Return 400 BAD_REQUEST when a field scorer references a missing field (#127229)

Currently it returns 5xx, when it's entirely an error in the request

Author: thecoop

docs/changelog/127229.yaml

@@ -0,0 +1,6 @@
+pr: 127229
+summary: Return BAD_REQUEST when a field scorer references a missing field
+area: Ranking
+type: bug
+issues:
+ - 127162

plugins/examples/rescore/src/yamlRestTest/resources/rest-api-spec/test/example-rescore/30_factor_field.yml

@@ -48,6 +48,24 @@ setup:
   - match: { hits.hits.1._score: 20 }
   - match: { hits.hits.2._score: 10 }
 
+---
+"referencing a missing field returns bad request":
+  - requires:
+      cluster_features: [ "search.rescorer.missing.field.bad.request" ]
+      reason: "Testing the behaviour change with this feature"
+  - do:
+      catch: bad_request
+      search:
+        index: test
+        body:
+          rescore:
+            example:
+              factor: 1
+              factor_field: missing
+  - match: { status: 400 }
+  - match: { error.root_cause.0.type: "illegal_argument_exception" }
+  - match: { error.root_cause.0.reason: "Missing value for field [missing]" }
+
 ---
 "sorted based on a numeric field and rescored based on a factor field using a window size":
   - do:

server/src/main/java/org/elasticsearch/common/lucene/search/function/FieldValueFactorFunction.java

@@ -11,7 +11,6 @@
 
 import org.apache.lucene.index.LeafReaderContext;
 import org.apache.lucene.search.Explanation;
-import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.io.stream.Writeable;
@@ -73,7 +72,7 @@ public double score(int docId, float subQueryScore) throws IOException {
                     if (missing != null) {
                         value = missing;
                     } else {
-                        throw new ElasticsearchException("Missing value for field [" + field + "]");
+                        throw new IllegalArgumentException("Missing value for field [" + field + "]");
                     }
                 }
                 double val = value * boostFactor;

server/src/main/java/org/elasticsearch/search/SearchFeatures.java

@@ -28,9 +28,10 @@ public Set<NodeFeature> getFeatures() {
     public static final NodeFeature COMPLETION_FIELD_SUPPORTS_DUPLICATE_SUGGESTIONS = new NodeFeature(
         "search.completion_field.duplicate.support"
     );
+    public static final NodeFeature RESCORER_MISSING_FIELD_BAD_REQUEST = new NodeFeature("search.rescorer.missing.field.bad.request");
 
     @Override
     public Set<NodeFeature> getTestFeatures() {
-        return Set.of(RETRIEVER_RESCORER_ENABLED, COMPLETION_FIELD_SUPPORTS_DUPLICATE_SUGGESTIONS);
+        return Set.of(RETRIEVER_RESCORER_ENABLED, COMPLETION_FIELD_SUPPORTS_DUPLICATE_SUGGESTIONS, RESCORER_MISSING_FIELD_BAD_REQUEST);
     }
 }