Fix thread leak and deadlock on terminating slaves

src/main/java/org/jenkinsci/plugins/mesos/JenkinsScheduler.java

@@ -211,6 +211,9 @@ public synchronized void terminateJenkinsSlave(String name) {
            if(request.request.slave.name.equals(name)) {
              LOGGER.info("Removing enqueued mesos task " + name);
              requests.remove(request);
+             // Also signal the Thread of the MesosComputerLauncher.launch() to exit from latch.await()
+             // Otherwise the Thread will stay in WAIT forever -> Leak!
+             request.result.failed(request.request.slave);
              return;
            }
         }

src/main/java/org/jenkinsci/plugins/mesos/MesosRetentionStrategy.java

@@ -16,6 +16,7 @@
 
 import static hudson.util.TimeUnit2.MINUTES;
 
+import java.util.concurrent.locks.ReentrantLock;
 import java.util.logging.Logger;
 
 import org.kohsuke.stapler.DataBoundConstructor;
@@ -35,6 +36,7 @@ public class MesosRetentionStrategy extends RetentionStrategy<MesosComputer> {
    * terminated.
    */
   public final int idleTerminationMinutes;
+  private ReentrantLock checkLock = new ReentrantLock(false);
 
   private static final Logger LOGGER = Logger
       .getLogger(MesosRetentionStrategy.class.getName());
@@ -43,8 +45,21 @@ public MesosRetentionStrategy(int idleTerminationMinutes) {
     this.idleTerminationMinutes = idleTerminationMinutes;
   }
 
+
   @Override
-  public synchronized long check(MesosComputer c) {
+  public long check(MesosComputer c) {
+    if (!checkLock.tryLock()) {
+      return 1;
+    } else {
+      try {
+        return checkInternal(c);
+      } finally {
+        checkLock.unlock();
+      }
+    }
+  }
+
+  private long checkInternal(MesosComputer c) {
     if (c.getNode() == null) {
       return 1;
     }