[8.x] Fix failing test(s) in `TimeSeriesDataStreamsIT` (#123378) (#123472)

When these tests were run around midnight, the use of
`DataStream#getDefaultBackingIndexName` could result in a potential
mismatch in the generated index name and the one that the cluster
actually created. Instead, we need to obtain the backing index and
extract the desired index name from there.

Fixes #123086
Relates #123376

x-pack/plugin/ilm/qa/multi-node/src/javaRestTest/java/org/elasticsearch/xpack/ilm/TimeSeriesDataStreamsIT.java

@@ -9,7 +9,6 @@ package org.elasticsearch.xpack.ilm;
 
 import org.elasticsearch.client.Request;
 import org.elasticsearch.client.Response;
-import org.elasticsearch.cluster.metadata.DataStream;
 import org.elasticsearch.cluster.metadata.IndexMetadata;
 import org.elasticsearch.cluster.metadata.Template;
 import org.elasticsearch.common.xcontent.XContentHelper;
@@ -80,20 +79,15 @@ public class TimeSeriesDataStreamsIT extends ESRestTestCase {
 
         indexDocument(client(), dataStream, true);
 
-        assertBusy(() -> assertTrue(indexExists(DataStream.getDefaultBackingIndexName(dataStream, 2))));
-        assertBusy(
-            () -> assertTrue(
-                Boolean.parseBoolean(
-                    (String) getIndexSettingsAsMap(DataStream.getDefaultBackingIndexName(dataStream, 2)).get("index.hidden")
-                )
-            )
-        );
-        assertBusy(
-            () -> assertThat(
-                getStepKeyForIndex(client(), DataStream.getDefaultBackingIndexName(dataStream, 1)),
-                equalTo(PhaseCompleteStep.finalStep("hot").getKey())
-            )
-        );
+        assertBusy(() -> {
+            final var backingIndices = getBackingIndices(client(), dataStream);
+            assertEquals(2, backingIndices.size());
+            assertTrue(Boolean.parseBoolean((String) getIndexSettingsAsMap(backingIndices.get(1)).get("index.hidden")));
+        });
+        assertBusy(() -> {
+            final var backingIndices = getBackingIndices(client(), dataStream);
+            assertEquals(PhaseCompleteStep.finalStep("hot").getKey(), getStepKeyForIndex(client(), backingIndices.get(0)));
+        });
     }
 
     public void testRolloverIsSkippedOnManualDataStreamRollover() throws Exception {
@@ -103,7 +97,7 @@ public class TimeSeriesDataStreamsIT extends ESRestTestCase {
 
         indexDocument(client(), dataStream, true);
 
-        String firstGenerationIndex = DataStream.getDefaultBackingIndexName(dataStream, 1);
+        String firstGenerationIndex = getBackingIndices(client(), dataStream).get(0);
         assertBusy(
             () -> assertThat(getStepKeyForIndex(client(), firstGenerationIndex).name(), equalTo(WaitForRolloverReadyStep.NAME)),
             30,
@@ -111,7 +105,10 @@ public class TimeSeriesDataStreamsIT extends ESRestTestCase {
         );
 
         rolloverMaxOneDocCondition(client(), dataStream);
-        assertBusy(() -> assertThat(indexExists(DataStream.getDefaultBackingIndexName(dataStream, 2)), is(true)), 30, TimeUnit.SECONDS);
+        assertBusy(() -> {
+            final var backingIndices = getBackingIndices(client(), dataStream);
+            assertEquals(2, backingIndices.size());
+        }, 30, TimeUnit.SECONDS);
 
         // even though the first index doesn't have 2 documents to fulfill the rollover condition, it should complete the rollover action
         // because it's not the write index anymore
@@ -128,7 +125,7 @@ public class TimeSeriesDataStreamsIT extends ESRestTestCase {
         createComposableTemplate(client(), template, dataStream + "*", getTemplate(policyName));
         indexDocument(client(), dataStream, true);
 
-        String backingIndexName = DataStream.getDefaultBackingIndexName(dataStream, 1);
+        String backingIndexName = getBackingIndices(client(), dataStream).get(0);
         assertBusy(
             () -> assertThat(
                 "original index must wait in the " + CheckNotDataStreamWriteIndexStep.NAME + " until it is not the write index anymore",
@@ -142,8 +139,11 @@ public class TimeSeriesDataStreamsIT extends ESRestTestCase {
         // Manual rollover the original index such that it's not the write index in the data stream anymore
         rolloverMaxOneDocCondition(client(), dataStream);
         // Wait for rollover to happen
-        String rolloverIndex = DataStream.getDefaultBackingIndexName(dataStream, 2);
-        assertBusy(() -> assertTrue("the rollover action created the rollover index", indexExists(rolloverIndex)), 30, TimeUnit.SECONDS);
+        assertBusy(
+            () -> assertEquals("the rollover action created the rollover index", 2, getBackingIndices(client(), dataStream).size()),
+            30,
+            TimeUnit.SECONDS
+        );
 
         String shrunkenIndex = waitAndGetShrinkIndexName(client(), backingIndexName);
         assertBusy(() -> assertTrue(indexExists(shrunkenIndex)), 30, TimeUnit.SECONDS);
@@ -159,7 +159,7 @@ public class TimeSeriesDataStreamsIT extends ESRestTestCase {
         createComposableTemplate(client(), template, dataStream + "*", getTemplate(policyName));
         indexDocument(client(), dataStream, true);
 
-        String backingIndexName = DataStream.getDefaultBackingIndexName(dataStream, 1);
+        String backingIndexName = getBackingIndices(client(), dataStream).get(0);
         String restoredIndexName = SearchableSnapshotAction.FULL_RESTORED_INDEX_PREFIX + backingIndexName;
 
         assertBusy(
@@ -190,7 +190,7 @@ public class TimeSeriesDataStreamsIT extends ESRestTestCase {
         createComposableTemplate(client(), template, dataStream + "*", getTemplate(policyName));
         indexDocument(client(), dataStream, true);
 
-        String backingIndexName = DataStream.getDefaultBackingIndexName(dataStream, 1);
+        String backingIndexName = getBackingIndices(client(), dataStream).get(0);
         assertBusy(
             () -> assertThat(
                 "index must wait in the " + CheckNotDataStreamWriteIndexStep.NAME + " until it is not the write index anymore",
@@ -220,7 +220,7 @@ public class TimeSeriesDataStreamsIT extends ESRestTestCase {
         createComposableTemplate(client(), template, dataStream + "*", getTemplate(policyName));
         indexDocument(client(), dataStream, true);
 
-        String backingIndexName = DataStream.getDefaultBackingIndexName(dataStream, 1);
+        String backingIndexName = getBackingIndices(client(), dataStream).get(0);
         assertBusy(
             () -> assertThat(
                 "index must wait in the " + CheckNotDataStreamWriteIndexStep.NAME + " until it is not the write index anymore",
@@ -249,7 +249,7 @@ public class TimeSeriesDataStreamsIT extends ESRestTestCase {
         createComposableTemplate(client(), template, dataStream + "*", getTemplate(policyName));
         indexDocument(client(), dataStream, true);
 
-        String backingIndexName = DataStream.getDefaultBackingIndexName(dataStream, 1);
+        String backingIndexName = getBackingIndices(client(), dataStream).get(0);
         assertBusy(
             () -> assertThat(
                 "index must wait in the " + CheckNotDataStreamWriteIndexStep.NAME + " until it is not the write index anymore",