made the code more robust (against SCM computing null change set), and fixed...

made the code more robust (against SCM computing null change set), and fixed the race condition (if GC happens during this operation.)

core/src/main/java/hudson/model/AbstractBuild.java

@@ -766,17 +766,21 @@ public abstract class AbstractBuild<P extends AbstractProject<P,R>,R extends Abs
             }
         }
 
-        if (changeSet == null || changeSet.get() == null) // cached value
-            try {
-                changeSet = new WeakReference<ChangeLogSet<? extends Entry>>(calcChangeSet());
-            } finally {
-                // defensive check. if the calculation fails (such as through an exception),
-                // set a dummy value so that it'll work the next time. the exception will
-                // be still reported, giving the plugin developer an opportunity to fix it.
-                if (changeSet==null)
-                    changeSet=new WeakReference<ChangeLogSet<? extends Entry>>(ChangeLogSet.createEmpty(this));
-            }
-        return changeSet.get();
+        ChangeLogSet<? extends Entry> cs = null;
+        if (changeSet!=null)
+            cs = changeSet.get();
+
+        if (cs==null)
+            cs = calcChangeSet();
+
+        // defensive check. if the calculation fails (such as through an exception),
+        // set a dummy value so that it'll work the next time. the exception will
+        // be still reported, giving the plugin developer an opportunity to fix it.
+        if (cs==null)
+            cs = ChangeLogSet.createEmpty(this);
+
+        changeSet = new WeakReference<ChangeLogSet<? extends Entry>>(cs);
+        return cs;
     }
 
     /**