Fix TSDBIndexingIT#testTrimId() test failure. (elastic#112194)

Sometimes initial indexing results into exactly one segment.
However, multiple segments are needed to perform the force merge that purges stored fields for _id field in a later stage of the test.

This change tweaks the test such that an extra update is performed after initial indexing. This should always create an extra segment, so that this test can actual purge stored fields for _id field.

Closes elastic#112124

Author: martijnvg

modules/data-streams/src/internalClusterTest/java/org/elasticsearch/datastreams/TSDBIndexingIT.java

@@ -22,6 +22,7 @@
 import org.elasticsearch.action.get.GetRequest;
 import org.elasticsearch.action.index.IndexRequest;
 import org.elasticsearch.action.search.SearchRequest;
+import org.elasticsearch.action.support.WriteRequest;
 import org.elasticsearch.cluster.metadata.ComponentTemplate;
 import org.elasticsearch.cluster.metadata.ComposableIndexTemplate;
 import org.elasticsearch.cluster.metadata.Template;
@@ -35,6 +36,7 @@
 import org.elasticsearch.index.query.RangeQueryBuilder;
 import org.elasticsearch.indices.InvalidIndexTemplateException;
 import org.elasticsearch.plugins.Plugin;
+import org.elasticsearch.rest.RestStatus;
 import org.elasticsearch.search.builder.SearchSourceBuilder;
 import org.elasticsearch.test.ESSingleNodeTestCase;
 import org.elasticsearch.test.InternalSettingsPlugin;
@@ -457,6 +459,16 @@ public void testTrimId() throws Exception {
                 indexName = bulkResponse.getItems()[0].getIndex();
             }
             client().admin().indices().refresh(new RefreshRequest(dataStreamName)).actionGet();
+
+            // In rare cases we can end up with a single segment shard, which means we can't trim away the _id later.
+            // So update an existing doc to create a new segment without adding a new document after force merging:
+            var indexRequest = new IndexRequest(indexName).setIfPrimaryTerm(1L)
+                .setIfSeqNo((numBulkRequests * numDocsPerBulk) - 1)
+                .setRefreshPolicy(WriteRequest.RefreshPolicy.IMMEDIATE);
+            indexRequest.source(DOC.replace("$time", formatInstant(time.minusMillis(1))), XContentType.JSON);
+            var res = client().index(indexRequest).actionGet();
+            assertThat(res.status(), equalTo(RestStatus.OK));
+            assertThat(res.getVersion(), equalTo(2L));
         }
 
         // Check whether there are multiple segments:
@@ -494,7 +506,7 @@ public void testTrimId() throws Exception {
             assertThat(retentionLeasesStats.retentionLeases().leases(), hasSize(1));
             assertThat(
                 retentionLeasesStats.retentionLeases().leases().iterator().next().retainingSequenceNumber(),
-                equalTo((long) numBulkRequests * numDocsPerBulk)
+                equalTo((long) numBulkRequests * numDocsPerBulk + 1)
             );
         });