Fix ConcurrentModificationException in getDeclaredResources and getResourcesFromProject

Add synchronized(syncResources) blocks around iterations in
getDeclaredResources() and getResourcesFromProject() to prevent
ConcurrentModificationException when the resources list is modified
concurrently by another thread.

Also changes both methods to iterate over this.resources directly
(inside the sync block) instead of this.getResources() which returns
an unprotected reference.

Fixes #933

Author: mPokornyETM

src/main/java/org/jenkins/plugins/lockableresources/LockableResourcesManager.java

@@ -155,13 +155,15 @@ public List<LockableResource> getReadOnlyResources() {
     /** Get declared resources, means only defined in config file (xml or JCaC yaml). */
     @Restricted(NoExternalUse.class)
     public List<LockableResource> getDeclaredResources() {
-        ArrayList<LockableResource> declaredResources = new ArrayList<>();
-        for (LockableResource r : this.getResources()) {
-            if (!r.isEphemeral() && !r.isNodeResource()) {
-                declaredResources.add(r);
+        synchronized (syncResources) {
+            ArrayList<LockableResource> declaredResources = new ArrayList<>();
+            for (LockableResource r : this.resources) {
+                if (!r.isEphemeral() && !r.isNodeResource()) {
+                    declaredResources.add(r);
+                }
             }
+            return declaredResources;
         }
-        return declaredResources;
     }
 
     // ---------------------------------------------------------------------------
@@ -225,14 +227,16 @@ public void setDeclaredResources(List<LockableResource> declaredResources) {
     /** Get all resources used by project. */
     @Restricted(NoExternalUse.class)
     public List<LockableResource> getResourcesFromProject(String fullName) {
-        List<LockableResource> matching = new ArrayList<>();
-        for (LockableResource r : this.getResources()) {
-            String rName = r.getQueueItemProject();
-            if (rName != null && rName.equals(fullName)) {
-                matching.add(r);
+        synchronized (syncResources) {
+            List<LockableResource> matching = new ArrayList<>();
+            for (LockableResource r : this.resources) {
+                String rName = r.getQueueItemProject();
+                if (rName != null && rName.equals(fullName)) {
+                    matching.add(r);
+                }
             }
+            return matching;
         }
-        return matching;
     }
 
     // ---------------------------------------------------------------------------