[8.19] Add TEST MergeWithLowDiskSpaceIT testRelocationWhileForceMerging (#131767) (#131779)

* Add TEST MergeWithLowDiskSpaceIT testRelocationWhileForceMerging (#131767)

This adds a test that covers relocation for shards that are running a
force merge.

Relates https://github.com/elastic/elasticsearch/issues/93503

* Fix MergeWithLowDiskSpaceIT testRelocationWhileForceMerging (#131806)

The index settings are randomized in the test, but this test suite doesn't work when indices have a custom data path.

* Fix compilation

server/src/internalClusterTest/java/org/elasticsearch/index/engine/MergeWithLowDiskSpaceIT.java

@@ -10,16 +10,22 @@
 package org.elasticsearch.index.engine;
 
 import org.elasticsearch.action.ActionFuture;
+import org.elasticsearch.action.admin.cluster.health.ClusterHealthResponse;
 import org.elasticsearch.action.admin.cluster.node.stats.NodeStats;
 import org.elasticsearch.action.admin.cluster.node.stats.NodesStatsResponse;
+import org.elasticsearch.action.admin.cluster.reroute.ClusterRerouteUtils;
+import org.elasticsearch.action.admin.indices.segments.IndicesSegmentResponse;
 import org.elasticsearch.action.admin.indices.segments.ShardSegments;
 import org.elasticsearch.action.admin.indices.stats.IndicesStatsResponse;
 import org.elasticsearch.action.support.broadcast.BroadcastResponse;
 import org.elasticsearch.cluster.DiskUsageIntegTestCase;
 import org.elasticsearch.cluster.metadata.IndexMetadata;
 import org.elasticsearch.cluster.routing.allocation.DiskThresholdSettings;
+import org.elasticsearch.cluster.routing.allocation.command.MoveAllocationCommand;
+import org.elasticsearch.common.Priority;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.util.concurrent.EsExecutors;
+import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.index.IndexNotFoundException;
 import org.elasticsearch.indices.IndicesService;
 import org.elasticsearch.test.ESIntegTestCase;
@@ -28,16 +34,20 @@ import org.junit.BeforeClass;
 
 import java.util.List;
 import java.util.Locale;
+import java.util.concurrent.TimeUnit;
 import java.util.stream.IntStream;
 
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAcked;
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertNoFailures;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.greaterThan;
+import static org.hamcrest.Matchers.iterableWithSize;
 import static org.hamcrest.Matchers.lessThan;
+import static org.hamcrest.Matchers.not;
 
 @ESIntegTestCase.ClusterScope(scope = ESIntegTestCase.Scope.TEST, numDataNodes = 0)
 public class MergeWithLowDiskSpaceIT extends DiskUsageIntegTestCase {
+    private final TimeValue ACCEPTABLE_RELOCATION_TIME = new TimeValue(5, TimeUnit.MINUTES);
     protected static long MERGE_DISK_HIGH_WATERMARK_BYTES;
 
     @BeforeClass
@@ -235,6 +245,106 @@ public class MergeWithLowDiskSpaceIT extends DiskUsageIntegTestCase {
         assertAcked(indicesAdmin().prepareDelete(indexName).get());
     }
 
+    public void testRelocationWhileForceMerging() throws Exception {
+        final String node1 = internalCluster().startNode();
+        ensureStableCluster(1);
+        setTotalSpace(node1, Long.MAX_VALUE);
+        String indexName = randomAlphaOfLength(10).toLowerCase(Locale.ROOT);
+        createIndex(
+            indexName,
+            Settings.builder().put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0).put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, 1).build()
+        );
+        // get current disk space usage (for all indices on the node)
+        IndicesStatsResponse stats = indicesAdmin().prepareStats().clear().setStore(true).get();
+        long usedDiskSpaceAfterIndexing = stats.getTotal().getStore().sizeInBytes();
+        // restrict the total disk space such that the next merge does not have sufficient disk space
+        long insufficientTotalDiskSpace = usedDiskSpaceAfterIndexing + MERGE_DISK_HIGH_WATERMARK_BYTES - randomLongBetween(1L, 10L);
+        setTotalSpace(node1, insufficientTotalDiskSpace);
+        // node stats' FS stats should report that there is insufficient disk space available
+        assertBusy(() -> {
+            NodesStatsResponse nodesStatsResponse = client().admin().cluster().prepareNodesStats().setFs(true).get();
+            assertThat(nodesStatsResponse.getNodes().size(), equalTo(1));
+            NodeStats nodeStats = nodesStatsResponse.getNodes().get(0);
+            assertThat(nodeStats.getFs().getTotal().getTotal().getBytes(), equalTo(insufficientTotalDiskSpace));
+            assertThat(nodeStats.getFs().getTotal().getAvailable().getBytes(), lessThan(MERGE_DISK_HIGH_WATERMARK_BYTES));
+        });
+        int indexingRounds = randomIntBetween(5, 10);
+        while (indexingRounds-- > 0) {
+            indexRandom(
+                true,
+                true,
+                true,
+                false,
+                IntStream.range(1, randomIntBetween(5, 10))
+                    .mapToObj(i -> prepareIndex(indexName).setSource("field", randomAlphaOfLength(50)))
+                    .toList()
+            );
+        }
+        // the max segments argument makes it a blocking call
+        ActionFuture<BroadcastResponse> forceMergeBeforeRelocationFuture = indicesAdmin().prepareForceMerge(indexName)
+            .setMaxNumSegments(1)
+            .execute();
+        ThreadPoolMergeExecutorService threadPoolMergeExecutorService = internalCluster().getInstance(IndicesService.class, node1)
+            .getThreadPoolMergeExecutorService();
+        assertBusy(() -> {
+            // merge executor says merging is blocked due to insufficient disk space while there is a single merge task enqueued
+            assertThat(threadPoolMergeExecutorService.getMergeTasksQueueLength(), equalTo(1));
+            assertTrue(threadPoolMergeExecutorService.isMergingBlockedDueToInsufficientDiskSpace());
+            // indices stats also says that no merge is currently running (blocked merges are NOT considered as "running")
+            IndicesStatsResponse indicesStatsResponse = client().admin().indices().prepareStats(indexName).setMerge(true).get();
+            long currentMergeCount = indicesStatsResponse.getIndices().get(indexName).getPrimaries().merge.getCurrent();
+            assertThat(currentMergeCount, equalTo(0L));
+        });
+        // the force merge call is still blocked
+        assertFalse(forceMergeBeforeRelocationFuture.isCancelled());
+        assertFalse(forceMergeBeforeRelocationFuture.isDone());
+        // merge executor still confirms merging is blocked due to insufficient disk space
+        assertTrue(threadPoolMergeExecutorService.isMergingBlockedDueToInsufficientDiskSpace());
+        IndicesSegmentResponse indicesSegmentResponseBeforeRelocation = indicesAdmin().prepareSegments(indexName).get();
+        // the index should have more than 1 segments at this stage
+        assertThat(
+            indicesSegmentResponseBeforeRelocation.getIndices().get(indexName).iterator().next().shards()[0].getSegments(),
+            iterableWithSize(greaterThan(1))
+        );
+        // start another node
+        final String node2 = internalCluster().startNode();
+        ensureStableCluster(2);
+        setTotalSpace(node2, Long.MAX_VALUE);
+        // relocate the shard from node1 to node2
+        ClusterRerouteUtils.reroute(client(), new MoveAllocationCommand(indexName, 0, node1, node2));
+        ClusterHealthResponse clusterHealthResponse = clusterAdmin().prepareHealth(TEST_REQUEST_TIMEOUT)
+            .setWaitForEvents(Priority.LANGUID)
+            .setWaitForNoRelocatingShards(true)
+            .setTimeout(ACCEPTABLE_RELOCATION_TIME)
+            .get();
+        assertThat(clusterHealthResponse.isTimedOut(), equalTo(false));
+        // the force merge call is now unblocked
+        assertBusy(() -> {
+            assertTrue(forceMergeBeforeRelocationFuture.isDone());
+            assertFalse(forceMergeBeforeRelocationFuture.isCancelled());
+        });
+        // there is some merging going on in the {@code PostRecoveryMerger} after recovery, but that's not guaranteeing us a single segment,
+        // so let's trigger a force merge to 1 segment again (this one should succeed promptly)
+        indicesAdmin().prepareForceMerge(indexName).setMaxNumSegments(1).get();
+        IndicesSegmentResponse indicesSegmentResponseAfterRelocation = indicesAdmin().prepareSegments(indexName).get();
+        // assert there's only one segment now
+        assertThat(
+            indicesSegmentResponseAfterRelocation.getIndices().get(indexName).iterator().next().shards()[0].getSegments(),
+            iterableWithSize(1)
+        );
+        // also assert that the shard was indeed moved to a different node
+        assertThat(
+            indicesSegmentResponseAfterRelocation.getIndices().get(indexName).iterator().next().shards()[0].getShardRouting()
+                .currentNodeId(),
+            not(
+                equalTo(
+                    indicesSegmentResponseBeforeRelocation.getIndices().get(indexName).iterator().next().shards()[0].getShardRouting()
+                        .currentNodeId()
+                )
+            )
+        );
+    }
+
     public void setTotalSpace(String dataNodeName, long totalSpace) {
         getTestFileStore(dataNodeName).setTotalSpace(totalSpace);
         refreshClusterInfo();