Deprecate negative weight in Function Score Query

docs/reference/migration/migrate_6_0/search.asciidoc

@@ -99,6 +99,8 @@ the `match` query but is supported for `match_phrase` and `match_phrase_prefix`.
 * The deprecated multi term rewrite parameters `constant_score_auto`, `constant_score_filter` (synonyms for `constant_score`)
 have been removed.
 
+*  Setting a negative `weight` in Function Score Query is deprecated.
+
 ==== Search shards API
 
 The search shards API no longer accepts the `type` url parameter, which didn't

server/src/main/java/org/elasticsearch/index/query/functionscore/ScoreFunctionBuilder.java

@@ -19,12 +19,13 @@
 
 package org.elasticsearch.index.query.functionscore;
 
+import org.apache.logging.log4j.LogManager;
 import org.elasticsearch.common.io.stream.NamedWriteable;
 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.search.function.ScoreFunction;
 import org.elasticsearch.common.lucene.search.function.WeightFactorFunction;
-import org.elasticsearch.common.xcontent.ToXContent.Params;
 import org.elasticsearch.common.xcontent.ToXContentFragment;
 import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.index.query.QueryShardContext;
@@ -34,6 +35,8 @@
 
 public abstract class ScoreFunctionBuilder<FB extends ScoreFunctionBuilder<FB>> implements ToXContentFragment, NamedWriteable {
 
+    private static final DeprecationLogger DEPRECATION_LOGGER = new DeprecationLogger(LogManager.getLogger(ScoreFunctionBuilder.class));
+
     private Float weight;
 
     /**
@@ -46,7 +49,7 @@ public ScoreFunctionBuilder() {
      * Read from a stream.
      */
     public ScoreFunctionBuilder(StreamInput in) throws IOException {
-        weight = in.readOptionalFloat();
+        weight = checkWeight(in.readOptionalFloat());
     }
 
     @Override
@@ -70,10 +73,18 @@ public final void writeTo(StreamOutput out) throws IOException {
      */
     @SuppressWarnings("unchecked")
     public final FB setWeight(float weight) {
-        this.weight = weight;
+        this.weight = checkWeight(weight);
         return (FB) this;
     }
 
+    private Float checkWeight(Float weight) {
+        if (weight != null && Float.compare(weight, 0) < 0) {
+            DEPRECATION_LOGGER.deprecated("Setting a negative [weight] in Function Score Query is deprecated "
+                + "and will throw an error in the next major version");
+        }
+        return weight;
+    }
+
     /**
      * The weight applied to the function before combining.
      */

server/src/test/java/org/elasticsearch/index/query/functionscore/FunctionScoreQueryBuilderTests.java

@@ -20,6 +20,7 @@
 package org.elasticsearch.index.query.functionscore;
 
 import com.fasterxml.jackson.core.JsonParseException;
+
 import org.apache.lucene.index.Term;
 import org.apache.lucene.search.MatchAllDocsQuery;
 import org.apache.lucene.search.Query;
@@ -285,6 +286,13 @@ public void testIllegalArguments() {
         expectThrows(IllegalArgumentException.class, () -> builder.boostMode(null));
     }
 
+    public void testDeprecatedArgumanets() {
+        float weight = -1 * randomFloat();
+        new FunctionScoreQueryBuilder.FilterFunctionBuilder(new WeightBuilder().setWeight(weight));
+        assertWarnings("Setting a negative [weight] in Function Score Query is deprecated "
+            + "and will throw an error in the next major version");
+    }
+
     public void testParseFunctionsArray() throws IOException {
         String functionScoreQuery = "{\n" +
             "    \"function_score\":{\n" +