Deprecate negative query boost (#34486) (#34512)

This change deprecates negative query boosts. Negative scores are not allowed in Lucene 8 so
it is easier to just disallow negative boosts entirely.

Relates #33309

Author: jimczi

docs/reference/migration/migrate_6_0/search.asciidoc

@@ -250,3 +250,9 @@ can set the <<query-dsl-multi-term-rewrite, rewrite method>> of the multi term q
 rewrite. Or, if you use `span_multi` on `prefix` query only, you can activate the
 <<index-prefix-config,`index_prefixes`>> field option of the `text` field instead. This will
 rewrite any prefix query on the field to a a single term query that matches the indexed prefix.
+
+[float]
+==== Negative boosts are deprecated
+
+Setting a negative `boost` in a query is deprecated and will throw an error in the next version.
+To deboost a specific query you can use a `boost` comprise between 0 and 1.

server/src/main/java/org/elasticsearch/index/query/AbstractQueryBuilder.java

@@ -19,6 +19,8 @@
 
 package org.elasticsearch.index.query;
 
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
 import org.apache.lucene.search.BoostQuery;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.spans.SpanBoostQuery;
@@ -29,6 +31,7 @@
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.common.logging.DeprecationLogger;
 import org.elasticsearch.common.lucene.BytesRefs;
 import org.elasticsearch.common.xcontent.AbstractObjectParser;
 import org.elasticsearch.common.xcontent.NamedObjectNotFoundException;
@@ -50,6 +53,9 @@
  */
 public abstract class AbstractQueryBuilder<QB extends AbstractQueryBuilder<QB>> implements QueryBuilder {
 
+    private static final Logger logger = LogManager.getLogger(AbstractQueryBuilder.class);
+    private static final DeprecationLogger deprecationLogger = new DeprecationLogger(logger);
+
     /** Default for boost to apply to resulting Lucene query. Defaults to 1.0*/
     public static final float DEFAULT_BOOST = 1.0f;
     public static final ParseField NAME_FIELD = new ParseField("_name");
@@ -159,6 +165,10 @@ public final float boost() {
     @SuppressWarnings("unchecked")
     @Override
     public final QB boost(float boost) {
+        if (Float.compare(boost, 0f) < 0) {
+            deprecationLogger.deprecatedAndMaybeLog("negative boost", "setting a negative [boost] on a query " +
+                "is deprecated and will throw an error in the next version. You can use a value between 0 and 1 to deboost.");
+        }
         this.boost = boost;
         return (QB) this;
     }

test/framework/src/main/java/org/elasticsearch/test/AbstractQueryTestCase.java

@@ -77,7 +77,6 @@
 import static org.hamcrest.Matchers.greaterThan;
 import static org.hamcrest.Matchers.instanceOf;
 
-
 public abstract class AbstractQueryTestCase<QB extends AbstractQueryBuilder<QB>> extends AbstractBuilderTestCase {
 
     private static final int NUMBER_OF_TESTQUERIES = 20;
@@ -101,6 +100,13 @@ public final QB createTestQueryBuilder() {
      */
     protected abstract QB doCreateTestQueryBuilder();
 
+    public void testNegativeBoosts() {
+        QB testQuery = createTestQueryBuilder();
+        testQuery.boost(-0.5f);
+        assertWarnings("setting a negative [boost] on a query" +
+            " is deprecated and will throw an error in the next version. You can use a value between 0 and 1 to deboost.");
+    }
+
     /**
      * Generic test that creates new query from the test query and checks both for equality
      * and asserts equality on the two queries.