[SPARK-40932][CORE] Fix issue messages for allGather are overridden

### What changes were proposed in this pull request?

The messages returned by allGather may be overridden by the following barrier APIs, eg,

``` scala
      val messages: Array[String] = context.allGather("ABC")
      context.barrier()
```

the  `messages` may be like Array("", ""), but we're expecting Array("ABC", "ABC")

The root cause of this issue is the [messages got by allGather](https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/BarrierTaskContext.scala#L102) pointing to the [original message](https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/BarrierCoordinator.scala#L107) in the local mode. So when the following barrier APIs changed the messages, then the allGather message will be changed accordingly.
Finally, users can't get the correct result.

This PR fixed this issue by sending back the cloned messages.

### Why are the changes needed?

The bug mentioned in this description may block some external SPARK ML libraries which heavily depend on the spark barrier API to do some synchronization. If the barrier mechanism can't guarantee the correctness of the barrier APIs, it will be a disaster for external SPARK ML libraries.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?

I added a unit test, with this PR, the unit test can pass

Closes apache#38410 from wbo4958/allgather-issue.

Authored-by: Bobby Wang <wbo4958@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

core/src/main/scala/org/apache/spark/BarrierCoordinator.scala

@@ -176,7 +176,7 @@ private[spark] class BarrierCoordinator(
         logInfo(s"Barrier sync epoch $barrierEpoch from $barrierId received update from Task " +
           s"$taskId, current progress: ${requesters.size}/$numTasks.")
         if (requesters.size == numTasks) {
-          requesters.foreach(_.reply(messages))
+          requesters.foreach(_.reply(messages.clone()))
           // Finished current barrier() call successfully, clean up ContextBarrierState and
           // increase the barrier epoch.
           logInfo(s"Barrier sync epoch $barrierEpoch from $barrierId received all updates from " +

core/src/test/scala/org/apache/spark/scheduler/BarrierTaskContextSuite.scala

@@ -367,4 +367,27 @@ class BarrierTaskContextSuite extends SparkFunSuite with LocalSparkContext with
     // double check we kill task success
     assert(System.currentTimeMillis() - startTime < 5000)
   }
+
+  test("SPARK-40932, messages of allGather should not been overridden " +
+    "by the following barrier APIs") {
+
+    sc = new SparkContext(new SparkConf().setAppName("test").setMaster("local[2]"))
+    sc.setLogLevel("INFO")
+    val rdd = sc.makeRDD(1 to 10, 2)
+    val rdd2 = rdd.barrier().mapPartitions { it =>
+      val context = BarrierTaskContext.get()
+      // Sleep for a random time before global sync.
+      Thread.sleep(Random.nextInt(1000))
+      // Pass partitionId message in
+      val message: String = context.partitionId().toString
+      val messages: Array[String] = context.allGather(message)
+      context.barrier()
+      Iterator.single(messages.toList)
+    }
+    val messages = rdd2.collect()
+    // All the task partitionIds are shared across all tasks
+    assert(messages.length === 2)
+    assert(messages.forall(_ == List("0", "1")))
+  }
+
 }