Handle missing and multiple values in script

Previously in script for numeric fields, there was no way to check if
a document is missing a value. Also certain operations on multiple-
values fields were missing.

This PR adds the following:

- add the following functions for multiple-valued numeric fields:
    doc['field'].min returns the minumum amoung values
    doc['field'].max returns the maximum amoung values
    doc['field'].sum returns the sum of amoung values
    doc['field'].avg returns the average of values

- return null for doc['field'] if a document is missing a 'field1':
    Now we can do this:
    if (doc['field'] == null) {return -1;} return doc['field'].value; or
    doc['field']?.value ?: -1
    This new behaviour will only work if the following system property is set:
    `export ES_JAVA_OPTS="-Des.script.null_for_missing_value=true"'

Closes elastic#29286

docs/CHANGELOG.asciidoc

@@ -12,6 +12,7 @@
 === Breaking Java Changes
 
 === Deprecations
+Returning 0 for missing numeric fields in script is deprecated. PR: 29611
 
 === New Features 
 

docs/painless/painless-getting-started.asciidoc

@@ -121,12 +121,22 @@ GET hockey/_search
 // CONSOLE
 
 [float]
-===== Missing and multiple values
+===== Missing values
 
-If a document is missing a field `field`, `doc['field']` for this document
-will return null.
+Currently by default, if a document is missing a numeric field `field`,
+`doc['field'].value` returns `0` for this document. This default behaviour
+will be changed in the next major version of elasticsearch:
+if a document is missing a field `field`, `doc['field']` for this document
+will return `null`. You can set a system property
+`export ES_JAVA_OPTS="-Des.script.null_for_missing_value=true"' on a node
+to make this node's behaviour compatible with the future major version.
+Otherwise, every time the node starts a deprecation warning will remind you
+about this forthcoming change.
 
-There is also a number of operations designed for numeric fields,
+
+===== Multiple values
+
+There is a number of operations designed for numeric fields,
 if a document has multiple values in such a field:
 
 - `doc['field'].min` - gets the minimum value among values

modules/lang-painless/build.gradle

@@ -25,6 +25,7 @@ esplugin {
 }
 
 integTestCluster {
+  systemProperty 'es.script.null_for_missing_value', 'true'
   module project.project(':modules:mapper-extras')
 }
 

server/src/main/java/org/elasticsearch/node/Node.java

@@ -716,6 +716,10 @@ public void onTimeout(TimeValue timeout) {
             writePortsFile("transport", transport.boundAddress());
         }
 
+        if (!ScriptModule.NULL_FOR_MISSING_VALUE)
+            logger.warn("Script: returning 0 for missing numeric values is deprecated. " +
+                "Set system property '-Des.script.null_for_missing_value=true' to make behaviour compatible with future major versions.");
+
         logger.info("started");
 
         pluginsService.filterPlugins(ClusterPlugin.class).forEach(ClusterPlugin::onNodeStarted);

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

@@ -27,6 +27,7 @@
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
+import org.elasticsearch.common.Booleans;
 import org.elasticsearch.common.settings.ClusterSettings;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.plugins.ScriptPlugin;
@@ -52,6 +53,9 @@ public class ScriptModule {
         ).collect(Collectors.toMap(c -> c.name, Function.identity()));
     }
 
+    public static final boolean NULL_FOR_MISSING_VALUE =
+        Booleans.parseBoolean(System.getProperty("es.script.null_for_missing_value", "false"));
+
     private final ScriptService scriptService;
 
     public ScriptModule(Settings settings, List<ScriptPlugin> scriptPlugins) {

server/src/main/java/org/elasticsearch/search/lookup/LeafDocLookup.java

@@ -25,6 +25,7 @@
 import org.elasticsearch.index.fielddata.ScriptDocValues;
 import org.elasticsearch.index.mapper.MappedFieldType;
 import org.elasticsearch.index.mapper.MapperService;
+import org.elasticsearch.script.ScriptModule;
 
 import java.io.IOException;
 import java.security.AccessController;
@@ -92,7 +93,7 @@ public ScriptDocValues<?> run() {
         }
         try {
             boolean docHasValues = scriptValues.setNextDocId(docId);
-            if (!docHasValues) return null;
+            if (!docHasValues && ScriptModule.NULL_FOR_MISSING_VALUE) return null;
         } catch (IOException e) {
             throw ExceptionsHelper.convertToElastic(e);
         }