(2.6) dcache-webadmin: put limit on entries in alarms table

In order to avoid OOM.

This is hard-coded as effectively the capacity to hold more than 10000 entries in memory should not be required.  One can always adjust the numbers or refine the query in order to retrieve subsets of a larger set.

Testing: Rechecked interactively.

Target: 2.6
Patch: http://rb.dcache.org/r/6344/
Bug: http://rt.dcache.org/Ticket/Display.html?id=8084
Require-book: no
Require-notes: no
Acked-by: Karsten
Committed: 0bd6afa

modules/webadmin/src/main/java/org/dcache/webadmin/model/util/AlarmJDOUtils.java

@@ -83,6 +83,7 @@ LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
  * @author arossi
  */
 public class AlarmJDOUtils {
+    private static final int MAXIMUM_QUERY_RESULTS = 10000;
 
     public static class AlarmDAOFilter {
         private String filter;
@@ -143,17 +144,24 @@ public static Query createQuery(PersistenceManager pm, AlarmDAOFilter filter) {
         Integer to;
         Query query = pm.newQuery(LogEntry.class);
 
+        /*
+         * 2013/12/11 -- added a range limit guard.  This can be hard-coded
+         * as effectively the capacity to hold more than 10000 entries in
+         * memory should not be required.  One can always adjust the numbers or
+         * refine the query.
+         */
         if (filter != null) {
             expression = filter.filter;
             parameters = filter.parameters;
             filter.normalizeRange();
-            from = filter.rangeStart;
-            to = filter.rangeEnd;
+            from = filter.rangeStart == null ? 0
+                            : filter.rangeStart;
+            int limit = from + MAXIMUM_QUERY_RESULTS;
+            to   = filter.rangeEnd   == null ? limit
+                            : Math.min(filter.rangeEnd, limit);
             query.setFilter(expression);
             query.declareParameters(parameters);
-            if (from != null) {
-                query.setRange(from, to);
-            }
+            query.setRange(from, to);
         } else {
             expression = null;
             parameters = null;