[FIXED JENKINS-13735]

Added test whether the currently checked slave computer actually can take the buildable item before flagging it as needed (avoids powering up and connecting to slaves for jobs they can't build).

changelog.html

@@ -55,6 +55,9 @@ Upcoming changes</a>
 <!-- Record your changes in the trunk here. -->
 <div id="trunk" style="display:none"><!--=TRUNK-BEGIN=-->
 <ul class=image>
+  <li class=bug>
+    With 'on-demand' retention strategy, wrong slave can be started for jobs restricted to specific slave.
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-13735">issue 13735</a>)
   <li class=bug>
     Fixed encoding handling in e-mail headers.
     (<a href="https://github.com/jenkinsci/jenkins/pull/486">pull 486</a>)

core/src/main/java/hudson/slaves/RetentionStrategy.java

@@ -199,13 +199,16 @@ public abstract class RetentionStrategy<T extends Computer> extends AbstractDesc
                 for (Computer o : Jenkins.getInstance().getComputers()) {
                     if ((o.isOnline() || o.isConnecting()) && o.isPartiallyIdle()) {
                         final int idleExecutors = o.countIdle();
-                        availableComputers.put(o, idleExecutors);
+                        if (idleExecutors>0)
+                            availableComputers.put(o, idleExecutors);
                     }
                 }
 
                 boolean needComputer = false;
                 long demandMilliseconds = 0;
                 for (Queue.BuildableItem item : Queue.getInstance().getBuildableItems()) {
+                    // can any of the currently idle executors take this task?
+                    // assume the answer is no until we can find such an executor
                     boolean needExecutor = true;
                     for (Computer o : Collections.unmodifiableSet(availableComputers.keySet())) {
                         if (o.getNode().canTake(item) == null) {
@@ -213,12 +216,15 @@ public abstract class RetentionStrategy<T extends Computer> extends AbstractDesc
                             final int availableExecutors = availableComputers.remove(o);
                             if (availableExecutors > 1) {
                                 availableComputers.put(o, availableExecutors - 1);
+                            } else {
+                                availableComputers.remove(o);
                             }
                             break;
                         }
                     }
 
-                    if (needExecutor) {
+                    // this 'item' cannot be built by any of the existing idle nodes, but it can be built by 'c'
+                    if (needExecutor && c.getNode().canTake(item) == null) {
                         demandMilliseconds = System.currentTimeMillis() - item.buildableStartMilliseconds;
                         needComputer = demandMilliseconds > inDemandDelay * 1000 * 60 /*MINS->MILLIS*/;
                         break;