Use read/write engine lock to guard operations against resets (elastic#124635)

Today shard's engine mutation are guarded by an engineMutex object monitor. But we would like to be able to execute one or more operations on an engine instance, without this instance being resetted during the execution of the operation.

In order to do that, this change replaces the engineMutex by a reentrant read/write lock and introduces two new methods IndexShard#withEngine() and IndexShard#withEngineOrNull() that can be used to execute an operation while avoiding the current engine instance to be reset. It does not prevent it to be closed during execution though.

Relates ES-10826

Co-authored-by: Francisco Fernández Castaño <[email protected]>

Author: 2 people

server/src/main/java/org/elasticsearch/index/engine/Engine.java

@@ -2371,7 +2371,7 @@ public record FlushResult(boolean flushPerformed, long generation) {
     }
 
     /**
-     * Ensures the engine is in a state that it can be closed by a call to {@link IndexShard#resetEngine()}.
+     * Ensures the engine is in a state that it can be closed by a call to {@link IndexShard#resetEngine(Consumer<Engine>)}.
      *
      * In general, resetting the engine should be done with care, to consider any
      * in-progress operations and listeners (e.g., primary term and generation listeners).