Experimental change to prevent offlining of slave

Check to see if any items in the queue (including blocked items) ask for this node explicitly.
This is the case for individual Ivy/Maven Module builds which must build on the same node as their parent.
If there are any then don't offline this node.

src/main/java/hudson/plugins/ec2/EC2RetentionStrategy.java

@@ -32,6 +32,7 @@
 import hudson.model.ExecutorListener;
 import hudson.model.Queue;
 import hudson.plugins.ec2.util.MinimumInstanceChecker;
+import hudson.model.Label;
 import hudson.slaves.RetentionStrategy;
 import jenkins.model.Jenkins;
 
@@ -197,7 +198,8 @@ private long internalCheck(EC2Computer computer) {
                 // TODO: really think about the right strategy here, see
                 // JENKINS-23792
 
-                if (idleMilliseconds > TimeUnit.MINUTES.toMillis(idleTerminationMinutes)) {
+                if (idleMilliseconds > TimeUnit.MINUTES.toMillis(idleTerminationMinutes) &&
+                                        !itemsInQueueForThisSlave(computer)){
 
                     LOGGER.info("Idle timeout of " + computer.getName() + " after "
                             + TimeUnit.MILLISECONDS.toMinutes(idleMilliseconds) +
@@ -216,7 +218,7 @@ private long internalCheck(EC2Computer computer) {
                 // if we have less "free" (aka already paid for) time left than
                 // our idle time, stop/terminate the instance
                 // See JENKINS-23821
-                if (freeSecondsLeft <= TimeUnit.MINUTES.toSeconds(Math.abs(idleTerminationMinutes))) {
+                if (freeSecondsLeft <= TimeUnit.MINUTES.toSeconds(Math.abs(idleTerminationMinutes)) && !itemsInQueueForThisSlave(computer)) {
                     LOGGER.info("Idle timeout of " + computer.getName() + " after "
                             + TimeUnit.MILLISECONDS.toMinutes(idleMilliseconds) + " idle minutes, with "
                             + TimeUnit.SECONDS.toMinutes(freeSecondsLeft)
@@ -231,6 +233,28 @@ private long internalCheck(EC2Computer computer) {
         return 1;
     }
 
+    /*
+     * Checks if there are any items in the queue that are waiting for this node explicitly.
+     * This prevents a node from being taken offline while there are Ivy/Maven Modules waiting to build.
+     * Need to check entire queue as some modules may be blocked by upstream dependencies.
+     * Accessing the queue in this way can block other threads, so only perform this check just prior
+     * to timing out the slave.
+     */
+    private boolean itemsInQueueForThisSlave(EC2Computer c) {
+        final Label selfLabel = c.getNode().getSelfLabel();
+        Queue.Item[] items = Jenkins.getInstance().getQueue().getItems();
+        for (int i = 0; i < items.length; i++) {
+            Queue.Item item = items[i];
+            final Label assignedLabel = item.getAssignedLabel();
+            if (assignedLabel == selfLabel) {
+                LOGGER.fine("Preventing idle timeout of " + c.getName()
+                        + " as there is at least one item in the queue explicitly waiting for this slave");
+                return true;
+            }
+        }
+        return false;
+    }
+
     /**
      * Called when a new {@link EC2Computer} object is introduced (such as when Hudson started, or when
      * a new agent is added.)