +793,7 @@ public class IndexShard extends AbstractIndexShardComponent implements IndicesCl
     }
 
     public void forceMerge(ForceMergeRequest forceMerge) throws IOException {
-        verifyStarted();
+        verifyActive();
         if (logger.isTraceEnabled()) {
             logger.trace("force merge with {}", forceMerge);
         }
@@ -805,7 +805,7 @@ public class IndexShard extends AbstractIndexShardComponent implements IndicesCl
      * Upgrades the shard to the current version of Lucene and returns the minimum segment version
      */
     public org.apache.lucene.util.Version upgrade(UpgradeRequest upgrade) throws IOException {
-        verifyStarted();
+        verifyActive();
         if (logger.isTraceEnabled()) {
             logger.trace("upgrade with {}", upgrade);
         }
@@ -1148,13 +1148,6 @@ public class IndexShard extends AbstractIndexShardComponent implements IndicesCl
         }
     }
 
-    protected final void verifyStartedOrRecovering() throws IllegalIndexShardStateException {
-        IndexShardState state = this.state; // one time volatile read
-        if (state != IndexShardState.STARTED && state != IndexShardState.RECOVERING && state != IndexShardState.POST_RECOVERY) {
-            throw new IllegalIndexShardStateException(shardId, state, "operation only allowed when started/recovering");
-        }
-    }
-
     private void verifyNotClosed() throws IllegalIndexShardStateException {
         verifyNotClosed(null);
     }
@@ -1170,10 +1163,10 @@ public class IndexShard extends AbstractIndexShardComponent implements IndicesCl
         }
     }
 
-    protected final void verifyStarted() throws IllegalIndexShardStateException {
+    protected final void verifyActive() throws IllegalIndexShardStateException {
         IndexShardState state = this.state; // one time volatile read
-        if (state != IndexShardState.STARTED) {
-            throw new IndexShardNotStartedException(shardId, state);
+        if (state != IndexShardState.STARTED && state != IndexShardState.RELOCATED) {
+            throw new IllegalIndexShardStateException(shardId, state, "operation only allowed when shard is active");
         }
     }