[SPARK-21991][LAUNCHER] Fix race condition in LauncherServer#acceptCo…

…nnections

## What changes were proposed in this pull request?
This patch changes the order in which _acceptConnections_ starts the client thread and schedules the client timeout action ensuring that the latter has been scheduled before the former get a chance to cancel it.

## How was this patch tested?
Due to the non-deterministic nature of the patch I wasn't able to add a new test for this issue.

Author: Andrea zito <andrea.zito@u-hopper.com>

Closes #19217 from nivox/SPARK-21991.

(cherry picked from commit 6ea8a56)

launcher/src/main/java/org/apache/spark/launcher/LauncherServer.java

@@ -237,20 +237,20 @@ public void run() {
         };
         ServerConnection clientConnection = new ServerConnection(client, timeout);
         Thread clientThread = factory.newThread(clientConnection);
-        synchronized (timeout) {
-          clientThread.start();
-          synchronized (clients) {
-            clients.add(clientConnection);
-          }
-          long timeoutMs = getConnectionTimeout();
-          // 0 is used for testing to avoid issues with clock resolution / thread scheduling,
-          // and force an immediate timeout.
-          if (timeoutMs > 0) {
-            timeoutTimer.schedule(timeout, getConnectionTimeout());
-          } else {
-            timeout.run();
-          }
+        synchronized (clients) {
+          clients.add(clientConnection);
+        }
+
+        long timeoutMs = getConnectionTimeout();
+        // 0 is used for testing to avoid issues with clock resolution / thread scheduling,
+        // and force an immediate timeout.
+        if (timeoutMs > 0) {
+          timeoutTimer.schedule(timeout, timeoutMs);
+        } else {
+          timeout.run();
         }
+
+        clientThread.start();
       }
     } catch (IOException ioe) {
       if (running) {