Scripting: Add back lookup vars in score script (#34833) (#35386)

The lookup vars under params (namely _fields and _source) were
inadvertently removed when scoring scripts were converted to using
script contexts. This commit adds them back, along with deprecation
warnings for those that should not be used.

Author: jdconrad

server/src/main/java/org/elasticsearch/script/ScoreScript.java

@@ -26,14 +26,32 @@
 
 import java.io.IOException;
 import java.io.UncheckedIOException;
+import java.util.Collections;
+import java.util.HashMap;
 import java.util.Map;
 import java.util.function.DoubleSupplier;
 
 /**
  * A script used for adjusting the score on a per document basis.
  */
 public abstract class ScoreScript {
-    
+
+    private static final Map<String, String> DEPRECATIONS;
+    static {
+        Map<String, String> deprecations = new HashMap<>();
+        deprecations.put(
+            "doc",
+            "Accessing variable [doc] via [params.doc] from within a score script " +
+                "is deprecated in favor of directly accessing [doc]."
+        );
+        deprecations.put(
+            "_doc",
+            "Accessing variable [doc] via [params._doc] from within a score script " +
+                "is deprecated in favor of directly accessing [doc]."
+        );
+        DEPRECATIONS = Collections.unmodifiableMap(deprecations);
+    }
+
     public static final String[] PARAMETERS = new String[]{};
 
     /** The generic runtime parameters for the script. */
@@ -45,8 +63,18 @@ public abstract class ScoreScript {
     private DoubleSupplier scoreSupplier = () -> 0.0;
 
     public ScoreScript(Map<String, Object> params, SearchLookup lookup, LeafReaderContext leafContext) {
-        this.params = params;
-        this.leafLookup = lookup.getLeafSearchLookup(leafContext);
+        // null check needed b/c of expression engine subclass
+        if (lookup == null) {
+            assert params == null;
+            assert leafContext == null;
+            this.params = null;
+            this.leafLookup = null;
+        } else {
+            this.leafLookup = lookup.getLeafSearchLookup(leafContext);
+            params = new HashMap<>(params);
+            params.putAll(leafLookup.asMap());
+            this.params = new ParameterMap(params, DEPRECATIONS);
+        }
     }
 
     public abstract double execute();