Run merge-on-full-flush even though no changes got flushed. (#12549)

Currently, merge-on-full-flush only checks if merges need to run if changes
have been flushed to disk. This prevents from having different merging logic
for refreshes and commits, since the merge policy would not be checked upon
commit if no new documents got indexed since the previous refresh.

lucene/core/src/java/org/apache/lucene/index/IndexWriter.java

@@ -3680,7 +3680,7 @@ private long prepareCommitInternal() throws IOException {
               // merge completes which would otherwise have
               // removed the files we are now syncing.
               deleter.incRef(toCommit.files(false));
-              if (anyChanges && maxCommitMergeWaitMillis > 0) {
+              if (maxCommitMergeWaitMillis > 0) {
                 // we can safely call preparePointInTimeMerge since writeReaderPool(true) above
                 // wrote all
                 // necessary files to disk and checkpointed them.
@@ -4232,7 +4232,6 @@ private boolean doFlush(boolean applyAllDeletes) throws IOException {
           flushSuccess = true;
         } finally {
           assert Thread.holdsLock(fullFlushLock);
-          ;
           docWriter.finishFullFlush(flushSuccess);
           processEvents(false);
         }

lucene/core/src/test/org/apache/lucene/index/TestDirectoryReaderReopen.java

@@ -654,7 +654,8 @@ public void testOpenIfChangedNRTToCommit() throws Exception {
   public void testOverDecRefDuringReopen() throws Exception {
     MockDirectoryWrapper dir = newMockDirectory();
 
-    IndexWriterConfig iwc = new IndexWriterConfig(new MockAnalyzer(random()));
+    IndexWriterConfig iwc =
+        new IndexWriterConfig(new MockAnalyzer(random())).setMergePolicy(NoMergePolicy.INSTANCE);
     iwc.setCodec(TestUtil.getDefaultCodec());
     IndexWriter w = new IndexWriter(dir, iwc);
     Document doc = new Document();

lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java

@@ -519,11 +519,10 @@ public void testFlushWithNoMerging() throws IOException {
     doc.add(newField("field", "aaa", customType));
     for (int i = 0; i < 19; i++) writer.addDocument(doc);
     writer.flush(false, true);
-    writer.close();
-    SegmentInfos sis = SegmentInfos.readLatestCommit(dir);
     // Since we flushed w/o allowing merging we should now
     // have 10 segments
-    assertEquals(10, sis.size());
+    assertEquals(10, writer.getSegmentCount());
+    writer.close();
     dir.close();
   }
 

lucene/core/src/test/org/apache/lucene/index/TestIndexWriterDelete.java

@@ -1314,7 +1314,7 @@ public void testTryDeleteDocument() throws Exception {
     w.addDocument(doc);
     w.close();
 
-    iwc = new IndexWriterConfig(new MockAnalyzer(random()));
+    iwc = new IndexWriterConfig(new MockAnalyzer(random())).setMergePolicy(NoMergePolicy.INSTANCE);
     iwc.setOpenMode(IndexWriterConfig.OpenMode.APPEND);
     w = new IndexWriter(d, iwc);
     IndexReader r = DirectoryReader.open(w, false, false);

lucene/core/src/test/org/apache/lucene/index/TestIndexWriterMergePolicy.java

@@ -394,14 +394,13 @@ public void testMergeOnCommit() throws IOException {
             .setMaxFullFlushMergeWaitMillis(Integer.MAX_VALUE);
 
     IndexWriter writerWithMergePolicy = new IndexWriter(dir, iwc);
-    writerWithMergePolicy.commit(); // No changes. Commit doesn't trigger a merge.
 
+    // No changes. Refresh doesn't trigger a merge.
     DirectoryReader unmergedReader = DirectoryReader.open(writerWithMergePolicy);
     assertEquals(5, unmergedReader.leaves().size());
     unmergedReader.close();
 
-    TestIndexWriter.addDoc(writerWithMergePolicy);
-    writerWithMergePolicy.commit(); // Doc added, do merge on commit.
+    writerWithMergePolicy.commit(); // Do merge on commit.
     assertEquals(1, writerWithMergePolicy.getSegmentCount()); //
 
     DirectoryReader mergedReader = DirectoryReader.open(writerWithMergePolicy);
@@ -410,8 +409,8 @@ public void testMergeOnCommit() throws IOException {
 
     try (IndexReader reader = DirectoryReader.open(writerWithMergePolicy)) {
       IndexSearcher searcher = new IndexSearcher(reader);
-      assertEquals(6, reader.numDocs());
-      assertEquals(6, searcher.count(new MatchAllDocsQuery()));
+      assertEquals(5, reader.numDocs());
+      assertEquals(5, searcher.count(new MatchAllDocsQuery()));
     }
 
     writerWithMergePolicy.close();
@@ -445,16 +444,14 @@ public void testMergeOnCommitWithEventListener() throws IOException {
             .setIndexWriterEventListener(eventListener);
 
     IndexWriter writerWithMergePolicy = new IndexWriter(dir, iwc);
-    writerWithMergePolicy.commit(); // No changes. Commit doesn't trigger a merge.
 
+    // No changes. Refresh doesn't trigger a merge.
     DirectoryReader unmergedReader = DirectoryReader.open(writerWithMergePolicy);
     assertEquals(5, unmergedReader.leaves().size());
     unmergedReader.close();
 
-    TestIndexWriter.addDoc(writerWithMergePolicy);
-
     assertFalse(eventListener.isEventsRecorded());
-    writerWithMergePolicy.commit(); // Doc added, do merge on commit.
+    writerWithMergePolicy.commit(); // Do merge on commit.
     assertEquals(1, writerWithMergePolicy.getSegmentCount()); //
     assertTrue(eventListener.isEventsRecorded());
 

lucene/core/src/test/org/apache/lucene/index/TestPendingSoftDeletes.java

@@ -263,6 +263,7 @@ public void testUpdateAppliedOnlyOnce() throws IOException {
             newIndexWriterConfig()
                 .setSoftDeletesField("_soft_deletes")
                 .setMaxBufferedDocs(3) // make sure we write one segment
+                .setMergePolicy(NoMergePolicy.INSTANCE) // prevent deletes from triggering merges
                 .setRAMBufferSizeMB(IndexWriterConfig.DISABLE_AUTO_FLUSH));
     Document doc = new Document();
     doc.add(new StringField("id", "1", Field.Store.YES));
@@ -331,6 +332,7 @@ public void testResetOnUpdate() throws IOException {
             newIndexWriterConfig()
                 .setSoftDeletesField("_soft_deletes")
                 .setMaxBufferedDocs(3) // make sure we write one segment
+                .setMergePolicy(NoMergePolicy.INSTANCE) // prevent deletes from triggering merges
                 .setRAMBufferSizeMB(IndexWriterConfig.DISABLE_AUTO_FLUSH));
     Document doc = new Document();
     doc.add(new StringField("id", "1", Field.Store.YES));