Fix deadlock when afterDisconnect is called during launch() (#304)

Deadlock happens between:
(a) https://github.com/jenkinsci/ssh-slaves-plugin/blob/main/src/main/java/hudson/plugins/sshslaves/SSHLauncher.java#L953
(b) https://github.com/jenkinsci/ssh-slaves-plugin/blob/main/src/main/java/hudson/plugins/sshslaves/SSHLauncher.java#L413
Launch() is stuck here: https://github.com/jenkinsci/ssh-slaves-plugin/blob/main/src/main/java/hudson/plugins/sshslaves/SSHLauncher.java#L486 ,
and `callable` is stuck waiting here: https://github.com/jenkinsci/trilead-ssh2/blob/8e7118e74deacd7ef044df9076fcb9b95280c163/src/com/trilead/ssh2/channel/FifoBuffer.java#L212
After some time `afterDisconnect` is called which initiates tear down process. Teardown process calls launcherExecutorService.shutdown()
(here: https://github.com/jenkinsci/ssh-slaves-plugin/blob/main/src/main/java/hudson/plugins/sshslaves/SSHLauncher.java#L941) and continues to (a).
But because there is no writer writing to FifoBuffer, launch() never finishes and is stuck in (b), and because `shutdown()` only prevents new tasks being submitted
(https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/concurrent/ExecutorService.html) there is no one to interrupt lock.wait() in FifoBuffer, thus deadlock. The solutions is to use recommended way of shutting down ExecutorService from docs.

Author: tomekjarosik

src/main/java/hudson/plugins/sshslaves/SSHLauncher.java

@@ -505,11 +505,7 @@ public void launch(final SlaveComputer computer, final TaskListener listener) th
                 System.out.println(Messages.SSHLauncher_LaunchFailed(getTimestamp(),
                         nodeName, host));
             } finally {
-                ExecutorService srv = launcherExecutorService;
-                if (srv != null) {
-                    srv.shutdownNow();
-                    launcherExecutorService = null;
-                }
+              shutdownAndAwaitTerminationOfLauncher();
             }
         }
         if (node != null && getTrackCredentials()) {
@@ -934,12 +930,7 @@ public void afterDisconnect(SlaveComputer slaveComputer, final TaskListener list
             // Nothing to do here, the connection is not established
             return;
         }
-
-        ExecutorService srv = launcherExecutorService;
-        if (srv != null) {
-            // If the service is still running, shut it down and interrupt the operations if any
-            srv.shutdown();
-        }
+        shutdownAndAwaitTerminationOfLauncher();
 
         if (tearingDownConnection) {
             // tear down operation is in progress, do not even try to synchronize the call.
@@ -982,6 +973,30 @@ private void tearDownConnectionImpl(@NonNull SlaveComputer slaveComputer, final
         }
     }
 
+    private void shutdownAndAwaitTerminationOfLauncher() {
+      ExecutorService srv = launcherExecutorService;
+      if (srv == null) {
+          return;
+      }
+      srv.shutdown(); // Disable new tasks from being submitted
+      launcherExecutorService = null;
+      try {
+          // Wait a while for existing tasks to terminate
+          if (!srv.awaitTermination(10, TimeUnit.SECONDS)) {
+              srv.shutdownNow(); // Cancel currently executing tasks
+              // Wait a while for tasks to respond to being cancelled
+              if (!srv.awaitTermination(10, TimeUnit.SECONDS)) {
+                  LOGGER.log(WARNING, "launcherExecutorService thread pool did not terminate cleanly");
+              }
+          }
+      } catch (InterruptedException ie) {
+        // (Re-)Cancel if current thread also interrupted
+        srv.shutdownNow();
+        // Preserve interrupt status
+        Thread.currentThread().interrupt();
+      }
+    }
+
     /**
      * If the SSH connection as a whole is lost, report that information.
      */