LUCENE-5222: fix expression sort with executorService

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1524575 13f79535-47bb-0310-9956-ffa450edef68

lucene/core/src/java/org/apache/lucene/search/Sort.java

@@ -205,7 +205,7 @@ public class Sort {
   /** Whether the relevance score is needed to sort documents. */
   boolean needsScores() {
     for (SortField sortField : fields) {
-      if (sortField.getType() == SortField.Type.SCORE) {
+      if (sortField.needsScores()) {
         return true;
       }
     }

lucene/core/src/java/org/apache/lucene/search/SortField.java

@@ -402,4 +402,9 @@ public class SortField {
   public SortField rewrite(IndexSearcher searcher) throws IOException {
     return this;
   }
+  
+  /** Whether the relevance score is needed to sort documents. */
+  public boolean needsScores() {
+    return type == Type.SCORE;
+  }
 }

lucene/expressions/src/java/org/apache/lucene/expressions/ExpressionSortField.java

@@ -35,4 +35,9 @@ class ExpressionSortField extends SortField {
   public FieldComparator<?> getComparator(final int numHits, final int sortPos) throws IOException {
     return new ExpressionComparator(source, numHits);
   }
+
+  @Override
+  public boolean needsScores() {
+    return true; // TODO: maybe we can optimize by "figuring this out" somehow...
+  }
 }