Fix term stats when talking to ES 6

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/mixed_cluster/30_dfs_query.yml

@@ -0,0 +1,14 @@
+---
+"Perform a dfs_query_then_fetch search on a keyword field":
+  - do:
+      search:
+        search_type: dfs_query_then_fetch
+        index: keyword_index
+        rest_total_hits_as_int: true
+        body:
+          query:
+            match:
+              field:
+                query: value
+
+  - match: { hits.total: 3 }

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/old_cluster/30_dfs_query.yml

@@ -0,0 +1,43 @@
+---
+"Perform a dfs_query_then_fetch search on a keyword field":
+  - do:
+      indices.create:
+        index: keyword_index
+        body:
+          mappings:
+            properties:
+              field:
+                type: keyword
+  - do:
+      index:
+        index: keyword_index
+        body:
+          field: value
+        refresh: true
+
+  - do:
+      index:
+        index: keyword_index
+        body:
+          field: value
+        refresh: true
+
+  - do:
+      index:
+        index: keyword_index
+        body:
+          field: value
+        refresh: true
+
+  - do:
+      search:
+        search_type: dfs_query_then_fetch
+        index: keyword_index
+        rest_total_hits_as_int: true
+        body:
+          query:
+            match:
+              field:
+                query: value
+
+  - match: { hits.total: 3 }

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/upgraded_cluster/30_dfs_query.yml

@@ -0,0 +1,14 @@
+---
+"Perform a dfs_query_then_fetch search on a keyword field":
+  - do:
+      search:
+        search_type: dfs_query_then_fetch
+        index: keyword_index
+        rest_total_hits_as_int: true
+        body:
+          query:
+            match:
+              field:
+                query: value
+
+  - match: { hits.total: 3 }

server/src/main/java/org/elasticsearch/search/dfs/AggregatedDfs.java

@@ -14,6 +14,8 @@
 import org.apache.lucene.index.Term;
 import org.apache.lucene.search.CollectionStatistics;
 import org.apache.lucene.search.TermStatistics;
+import org.apache.lucene.util.BytesRef;
+import org.elasticsearch.Version;
 import org.elasticsearch.common.collect.HppcMaps;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
@@ -32,9 +34,19 @@ public AggregatedDfs(StreamInput in) throws IOException {
         termStatistics = HppcMaps.newMap(size);
         for (int i = 0; i < size; i++) {
             Term term = new Term(in.readString(), in.readBytesRef());
-            TermStatistics stats = new TermStatistics(in.readBytesRef(),
-                in.readVLong(),
-                DfsSearchResult.subOne(in.readVLong()));
+            BytesRef term2 = in.readBytesRef();
+            final long docFreq = in.readVLong();
+            assert docFreq >= 0;
+            long totalTermFreq = DfsSearchResult.subOne(in.readVLong());
+            if (in.getVersion().before(Version.V_7_0_0)) {
+                if (totalTermFreq == -1L) {
+                    // Lucene 7 and earlier used -1 to denote that this information wasn't stored by the codec
+                    // or that this field omitted term frequencies and positions. It used docFreq as fallback in that case
+                    // when calculating similarities. See LUCENE-8007 for more information.
+                    totalTermFreq = docFreq;
+                }
+            }
+            TermStatistics stats = new TermStatistics(term2, docFreq, totalTermFreq);
             termStatistics.put(term, stats);
         }
         fieldStatistics = DfsSearchResult.readFieldStats(in);

server/src/main/java/org/elasticsearch/search/dfs/DfsSearchResult.java

@@ -116,7 +116,7 @@ public static void writeFieldStats(StreamOutput out, ObjectObjectHashMap<String,
         for (ObjectObjectCursor<String, CollectionStatistics> c : fieldStatistics) {
             out.writeString(c.key);
             CollectionStatistics statistics = c.value;
-            assert statistics.maxDoc() >= 0;
+            assert statistics.maxDoc() > 0;
             out.writeVLong(statistics.maxDoc());
             if (out.getVersion().onOrAfter(Version.V_7_0_0)) {
                 // stats are always positive numbers
@@ -156,8 +156,8 @@ static ObjectObjectHashMap<String, CollectionStatistics> readFieldStats(StreamIn
             final String field = in.readString();
             assert field != null;
             final long maxDoc = in.readVLong();
-            final long docCount;
-            final long sumTotalTermFreq;
+            long docCount;
+            long sumTotalTermFreq;
             final long sumDocFreq;
             if (in.getVersion().onOrAfter(Version.V_7_0_0)) {
                 // stats are always positive numbers
@@ -168,6 +168,26 @@ static ObjectObjectHashMap<String, CollectionStatistics> readFieldStats(StreamIn
                 docCount = subOne(in.readVLong());
                 sumTotalTermFreq = subOne(in.readVLong());
                 sumDocFreq = subOne(in.readVLong());
+                if (sumTotalTermFreq == -1L) {
+                    // Lucene 7 and earlier used -1 to denote that this information wasn't stored by the codec
+                    // or that this field omitted term frequencies and positions. It used docFreq as fallback in that case
+                    // when calculating similarities. See LUCENE-8007 for more information.
+                    sumTotalTermFreq = sumDocFreq;
+                }
+                if (docCount == -1L) {
+                    // Lucene 7 and earlier used -1 to denote that this information wasn't stored by the codec
+                    // It used maxDoc as fallback in that case when calculating similarities. See LUCENE-8007 for more information.
+                    docCount = maxDoc;
+                }
+                if (docCount == 0L) {
+                    // empty stats object (LUCENE-8020)
+                    assert maxDoc == 0 && docCount == 0 && sumTotalTermFreq == 0 && sumDocFreq == 0:
+                        " maxDoc:" + maxDoc +
+                        " docCount:" + docCount +
+                        " sumTotalTermFreq:" + sumTotalTermFreq +
+                        " sumDocFreq:" + sumDocFreq;
+                    continue;
+                }
             }
             CollectionStatistics stats = new CollectionStatistics(field, maxDoc, docCount, sumTotalTermFreq, sumDocFreq);
             fieldStatistics.put(field, stats);
@@ -187,10 +207,18 @@ static TermStatistics[] readTermStats(StreamInput in, Term[] terms) throws IOExc
                 BytesRef term = terms[i].bytes();
                 final long docFreq = in.readVLong();
                 assert docFreq >= 0;
-                final long totalTermFreq = subOne(in.readVLong());
+                long totalTermFreq = subOne(in.readVLong());
                 if (docFreq == 0) {
                     continue;
                 }
+                if (in.getVersion().before(Version.V_7_0_0)) {
+                    if (totalTermFreq == -1L) {
+                        // Lucene 7 and earlier used -1 to denote that this information isn't stored by the codec
+                        // or that this field omits term frequencies and positions. It used docFreq as fallback in that case
+                        // when calculating similarities. See LUCENE-8007 for more information.
+                        totalTermFreq = docFreq;
+                    }
+                }
                 termStatistics[i] = new TermStatistics(term, docFreq, totalTermFreq);
             }
         }

server/src/test/java/org/elasticsearch/search/dfs/DfsSearchResultTests.java

@@ -0,0 +1,58 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.search.dfs;
+
+import com.carrotsearch.hppc.ObjectObjectHashMap;
+
+import org.apache.lucene.search.CollectionStatistics;
+import org.elasticsearch.Version;
+import org.elasticsearch.common.bytes.BytesReference;
+import org.elasticsearch.common.io.stream.BytesStreamOutput;
+import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.test.ESTestCase;
+import org.elasticsearch.test.VersionUtils;
+
+import java.io.IOException;
+
+public class DfsSearchResultTests extends ESTestCase {
+
+    /**
+     * checks inputs from 6.x that are difficult to simulate in a BWC mixed-cluster test, in particular the case
+     * where docCount == -1L which does not occur with the codecs that we typically use.
+     */
+    public void test6xSerialization() throws IOException {
+        Version version = VersionUtils.randomVersionBetween(random(), Version.V_6_8_0, Version.V_6_8_18);
+        BytesStreamOutput os = new BytesStreamOutput();
+        os.setVersion(version);
+        os.writeVInt(1);
+        String field = randomAlphaOfLength(10);
+        os.writeString(field);
+        long maxDoc = randomIntBetween(1, 5);
+        os.writeVLong(maxDoc);
+        long docCount = randomBoolean() ? -1 : randomIntBetween(1, (int) maxDoc);
+        os.writeVLong(DfsSearchResult.addOne(docCount));
+        long sumTotalTermFreq = randomBoolean() ? -1 : randomIntBetween(20, 30);
+        os.writeVLong(DfsSearchResult.addOne(sumTotalTermFreq));
+        long sumDocFreq = sumTotalTermFreq == -1 ? randomIntBetween(20, 30) : randomIntBetween(20, (int) sumTotalTermFreq);
+        os.writeVLong(DfsSearchResult.addOne(sumDocFreq));
+
+        try (StreamInput input = StreamInput.wrap(BytesReference.toBytes(os.bytes()))) {
+            input.setVersion(version);
+            ObjectObjectHashMap<String, CollectionStatistics> stats = DfsSearchResult.readFieldStats(input);
+            assertEquals(stats.size(), 1);
+            assertNotNull(stats.get(field));
+            CollectionStatistics cs = stats.get(field);
+            assertEquals(field, cs.field());
+            assertEquals(maxDoc, cs.maxDoc());
+            assertEquals(docCount == -1 ? maxDoc : docCount, cs.docCount());
+            assertEquals(sumDocFreq, cs.sumDocFreq());
+            assertEquals(sumTotalTermFreq == -1 ? sumDocFreq : sumTotalTermFreq, cs.sumTotalTermFreq());
+        }
+    }
+}