[minor] following 4270, add unit tests for the keys lost case (#5918)

hudi-flink-datasource/hudi-flink/src/test/java/org/apache/hudi/sink/TestWriteMergeOnRead.java

@@ -38,6 +38,31 @@ public class TestWriteMergeOnRead extends TestWriteCopyOnWrite {
     conf.setBoolean(FlinkOptions.COMPACTION_ASYNC_ENABLED, false);
   }
 
+  @Test
+  public void testIndexStateBootstrapWithMultiFilesInOneSlice() throws Exception {
+    // open the function and ingest data
+    preparePipeline(conf)
+        .consume(TestData.filterOddRows(TestData.DATA_SET_INSERT))
+        .assertEmptyDataFiles()
+        .checkpoint(1)
+        .assertNextEvent()
+        .checkpointComplete(1)
+        .consume(TestData.filterEvenRows(TestData.DATA_SET_INSERT))
+        .checkpoint(2)
+        .assertNextEvent()
+        .checkpointComplete(2)
+        .checkWrittenData(EXPECTED1, 4)
+        // write another commit but does not complete it
+        .consume(TestData.filterEvenRows(TestData.DATA_SET_INSERT))
+        .checkpoint(3)
+        .assertNextEvent()
+        .end();
+
+    // reset the config option
+    conf.setBoolean(FlinkOptions.INDEX_BOOTSTRAP_ENABLED, true);
+    validateIndexLoaded();
+  }
+
   @Test
   public void testIndexStateBootstrapWithCompactionScheduled() throws Exception {
     // sets up the delta commits as 1 to generate a new compaction plan.

hudi-flink-datasource/hudi-flink/src/test/java/org/apache/hudi/utils/TestData.java

@@ -64,6 +64,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Properties;
+import java.util.function.Predicate;
 import java.util.stream.Collectors;
 import java.util.stream.IntStream;
 
@@ -304,6 +305,24 @@ public class TestData {
     return inserts;
   }
 
+  public static List<RowData> filterOddRows(List<RowData> rows) {
+    return filterRowsByIndexPredicate(rows, i -> i % 2 != 0);
+  }
+
+  public static List<RowData> filterEvenRows(List<RowData> rows) {
+    return filterRowsByIndexPredicate(rows, i -> i % 2 == 0);
+  }
+
+  private static List<RowData> filterRowsByIndexPredicate(List<RowData> rows, Predicate<Integer> predicate) {
+    List<RowData> filtered = new ArrayList<>();
+    for (int i = 0; i < rows.size(); i++) {
+      if (predicate.test(i)) {
+        filtered.add(rows.get(i));
+      }
+    }
+    return filtered;
+  }
+
   private static Integer toIdSafely(Object id) {
     if (id == null) {
       return -1;