Fix CombiningSequence.close on single element sequences. (#2969)

Regression introduced by #2892.

common/src/main/java/io/druid/common/guava/CombiningSequence.java

@@ -140,9 +140,7 @@ public class CombiningSequence<T> implements Sequence<T>
       @Override
       public void close() throws IOException
       {
-        if (finalYielder != null) {
-          finalYielder.close();
-        }
+        yielder.close();
       }
     };
   }

common/src/test/java/io/druid/common/guava/CombiningSequenceTest.java

@@ -25,6 +25,7 @@ import com.google.common.collect.Iterators;
 import com.google.common.collect.Lists;
 import com.google.common.collect.Ordering;
 import com.metamx.common.Pair;
+import com.metamx.common.guava.ResourceClosingSequence;
 import com.metamx.common.guava.Sequence;
 import com.metamx.common.guava.Sequences;
 import com.metamx.common.guava.Yielder;
@@ -36,11 +37,14 @@ import org.junit.runner.RunWith;
 import org.junit.runners.Parameterized;
 
 import javax.annotation.Nullable;
+import java.io.Closeable;
 import java.io.IOException;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Iterator;
 import java.util.List;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
 
 @RunWith(Parameterized.class)
 public class CombiningSequenceTest
@@ -61,7 +65,7 @@ public class CombiningSequenceTest
   }
 
   @Test
-  public void testMerge() throws IOException
+  public void testMerge() throws Exception
   {
     List<Pair<Integer, Integer>> pairs = Arrays.asList(
         Pair.of(0, 1),
@@ -87,7 +91,7 @@ public class CombiningSequenceTest
   }
 
   @Test
-  public void testNoMergeOne() throws IOException
+  public void testNoMergeOne() throws Exception
   {
     List<Pair<Integer, Integer>> pairs = Arrays.asList(
         Pair.of(0, 1)
@@ -101,7 +105,7 @@ public class CombiningSequenceTest
   }
 
   @Test
-  public void testMergeMany() throws IOException
+  public void testMergeMany() throws Exception
   {
     List<Pair<Integer, Integer>> pairs = Arrays.asList(
         Pair.of(0, 6),
@@ -125,7 +129,7 @@ public class CombiningSequenceTest
   }
 
   @Test
-  public void testNoMergeTwo() throws IOException
+  public void testNoMergeTwo() throws Exception
   {
     List<Pair<Integer, Integer>> pairs = Arrays.asList(
         Pair.of(0, 1),
@@ -141,7 +145,7 @@ public class CombiningSequenceTest
   }
 
   @Test
-  public void testMergeTwo() throws IOException
+  public void testMergeTwo() throws Exception
   {
     List<Pair<Integer, Integer>> pairs = Arrays.asList(
         Pair.of(0, 1),
@@ -156,7 +160,7 @@ public class CombiningSequenceTest
   }
 
   @Test
-  public void testMergeSomeThingsMergedAtEnd() throws IOException
+  public void testMergeSomeThingsMergedAtEnd() throws Exception
   {
     List<Pair<Integer, Integer>> pairs = Arrays.asList(
         Pair.of(0, 1),
@@ -193,7 +197,7 @@ public class CombiningSequenceTest
   }
 
   private void testCombining(List<Pair<Integer, Integer>> pairs, List<Pair<Integer, Integer>> expected)
-      throws IOException
+      throws Exception
   {
     for (int limit = 0; limit < expected.size() + 1; limit++) {
       // limit = 0 doesn't work properly; it returns 1 element
@@ -211,11 +215,22 @@ public class CombiningSequenceTest
       List<Pair<Integer, Integer>> pairs,
       List<Pair<Integer, Integer>> expected,
       int limit
-  ) throws IOException
+  ) throws Exception
   {
+    // Test that closing works too
+    final CountDownLatch closed = new CountDownLatch(1);
+    final Closeable closeable = new Closeable()
+    {
+      @Override
+      public void close() throws IOException
+      {
+        closed.countDown();
+      }
+    };
+
     Sequence<Pair<Integer, Integer>> seq = Sequences.limit(
         new CombiningSequence<>(
-            Sequences.simple(pairs),
+            new ResourceClosingSequence<>(Sequences.simple(pairs), closeable),
             Ordering.natural().onResultOf(Pair.<Integer, Integer>lhsFn()),
             new BinaryFn<Pair<Integer, Integer>, Pair<Integer, Integer>, Pair<Integer, Integer>>()
             {
@@ -294,5 +309,7 @@ public class CombiningSequenceTest
     Assert.assertTrue(yielder.isDone());
     Assert.assertFalse(expectedVals.hasNext());
     yielder.close();
+
+    Assert.assertTrue("resource closed", closed.await(10000, TimeUnit.MILLISECONDS));
   }
 }