[SPARK-36532][CORE] Fix deadlock in CoarseGrainedExecutorBackend.onDisconnected to avoid executor shutdown hang

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

Instead of exiting the executor within the RpcEnv's thread, exit the executor in a separate thread.

### Why are the changes needed?

The current exit way in `onDisconnected` can cause the deadlock, which has the exact same root cause with https://github.com/apache/spark/pull/12012:

* `onDisconnected` -> `System.exit` are called in sequence in the thread of `MessageLoop.threadpool`
* `System.exit` triggers shutdown hooks and `executor.stop` is one of the hooks.
* `executor.stop` stops the `Dispatcher`, which waits for the `MessageLoop.threadpool`  to shutdown further.
* Thus, the thread which runs `System.exit` waits for hooks to be done, but the `MessageLoop.threadpool` in the hook waits that thread to finish. Finally, this mutual dependence results in the deadlock.

### Does this PR introduce _any_ user-facing change?

Yes, the executor shutdown won't hang.

### How was this patch tested?

Pass existing tests.

Closes #33759 from Ngone51/fix-executor-shutdown-hang.

Authored-by: yi.wu <yi.wu@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

core/src/main/scala/org/apache/spark/executor/CoarseGrainedExecutorBackend.scala

@@ -202,11 +202,17 @@ private[spark] class CoarseGrainedExecutorBackend(
       stopping.set(true)
       new Thread("CoarseGrainedExecutorBackend-stop-executor") {
         override def run(): Unit = {
-          // executor.stop() will call `SparkEnv.stop()` which waits until RpcEnv stops totally.
+          // `executor` can be null if there's any error in `CoarseGrainedExecutorBackend.onStart`
-          // However, if `executor.stop()` runs in some thread of RpcEnv, RpcEnv won't be able to
+          // or fail to create `Executor`.
-          // stop until `executor.stop()` returns, which becomes a dead-lock (See SPARK-14180).
+          if (executor == null) {
-          // Therefore, we put this line in a new thread.
+            System.exit(1)
-          executor.stop()
+          } else {
+            // executor.stop() will call `SparkEnv.stop()` which waits until RpcEnv stops totally.
+            // However, if `executor.stop()` runs in some thread of RpcEnv, RpcEnv won't be able to
+            // stop until `executor.stop()` returns, which becomes a dead-lock (See SPARK-14180).
+            // Therefore, we put this line in a new thread.
+            executor.stop()
+          }
         }
       }.start()
 
@@ -286,8 +292,7 @@ private[spark] class CoarseGrainedExecutorBackend(
       if (notifyDriver && driver.nonEmpty) {
         driver.get.send(RemoveExecutor(executorId, new ExecutorLossReason(reason)))
       }
-
+      self.send(Shutdown)
-      System.exit(code)
     } else {
       logInfo("Skip exiting executor since it's been already asked to exit before.")
     }