Fix failures in ShuffleForcedMergePolicyTests#testDiagnostics (#48627)

This commit fixes intermittent failures in ShuffleForcedMergePolicyTests#testDiagnostics
by setting a more restricted merge policy that ensures that extra merging will not happen
before the forced merge.

Author: jimczi

server/src/main/java/org/apache/lucene/index/ShuffleForcedMergePolicy.java

@@ -77,7 +77,7 @@ public CodecReader wrapForMerge(CodecReader reader) throws IOException {
 
                 @Override
                 public void setMergeInfo(SegmentCommitInfo info) {
-                    // Record that this merged segment is current as of this schemaGen:
+                    // record that this segment was merged with interleaved segments
                     Map<String, String> copy = new HashMap<>(info.info.getDiagnostics());
                     copy.put(SHUFFLE_MERGE_KEY, "");
                     info.info.setDiagnostics(copy);

server/src/test/java/org/apache/lucene/index/ShuffleForcedMergePolicyTests.java

@@ -37,7 +37,7 @@ public class ShuffleForcedMergePolicyTests extends BaseMergePolicyTestCase {
     public void testDiagnostics() throws IOException {
         try (Directory dir = newDirectory()) {
             IndexWriterConfig iwc = newIndexWriterConfig();
-            MergePolicy mp = new ShuffleForcedMergePolicy(newLogMergePolicy());
+            MergePolicy mp = new ShuffleForcedMergePolicy(newTieredMergePolicy());
             iwc.setMergePolicy(mp);
             boolean sorted = random().nextBoolean();
             if (sorted) {
@@ -57,16 +57,12 @@ public void testDiagnostics() throws IOException {
                 }
                 try (DirectoryReader reader = DirectoryReader.open(writer)) {
                     assertThat(reader.leaves().size(), greaterThan(2));
-                    assertSegmentReaders(reader, leaf -> {
-                        assertFalse(ShuffleForcedMergePolicy.isInterleavedSegment(leaf));
-                    });
+                    assertSegmentReaders(reader, leaf -> assertFalse(ShuffleForcedMergePolicy.isInterleavedSegment(leaf)));
                 }
                 writer.forceMerge(1);
                 try (DirectoryReader reader = DirectoryReader.open(writer)) {
                     assertThat(reader.leaves().size(), equalTo(1));
-                    assertSegmentReaders(reader, leaf -> {
-                        assertTrue(ShuffleForcedMergePolicy.isInterleavedSegment(leaf));
-                    });
+                    assertSegmentReaders(reader, leaf -> assertTrue(ShuffleForcedMergePolicy.isInterleavedSegment(leaf)));
                 }
             }
         }
@@ -84,8 +80,10 @@ protected MergePolicy mergePolicy() {
     }
 
     @Override
-    protected void assertSegmentInfos(MergePolicy policy, SegmentInfos infos) throws IOException {}
+    protected void assertSegmentInfos(MergePolicy policy, SegmentInfos infos) throws IOException {
+    }
 
     @Override
-    protected void assertMerge(MergePolicy policy, MergePolicy.MergeSpecification merge) throws IOException {}
+    protected void assertMerge(MergePolicy policy, MergePolicy.MergeSpecification merge) throws IOException {
+    }
 }