Fix quoted _exists_ query (#33019)

This change in the `query_string` query fixes the detection of the special
`_exists_` field when it is used with a quoted term.

Closes #28922

server/src/main/java/org/elasticsearch/index/search/QueryStringQueryParser.java

@@ -280,14 +280,14 @@ public class QueryStringQueryParser extends XQueryParser {
 
     @Override
     public Query getFieldQuery(String field, String queryText, boolean quoted) throws ParseException {
-        if (quoted) {
-            return getFieldQuery(field, queryText, getPhraseSlop());
-        }
-
         if (field != null && EXISTS_FIELD.equals(field)) {
             return existsQuery(queryText);
         }
 
+        if (quoted) {
+            return getFieldQuery(field, queryText, getPhraseSlop());
+        }
+
         // Detects additional operators '<', '<=', '>', '>=' to handle range query with one side unbounded.
         // It is required to use a prefix field operator to enable the detection since they are not treated
         // as logical operator by the query parser (e.g. age:>=10).
@@ -333,6 +333,10 @@ public class QueryStringQueryParser extends XQueryParser {
 
     @Override
     protected Query getFieldQuery(String field, String queryText, int slop) throws ParseException {
+        if (field != null && EXISTS_FIELD.equals(field)) {
+            return existsQuery(queryText);
+        }
+
         Map<String, Float> fields = extractMultiFields(field, true);
         if (fields.isEmpty()) {
             return newUnmappedFieldQuery(field);

server/src/test/java/org/elasticsearch/index/query/QueryStringQueryBuilderTests.java

@@ -998,6 +998,18 @@ public class QueryStringQueryBuilderTests extends AbstractQueryTestCase<QueryStr
         } else {
             assertThat(query, equalTo(new ConstantScoreQuery(new TermQuery(new Term("_field_names", STRING_FIELD_NAME)))));
         }
+
+        for (boolean quoted : new boolean[] {true, false}) {
+            String value = (quoted ? "\"" : "") + STRING_FIELD_NAME + (quoted ? "\"" : "");
+            queryBuilder = new QueryStringQueryBuilder("_exists_:" + value);
+            query = queryBuilder.toQuery(context);
+            if (context.getIndexSettings().getIndexVersionCreated().onOrAfter(Version.V_6_1_0)
+                && (context.fieldMapper(STRING_FIELD_NAME).omitNorms() == false)) {
+                assertThat(query, equalTo(new ConstantScoreQuery(new NormsFieldExistsQuery(STRING_FIELD_NAME))));
+            } else {
+                assertThat(query, equalTo(new ConstantScoreQuery(new TermQuery(new Term("_field_names", STRING_FIELD_NAME)))));
+            }
+        }
         QueryShardContext contextNoType = createShardContextWithNoType();
         query = queryBuilder.toQuery(contextNoType);
         assertThat(query, equalTo(new MatchNoDocsQuery()));