Make ILM force merging best effort (#43246)

It's possible for force merges kicked off by ILM to silently stop (due
to a node relocating for example). In which case, the segment count may
not reach what the user configured. In the subsequent `SegmentCountStep`
waiting for the expected segment count may wait indefinitely. Because of
this, this commit makes force merges "best effort" and then changes the
`SegmentCountStep` to simply report (at INFO level) if the merge was not
successful.

Relates to #42824
Resolves #43245

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/indexlifecycle/SegmentCountStep.java

@@ -5,10 +5,15 @@
  */
 package org.elasticsearch.xpack.core.indexlifecycle;
 
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
 import org.elasticsearch.action.ActionListener;
+import org.elasticsearch.action.admin.indices.segments.IndexSegments;
 import org.elasticsearch.action.admin.indices.segments.IndicesSegmentsRequest;
+import org.elasticsearch.action.admin.indices.segments.ShardSegments;
 import org.elasticsearch.client.Client;
 import org.elasticsearch.cluster.metadata.IndexMetaData;
+import org.elasticsearch.cluster.routing.ShardRouting;
 import org.elasticsearch.common.ParseField;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.xcontent.ConstructingObjectParser;
@@ -17,13 +22,17 @@
 
 import java.io.IOException;
 import java.util.Arrays;
+import java.util.List;
+import java.util.Map;
 import java.util.Objects;
-import java.util.stream.StreamSupport;
+import java.util.stream.Collectors;
 
 /**
  * This {@link Step} evaluates whether force_merge was successful by checking the segment count.
  */
 public class SegmentCountStep extends AsyncWaitStep {
+
+    private static final Logger logger = LogManager.getLogger(SegmentCountStep.class);
     public static final String NAME = "segment-count";
 
     private final int maxNumSegments;
@@ -41,10 +50,19 @@ public int getMaxNumSegments() {
     public void evaluateCondition(IndexMetaData indexMetaData, Listener listener) {
         getClient().admin().indices().segments(new IndicesSegmentsRequest(indexMetaData.getIndex().getName()),
             ActionListener.wrap(response -> {
-                long numberShardsLeftToMerge =
-                    StreamSupport.stream(response.getIndices().get(indexMetaData.getIndex().getName()).spliterator(), false)
-                        .filter(iss -> Arrays.stream(iss.getShards()).anyMatch(p -> p.getSegments().size() > maxNumSegments)).count();
-                listener.onResponse(numberShardsLeftToMerge == 0, new Info(numberShardsLeftToMerge));
+                IndexSegments segments = response.getIndices().get(indexMetaData.getIndex().getName());
+                List<ShardSegments> unmergedShards = segments.getShards().values().stream()
+                    .flatMap(iss -> Arrays.stream(iss.getShards()))
+                    .filter(shardSegments -> shardSegments.getSegments().size() > maxNumSegments)
+                    .collect(Collectors.toList());
+                if (unmergedShards.size() > 0) {
+                    Map<ShardRouting, Integer> unmergedShardCounts = unmergedShards.stream()
+                        .collect(Collectors.toMap(ShardSegments::getShardRouting, ss -> ss.getSegments().size()));
+                    logger.info("[{}] best effort force merge to [{}] segments did not succeed for {} shards: {}",
+                        indexMetaData.getIndex().getName(), maxNumSegments, unmergedShards.size(), unmergedShardCounts);
+                }
+                // Force merging is best effort, so always return true that the condition has been met.
+                listener.onResponse(true, new Info(unmergedShards.size()));
             }, listener::onFailure));
     }
 
@@ -90,8 +108,12 @@ public long getNumberShardsLeftToMerge() {
         @Override
         public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
             builder.startObject();
-            builder.field(MESSAGE.getPreferredName(),
-                    "Waiting for [" + numberShardsLeftToMerge + "] shards " + "to forcemerge");
+            if (numberShardsLeftToMerge == 0) {
+                builder.field(MESSAGE.getPreferredName(), "all shards force merged successfully");
+            } else {
+                builder.field(MESSAGE.getPreferredName(),
+                    "[" + numberShardsLeftToMerge + "] shards did not successfully force merge");
+            }
             builder.field(SHARDS_TO_MERGE.getPreferredName(), numberShardsLeftToMerge);
             builder.endObject();
             return builder;

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/indexlifecycle/SegmentCountStepTests.java

@@ -138,7 +138,7 @@ public void onFailure(Exception e) {
         assertEquals(new SegmentCountStep.Info(0L), conditionInfo.get());
     }
 
-    public void testIsConditionFails() {
+    public void testIsConditionIsTrueEvenWhenMoreSegments() {
         int maxNumSegments = randomIntBetween(3, 10);
         Index index = new Index(randomAlphaOfLengthBetween(1, 20), randomAlphaOfLengthBetween(1, 20));
         Client client = Mockito.mock(Client.class);
@@ -191,8 +191,8 @@ public void onFailure(Exception e) {
             }
         });
 
-        assertFalse(conditionMetResult.get());
-        assertEquals(new SegmentCountStep.Info(1L), conditionInfo.get());
+        assertTrue(conditionMetResult.get());
+        assertEquals(new SegmentCountStep.Info(0L), conditionInfo.get());
     }
 
     public void testThrowsException() {