Fix assertion in DiskThresholdDeciderIT (#127615) (#127643)

The test launches two concurrent restores and wants to verify that
the node with limited disk space is only assigned a single shard from
one of the indices.  The test was asserting that it had one shard from
the first index, but it is possible for it to get one shard from the
index copy instead.  This change allows the shard to be from either
index, but still asserts there is only one assignment to the tiny node.

Closes #127286

(cherry picked from commit 6263f44)

Author: JeremyDahlgren

muted-tests.yml

@@ -455,9 +455,6 @@ tests:
 - class: org.elasticsearch.search.SearchServiceSingleNodeTests
   method: testLookUpSearchContext
   issue: https://github.com/elastic/elasticsearch/issues/126813
-- class: org.elasticsearch.cluster.routing.allocation.decider.DiskThresholdDeciderIT
-  method: testRestoreSnapshotAllocationDoesNotExceedWatermarkWithMultipleRestores
-  issue: https://github.com/elastic/elasticsearch/issues/127286
 - class: org.elasticsearch.test.index.IndexVersionUtilsTests
   method: testIndexCompatibleVersionMatches
   issue: https://github.com/elastic/elasticsearch/issues/120760

server/src/internalClusterTest/java/org/elasticsearch/cluster/routing/allocation/decider/DiskThresholdDeciderIT.java

@@ -56,7 +56,6 @@
 import static org.hamcrest.Matchers.contains;
 import static org.hamcrest.Matchers.empty;
 import static org.hamcrest.Matchers.equalTo;
-import static org.hamcrest.Matchers.hasSize;
 import static org.hamcrest.Matchers.in;
 import static org.hamcrest.Matchers.is;
 import static org.hamcrest.Matchers.lessThanOrEqualTo;
@@ -242,7 +241,7 @@ public void testRestoreSnapshotAllocationDoesNotExceedWatermarkWithMultipleResto
                 clusterAdmin().prepareRestoreSnapshot(TEST_REQUEST_TIMEOUT, "repo", "snap")
                     .setWaitForCompletion(true)
                     .setRenamePattern(indexName)
-                    .setRenameReplacement(indexName + "-copy")
+                    .setRenameReplacement(copyIndexName)
                     .execute(ActionTestUtils.assertNoFailureListener(restoreSnapshotResponse -> {
                         final RestoreInfo restoreInfo = restoreSnapshotResponse.getRestoreInfo();
                         assertThat(restoreInfo.successfulShards(), is(snapshotInfo.totalShards()));
@@ -268,10 +267,19 @@ public void testRestoreSnapshotAllocationDoesNotExceedWatermarkWithMultipleResto
 
         // wait for all the shards to finish moving
         safeAwait(allShardsActiveListener);
-        ensureGreen(indexName, indexName + "-copy");
+        ensureGreen(indexName, copyIndexName);
 
         final var tinyNodeShardIds = getShardIds(dataNodeId, indexName);
-        assertThat(tinyNodeShardIds, hasSize(1));
+        final var tinyNodeShardIdsCopy = getShardIds(dataNodeId, copyIndexName);
+        assertThat(
+            "expected just one shard from one index on the tiny node, instead got "
+                + tinyNodeShardIds
+                + " from the original index and "
+                + tinyNodeShardIdsCopy
+                + " from the copy",
+            tinyNodeShardIds.size() + tinyNodeShardIdsCopy.size(),
+            is(1)
+        );
         assertThat(tinyNodeShardIds.iterator().next(), in(shardSizes.getShardIdsWithSizeSmallerOrEqual(usableSpace)));
     }