Fix Fast Vector Highlighter NPE on match phrase prefix (#25116)

The FVH fails with an NPE when a match phrase prefix is rewritten in an empty phrase query.
This change makes sure that the multi match query rewrites to a MatchNoDocsQuery (instead of an empty phrase query) when there is
a single term and that term does not expand to any term in the index.

Fixes #25088

1 files changed, 15 insertions, 1 deletions

diff --git a/core/src/test/java/org/elasticsearch/search/fetch/subphase/highlight/HighlighterSearchIT.java b/core/src/test/java/org/elasticsearch/search/fetch/subphase/highlight/HighlighterSearchIT.java
index c8c0314805..f7f9bbaeb4 100644
--- a/core/src/test/java/org/elasticsearch/search/fetch/subphase/highlight/HighlighterSearchIT.java
+++ b/core/src/test/java/org/elasticsearch/search/fetch/subphase/highlight/HighlighterSearchIT.java
@@ -1462,7 +1462,6 @@ public class HighlighterSearchIT extends ESIntegTestCase {
 
             assertHighlight(searchResponse, 0, "field0", 0, 1, equalTo("The quick <x>brown</x> fox jumps over the lazy dog"));
 
-
             source = searchSource()
                 .query(matchPhrasePrefixQuery("field0", "quick bro"))
                 .highlighter(highlight().field("field0").order("score").preTags("<x>").postTags("</x>").highlighterType(type));
@@ -1473,6 +1472,21 @@ public class HighlighterSearchIT extends ESIntegTestCase {
 
             logger.info("--> highlighting and searching on field1");
             source = searchSource()
+                .query(boolQuery()
+                    .should(matchPhrasePrefixQuery("field1", "test"))
+                    .should(matchPhrasePrefixQuery("field1", "bro"))
+                )
+                .highlighter(highlight().field("field1").order("score").preTags("<x>").postTags("</x>").highlighterType(type));
+
+            searchResponse = client().search(searchRequest("test").source(source)).actionGet();
+            assertThat(searchResponse.getHits().totalHits, equalTo(2L));
+            for (int i = 0; i < 2; i++) {
+                assertHighlight(searchResponse, i, "field1", 0, 1, anyOf(
+                    equalTo("The quick <x>browse</x> button is a fancy thing, right <x>bro</x>?"),
+                    equalTo("The quick <x>brown</x> fox jumps over the lazy dog")));
+            }
+
+            source = searchSource()
                 .query(matchPhrasePrefixQuery("field1", "quick bro"))
                 .highlighter(highlight().field("field1").order("score").preTags("<x>").postTags("</x>").highlighterType(type));