shubhamvishu commented on code in PR #12569:
URL: https://github.com/apache/lucene/pull/12569#discussion_r1329276879


##########
lucene/core/src/java/org/apache/lucene/search/TaskExecutor.java:
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@@ -22,18 +22,28 @@
 import java.util.Collection;
 import java.util.List;
 import java.util.Objects;
+import java.util.concurrent.Callable;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Executor;
 import java.util.concurrent.Future;
-import java.util.concurrent.RunnableFuture;
+import java.util.concurrent.FutureTask;
 import org.apache.lucene.util.IOUtils;
 import org.apache.lucene.util.ThreadInterruptedException;
 
 /**
  * Executor wrapper responsible for the execution of concurrent tasks. Used to 
parallelize search
- * across segments as well as query rewrite in some cases.
+ * across segments as well as query rewrite in some cases. Exposes a {@link 
#createTask(Callable)}
+ * method to create tasks given a {@link Callable}, as well as the {@link 
#invokeAll(Collection)}
+ * method to execute a set of tasks concurrently. Once all tasks are submitted 
to the executor, it
+ * blocks and wait for all tasks to be completed, and then returns a list with 
the obtained results.
+ * Ensures that the underlying executor is only used for top-level {@link 
#invokeAll(Collection)}
+ * calls, and not for potential {@link #invokeAll(Collection)} calls made from 
one of the tasks.
+ * This is to prevent deadlock with certain types of executors, as well as to 
limit the level of
+ * parallelism.

Review Comment:
   If have a question here, since we are limiting the levels of parallelism to 
1 will we be not affecting the executor implementations that won't deadlock due 
to this and instead wants to make use of multi level parallelism instead. Will 
we not be restricting those opportunities and maybe in future we would also 
want to multi-level parallelism?



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