Make _id terms optional in segment containing only noop

server/src/main/java/org/elasticsearch/common/lucene/uid/PerThreadIDVersionAndSeqNoLookup.java

@@ -28,6 +28,7 @@
 import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.util.Bits;
 import org.apache.lucene.util.BytesRef;
+import org.elasticsearch.common.lucene.Lucene;
 import org.elasticsearch.common.lucene.uid.VersionsAndSeqNoResolver.DocIdAndSeqNo;
 import org.elasticsearch.common.lucene.uid.VersionsAndSeqNoResolver.DocIdAndVersion;
 import org.elasticsearch.index.mapper.SeqNoFieldMapper;
@@ -66,15 +67,22 @@ final class PerThreadIDVersionAndSeqNoLookup {
      */
     PerThreadIDVersionAndSeqNoLookup(LeafReader reader, String uidField) throws IOException {
         this.uidField = uidField;
-        Terms terms = reader.terms(uidField);
+        final Terms terms = reader.terms(uidField);
         if (terms == null) {
-            throw new IllegalArgumentException("reader misses the [" + uidField + "] field");
+            // If a segment contains only no-ops, it does not have _uid but has both _soft_deletes and _tombstone fields.
+            final NumericDocValues softDeletesDV = reader.getNumericDocValues(Lucene.SOFT_DELETE_FIELD);
+            final NumericDocValues tombstoneDV = reader.getNumericDocValues(SeqNoFieldMapper.TOMBSTONE_NAME);
+            if (softDeletesDV == null || tombstoneDV == null) {
+                throw new IllegalArgumentException("reader does not have _uid terms but not a no-op segment; " +
+                    "_soft_deletes [" + softDeletesDV + "], _tombstone [" + tombstoneDV + "]");
+            }
+            termsEnum = null;
+        } else {
+            termsEnum = terms.iterator();
         }
-        termsEnum = terms.iterator();
         if (reader.getNumericDocValues(VersionFieldMapper.NAME) == null) {
-            throw new IllegalArgumentException("reader misses the [" + VersionFieldMapper.NAME + "] field");
+            throw new IllegalArgumentException("reader misses the [" + VersionFieldMapper.NAME + "] field; _uid terms [" + terms + "]");
         }
-
         Object readerKey = null;
         assert (readerKey = reader.getCoreCacheHelper().getKey()) != null;
         this.readerKey = readerKey;
@@ -111,7 +119,8 @@ public DocIdAndVersion lookupVersion(BytesRef id, LeafReaderContext context)
      * {@link DocIdSetIterator#NO_MORE_DOCS} is returned if not found
      * */
     private int getDocID(BytesRef id, Bits liveDocs) throws IOException {
-        if (termsEnum.seekExact(id)) {
+        // termsEnum can possibly be null here if this leaf contains only no-ops.
+        if (termsEnum != null && termsEnum.seekExact(id)) {
             int docID = DocIdSetIterator.NO_MORE_DOCS;
             // there may be more than one matching docID, in the case of nested docs, so we want the last one:
             docsEnum = termsEnum.postings(docsEnum, 0);

server/src/main/java/org/elasticsearch/index/engine/InternalEngine.java

@@ -1347,6 +1347,9 @@ private NoOpResult innerNoOp(final NoOp noOp) throws IOException {
                 try {
                     final ParsedDocument tombstone = engineConfig.getTombstoneDocSupplier().newNoopTombstoneDoc();
                     tombstone.updateSeqID(noOp.seqNo(), noOp.primaryTerm());
+                    // A noop tombstone does not require a _version but it's added to have a fully dense docvalues for the version field.
+                    // 1L is selected to optimize the compression because it might probably be the most common value in version field.
+                    tombstone.version().setLongValue(1L);
                     assert tombstone.docs().size() == 1 : "Tombstone should have a single doc [" + tombstone + "]";
                     final ParseContext.Document doc = tombstone.docs().get(0);
                     assert doc.getField(SeqNoFieldMapper.TOMBSTONE_NAME) != null

server/src/main/java/org/elasticsearch/index/mapper/DocumentMapper.java

@@ -178,8 +178,8 @@ public DocumentMapper(MapperService mapperService, Mapping mapping) {
             TypeFieldMapper.NAME, SeqNoFieldMapper.NAME, SeqNoFieldMapper.PRIMARY_TERM_NAME, SeqNoFieldMapper.TOMBSTONE_NAME);
         this.deleteTombstoneMetadataFieldMappers = Stream.of(mapping.metadataMappers)
             .filter(field -> deleteTombstoneMetadataFields.contains(field.name())).toArray(MetadataFieldMapper[]::new);
-        final Collection<String> noopTombstoneMetadataFields =
-            Arrays.asList(SeqNoFieldMapper.NAME, SeqNoFieldMapper.PRIMARY_TERM_NAME, SeqNoFieldMapper.TOMBSTONE_NAME);
+        final Collection<String> noopTombstoneMetadataFields = Arrays.asList(
+            VersionFieldMapper.NAME, SeqNoFieldMapper.NAME, SeqNoFieldMapper.PRIMARY_TERM_NAME, SeqNoFieldMapper.TOMBSTONE_NAME);
         this.noopTombstoneMetadataFieldMappers = Stream.of(mapping.metadataMappers)
             .filter(field -> noopTombstoneMetadataFields.contains(field.name())).toArray(MetadataFieldMapper[]::new);
     }

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

@@ -3781,6 +3781,61 @@ protected long doGenerateSeqNoForOperation(Operation operation) {
         }
     }
 
+    /**
+     * Verifies that a segment containing only no-ops can be used to look up _version and _seqno.
+     */
+    public void testSegmentContainsOnlyNoOps() throws Exception {
+        Engine.NoOpResult noOpResult = engine.noOp(new Engine.NoOp(1, primaryTerm.get(),
+            randomFrom(Engine.Operation.Origin.values()), randomNonNegativeLong(), "test"));
+        assertThat(noOpResult.getFailure(), nullValue());
+        engine.refresh("test");
+        Engine.DeleteResult deleteResult = engine.delete(replicaDeleteForDoc("id", 1, 2, randomNonNegativeLong()));
+        assertThat(deleteResult.getFailure(), nullValue());
+        engine.refresh("test");
+    }
+
+    /**
+     * A simple test to check that random combination of operations can coexist in segments and be lookup.
+     * This is needed as some fields in Lucene may not exist if a segment misses operation types and this code is to check for that.
+     * For example, a segment containing only no-ops does not have neither _uid or _version.
+     */
+    public void testRandomOperations() throws Exception {
+        int numOps = between(10, 100);
+        for (int i = 0; i < numOps; i++) {
+            String id = Integer.toString(randomIntBetween(1, 10));
+            ParsedDocument doc = createParsedDoc(id, null);
+            Engine.Operation.TYPE type = randomFrom(Engine.Operation.TYPE.values());
+            switch (type) {
+                case INDEX:
+                    Engine.IndexResult index = engine.index(replicaIndexForDoc(doc, between(1, 100), i, randomBoolean()));
+                    assertThat(index.getFailure(), nullValue());
+                    break;
+                case DELETE:
+                    Engine.DeleteResult delete = engine.delete(replicaDeleteForDoc(doc.id(), between(1, 100), i, randomNonNegativeLong()));
+                    assertThat(delete.getFailure(), nullValue());
+                    break;
+                case NO_OP:
+                    Engine.NoOpResult noOp = engine.noOp(new Engine.NoOp(i, primaryTerm.get(),
+                        randomFrom(Engine.Operation.Origin.values()), randomNonNegativeLong(), ""));
+                    assertThat(noOp.getFailure(), nullValue());
+                    break;
+                default:
+                    throw new IllegalStateException("Invalid op [" + type + "]");
+            }
+            if (randomBoolean()) {
+                engine.refresh("test");
+            }
+            if (randomBoolean()) {
+                engine.flush();
+            }
+            if (randomBoolean()) {
+                engine.forceMerge(randomBoolean(), between(1, 10), randomBoolean(), false, false);
+            }
+        }
+        List<Translog.Operation> operations = readAllOperationsInLucene(engine, createMapperService("test"));
+        assertThat(operations, hasSize(numOps));
+    }
+
     public void testMinGenerationForSeqNo() throws IOException, BrokenBarrierException, InterruptedException {
         engine.close();
         final int numberOfTriplets = randomIntBetween(1, 32);

server/src/test/java/org/elasticsearch/index/shard/IndexShardTests.java

@@ -3113,8 +3113,8 @@ public void testSupplyTombstoneDoc() throws Exception {
         assertThat(noopTombstone.docs(), hasSize(1));
         ParseContext.Document noopDoc = noopTombstone.docs().get(0);
         assertThat(noopDoc.getFields().stream().map(IndexableField::name).collect(Collectors.toList()),
-            containsInAnyOrder(SeqNoFieldMapper.NAME, SeqNoFieldMapper.NAME, SeqNoFieldMapper.PRIMARY_TERM_NAME,
-                SeqNoFieldMapper.TOMBSTONE_NAME));
+            containsInAnyOrder(VersionFieldMapper.NAME, SeqNoFieldMapper.TOMBSTONE_NAME,
+                SeqNoFieldMapper.NAME, SeqNoFieldMapper.NAME, SeqNoFieldMapper.PRIMARY_TERM_NAME));
         assertThat(noopDoc.getField(SeqNoFieldMapper.TOMBSTONE_NAME).numericValue().longValue(), equalTo(1L));
 
         closeShards(shard);

test/framework/src/main/java/org/elasticsearch/index/engine/EngineTestCase.java

@@ -315,7 +315,10 @@ public ParsedDocument newNoopTombstoneDoc() {
                 doc.add(seqID.primaryTerm);
                 seqID.tombstoneField.setLongValue(1);
                 doc.add(seqID.tombstoneField);
-                return new ParsedDocument(null, seqID, null, null, null, Collections.singletonList(doc), null, XContentType.JSON, null);
+                Field versionField = new NumericDocValuesField(VersionFieldMapper.NAME, 0);
+                doc.add(versionField);
+                return new ParsedDocument(versionField, seqID, null, null, null,
+                    Collections.singletonList(doc), null, XContentType.JSON, null);
             }
         };
     }
@@ -734,6 +737,7 @@ private static Translog.Operation readOperationInLucene(IndexSearcher searcher,
         final int segmentDocID = docID - leaves.get(leafIndex).docBase;
         final long seqNo = readNumericDV(leaves.get(leafIndex), SeqNoFieldMapper.NAME, segmentDocID);
         final long primaryTerm = readNumericDV(leaves.get(leafIndex), SeqNoFieldMapper.PRIMARY_TERM_NAME, segmentDocID);
+        final long version = readNumericDV(leaves.get(leafIndex), VersionFieldMapper.NAME, segmentDocID);
         final FieldsVisitor fields = new FieldsVisitor(true);
         searcher.doc(docID, fields);
         fields.postProcess(mapper);
@@ -743,11 +747,11 @@ private static Translog.Operation readOperationInLucene(IndexSearcher searcher,
             op = new Translog.NoOp(seqNo, primaryTerm, "");
             assert readNumericDV(leaves.get(leafIndex), Lucene.SOFT_DELETE_FIELD, segmentDocID) == 1
                 : "Noop operation but soft_deletes field is not set";
+            assert version == 1 : "Noop tombstone should have version 1L; actual version [" + version + "]";
         } else {
             final String id = fields.uid().id();
             final String type = fields.uid().type();
             final Term uid = new Term(IdFieldMapper.NAME, Uid.encodeId(id));
-            final long version = readNumericDV(leaves.get(leafIndex), VersionFieldMapper.NAME, segmentDocID);
             if (isTombstone) {
                 op = new Translog.Delete(type, id, uid, seqNo, primaryTerm, version, VersionType.INTERNAL);
                 assert readNumericDV(leaves.get(leafIndex), Lucene.SOFT_DELETE_FIELD, segmentDocID) == 1