Remove usage of IndexSearcher#Search(Query, Collector) from monitor p…

…ackage (#13735)

lucene/CHANGES.txt

@@ -27,6 +27,9 @@ API Changes
 * GITHUB#13737: Deprecate the FacetsCollector#search utility methods and add new corresponding method to
   FacetsCollectorManager that accept a FacetsCollectorManager as last argument in place of a Collector. (Luca Cavanna)
 
+* GITHUB#13735: Add CollectorManager#forSequentialExecution to make CollectorManager creation more convenient
+  for users of the deprecated IndexSearcher#search(Query, Collector). (Greg Miller)
+
 New Features
 ---------------------
 
@@ -71,6 +74,9 @@ Improvements
 
 * GITHUB#13201: Better cost estimation on MultiTermQuery over few terms. (Michael Froh)
 
+* GITHUB#13735: Migrate monitor package usage of deprecated IndexSearcher#search(Query, Collector)
+  to IndexSearcher#search(Query, CollectorManager). (Greg Miller)
+
 Optimizations
 ---------------------
 

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

@@ -18,6 +18,8 @@
 
 import java.io.IOException;
 import java.util.Collection;
+import java.util.concurrent.Executor;
+import org.apache.lucene.index.IndexReader;
 
 /**
  * A manager of collectors. This class is useful to parallelize execution of search requests and has
@@ -46,4 +48,36 @@ public interface CollectorManager<C extends Collector, T> {
    * called after collection is finished on all provided collectors.
    */
   T reduce(Collection<C> collectors) throws IOException;
+
+  /**
+   * Wrap a provided {@link Collector} with a thin {@code CollectorManager} wrapper for use with
+   * {@link IndexSearcher#search(Query, CollectorManager)} when doing single-threaded searching. The
+   * wrapping {@code CollectorManager} provides no {@link CollectorManager#reduce(Collection)}
+   * implementation, so the wrapped {@code Collector} needs to do all relevant work while
+   * collecting.
+   *
+   * <p>Note: This is only safe to use when {@code IndexSearcher} is created with no executor (see:
+   * {@link IndexSearcher#IndexSearcher(IndexReader, Executor)}).
+   */
+  static <C extends Collector> CollectorManager<C, ?> forSequentialExecution(C in) {
+    return new CollectorManager<C, Void>() {
+      private boolean newCollectorInvoked;
+
+      @Override
+      public C newCollector() {
+        if (newCollectorInvoked) {
+          throw new IllegalStateException(
+              "newCollector should be invoked at most once. Ensure your IndexSearcher has been created without an Executor.");
+        }
+        newCollectorInvoked = true;
+        return in;
+      }
+
+      @Override
+      public Void reduce(Collection<C> collectors) {
+        assert collectors.size() == 1 : "collectors should contain exactly one collector instance";
+        return null;
+      }
+    };
+  }
 }

lucene/monitor/src/java/org/apache/lucene/monitor/CollectingMatcher.java

@@ -19,6 +19,7 @@
 
 import java.io.IOException;
 import java.util.Map;
+import org.apache.lucene.search.CollectorManager;
 import org.apache.lucene.search.IndexSearcher;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.Scorable;
@@ -37,7 +38,9 @@ abstract class CollectingMatcher<T extends QueryMatch> extends CandidateMatcher<
   @Override
   protected void matchQuery(final String queryId, Query matchQuery, Map<String, String> metadata)
       throws IOException {
-    searcher.search(matchQuery, new MatchCollector(queryId, scoreMode));
+    searcher.search(
+        matchQuery,
+        CollectorManager.forSequentialExecution(new MatchCollector(queryId, scoreMode)));
   }
 
   /**