Add segment attributes to the `_segments` API. (#26157)

This contains information about whether high compression was enabled for instance.

Closes #26130

core/src/main/java/org/elasticsearch/action/admin/indices/segments/IndicesSegmentResponse.java

@@ -155,6 +155,9 @@ public class IndicesSegmentResponse extends BroadcastResponse implements ToXCont
                             }
                             builder.endArray();
                         }
+                        if (segment.attributes != null && segment.attributes.isEmpty() == false) {
+                            builder.field("attributes", segment.attributes);
+                        }
                         builder.endObject();
                     }
                     builder.endObject();

core/src/main/java/org/elasticsearch/index/engine/Engine.java

@@ -703,6 +703,7 @@ public abstract class Engine implements Closeable {
                 if (verbose) {
                     segment.ramTree = Accountables.namedAccountable("root", segmentReader);
                 }
+                segment.attributes = info.info.getAttributes();
                 // TODO: add more fine grained mem stats values to per segment info here
                 segments.put(info.info.name, segment);
             }

core/src/main/java/org/elasticsearch/index/engine/Segment.java

@@ -39,6 +39,7 @@ import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
+import java.util.Map;
 
 public class Segment implements Streamable {
 
@@ -55,6 +56,7 @@ public class Segment implements Streamable {
     public long memoryInBytes;
     public Sort segmentSort;
     public Accountable ramTree = null;
+    public Map<String, String> attributes;
 
     Segment() {
     }
@@ -128,6 +130,14 @@ public class Segment implements Streamable {
         return segmentSort;
     }
 
+    /**
+     * Return segment attributes.
+     * @see org.apache.lucene.index.SegmentInfo#getAttributes()
+     */
+    public Map<String, String> getAttributes() {
+        return attributes;
+    }
+
     @Override
     public boolean equals(Object o) {
         if (this == o) return true;
@@ -173,6 +183,11 @@ public class Segment implements Streamable {
         } else {
             segmentSort = null;
         }
+        if (in.getVersion().onOrAfter(Version.V_6_1_0) && in.readBoolean()) {
+            attributes = in.readMap(StreamInput::readString, StreamInput::readString);
+        } else {
+            attributes = null;
+        }
     }
 
     @Override
@@ -196,6 +211,13 @@ public class Segment implements Streamable {
         if (out.getVersion().onOrAfter(Version.V_6_0_0_alpha1)) {
             writeSegmentSort(out, segmentSort);
         }
+        if (out.getVersion().onOrAfter(Version.V_6_1_0)) {
+            boolean hasAttributes = attributes != null;
+            out.writeBoolean(hasAttributes);
+            if (hasAttributes) {
+                out.writeMap(attributes, StreamOutput::writeString, StreamOutput::writeString);
+            }
+        }
     }
 
     Sort readSegmentSort(StreamInput in) throws IOException {
@@ -329,6 +351,7 @@ public class Segment implements Streamable {
                 ", mergeId='" + mergeId + '\'' +
                 ", memoryInBytes=" + memoryInBytes +
                 (segmentSort != null ? ", sort=" + segmentSort : "") +
+                ", attributes=" + attributes +
                 '}';
     }
 }

core/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java

@@ -29,6 +29,7 @@ import org.apache.logging.log4j.core.filter.RegexFilter;
 import org.apache.lucene.analysis.Analyzer;
 import org.apache.lucene.analysis.standard.StandardAnalyzer;
 import org.apache.lucene.codecs.Codec;
+import org.apache.lucene.codecs.lucene50.Lucene50StoredFieldsFormat;
 import org.apache.lucene.document.Field;
 import org.apache.lucene.document.LongPoint;
 import org.apache.lucene.document.NumericDocValuesField;
@@ -138,6 +139,7 @@ import org.elasticsearch.test.IndexSettingsModule;
 import org.elasticsearch.threadpool.TestThreadPool;
 import org.elasticsearch.threadpool.ThreadPool;
 import org.hamcrest.MatcherAssert;
+import org.hamcrest.Matchers;
 import org.junit.After;
 import org.junit.Before;
 
@@ -495,6 +497,7 @@ public class InternalEngineTests extends ESTestCase {
             assertThat(segments.get(0).getDeletedDocs(), equalTo(0));
             assertThat(segments.get(0).isCompound(), equalTo(true));
             assertThat(segments.get(0).ramTree, nullValue());
+            assertThat(segments.get(0).getAttributes().keySet(), Matchers.contains(Lucene50StoredFieldsFormat.MODE_KEY));
 
             engine.flush();
 

core/src/test/java/org/elasticsearch/index/engine/SegmentTests.java

@@ -31,6 +31,7 @@ import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.test.ESTestCase;
 
 import java.io.IOException;
+import java.util.Collections;
 import java.util.Objects;
 
 public class SegmentTests extends ESTestCase {
@@ -81,6 +82,9 @@ public class SegmentTests extends ESTestCase {
         segment.mergeId = randomAlphaOfLengthBetween(1, 10);
         segment.memoryInBytes = randomNonNegativeLong();
         segment.segmentSort = randomIndexSort();
+        if (randomBoolean()) {
+            segment.attributes = Collections.singletonMap("foo", "bar");
+        }
         return segment;
     }