javanna commented on code in PR #12523:
URL: https://github.com/apache/lucene/pull/12523#discussion_r1343042425


##########
lucene/core/src/test/org/apache/lucene/search/TestIndexSearcher.java:
##########
@@ -267,11 +266,130 @@ protected LeafSlice[] slices(List<LeafReaderContext> 
leaves) {
             return slices.toArray(new LeafSlice[0]);
           }
         };
-    searcher.search(new MatchAllDocsQuery(), 10);
+    TopDocs topDocs = searcher.search(new MatchAllDocsQuery(), 10);
+    assertTrue(topDocs.totalHits.value > 0);
     if (leaves.size() <= 1) {
       assertEquals(0, numExecutions.get());
     } else {
       assertEquals(leaves.size(), numExecutions.get());
     }
   }
+
+  /**
+   * Tests that when IndexerSearcher runs concurrent searches on multiple 
slices if any Exception is
+   * thrown by one of the slice tasks, it will not return until all tasks have 
completed.
+   *
+   * <p>Without a larger refactoring of the Lucene IndexSearcher and/or 
TaskExecutor there isn't a
+   * clean deterministic way to test this. This test is probabilistic using 
short timeouts in the
+   * tasks that do not throw an Exception.
+   */
+  public void testMultipleSegmentsOnTheExecutorWithException() {
+    List<LeafReaderContext> leaves = reader.leaves();
+    int fixedThreads = leaves.size() == 1 ? 1 : leaves.size() / 2;
+
+    ExecutorService fixedThreadPoolExecutor =
+        Executors.newFixedThreadPool(fixedThreads, new 
NamedThreadFactory("concurrent-slices"));
+
+    IndexSearcher searcher =
+        new IndexSearcher(reader, fixedThreadPoolExecutor) {
+          @Override
+          protected LeafSlice[] slices(List<LeafReaderContext> leaves) {
+            ArrayList<LeafSlice> slices = new ArrayList<>();
+            for (LeafReaderContext ctx : leaves) {
+              slices.add(new LeafSlice(Arrays.asList(ctx)));
+            }
+            return slices.toArray(new LeafSlice[0]);
+          }
+        };
+
+    try {
+      AtomicInteger callsToScorer = new AtomicInteger(0);
+      int numExceptions = leaves.size() == 1 ? 1 : 
RandomizedTest.randomIntBetween(1, 2);
+      MatchAllOrThrowExceptionQuery query =
+          new MatchAllOrThrowExceptionQuery(numExceptions, callsToScorer);
+      RuntimeException exc = expectThrows(RuntimeException.class, () -> 
searcher.search(query, 10));
+      // if the TaskExecutor didn't wait for all tasks to finish, this assert 
would frequently fail
+      assertEquals(leaves.size(), callsToScorer.get());
+      assertThat(
+          exc.getMessage(), 
Matchers.containsString("MatchAllOrThrowExceptionQuery Exception"));
+    } finally {
+      TestUtil.shutdownExecutorService(fixedThreadPoolExecutor);
+    }
+  }
+
+  private static class MatchAllOrThrowExceptionQuery extends Query {
+
+    private final AtomicInteger numExceptionsToThrow;
+    private final Query delegate;
+    private final AtomicInteger callsToScorer;
+
+    /**
+     * Throws an Exception out of the {@code scorer} method the first {@code 
numExceptions} times it
+     * is called. Otherwise, it delegates all calls to the MatchAllDocsQuery.
+     *
+     * @param numExceptions number of exceptions to throw from scorer method
+     * @param callsToScorer where to record the number of times the {@code 
scorer} method has been
+     *     called
+     */
+    public MatchAllOrThrowExceptionQuery(int numExceptions, AtomicInteger 
callsToScorer) {
+      this.numExceptionsToThrow = new AtomicInteger(numExceptions);
+      this.callsToScorer = callsToScorer;
+      this.delegate = new MatchAllDocsQuery();
+    }
+
+    @Override
+    public Weight createWeight(IndexSearcher searcher, ScoreMode scoreMode, 
float boost)
+        throws IOException {
+      Weight matchAllWeight = delegate.createWeight(searcher, scoreMode, 
boost);
+
+      return new Weight(delegate) {
+        @Override
+        public boolean isCacheable(LeafReaderContext ctx) {
+          return matchAllWeight.isCacheable(ctx);
+        }
+
+        @Override
+        public Explanation explain(LeafReaderContext context, int doc) throws 
IOException {
+          return matchAllWeight.explain(context, doc);
+        }
+
+        @Override
+        public Scorer scorer(LeafReaderContext context) throws IOException {
+          if (numExceptionsToThrow.getAndDecrement() > 0) {
+            callsToScorer.getAndIncrement();
+            throw new RuntimeException("MatchAllOrThrowExceptionQuery 
Exception");
+          } else {
+            // A small sleep before incrementing the callsToScorer counter 
allows
+            // the task with the Exception to be thrown and if 
TaskExecutor.invokeAll
+            // does not wait until all tasks have finished, then the 
callsToScorer
+            // counter will not match the total number of tasks (or rather 
usually will
+            // not match, since there is a race condition that makes it 
probabilistic).
+            RandomizedTest.sleep(25);

Review Comment:
   Thinking more of this, perhaps using a single threaded executor, or even one 
that executes on the caller thread would make the test more repeatable? Do we 
need to leverage parallel execution to test the behaviour we are introducing? 
It seems like with a single thread we could even simplify the test and remove 
the latch? 



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