Add soft limit on allowed number of script fields in request (#26598)

Requesting to many script_fields in a search request can be costly
because of script execution. This change introduces a soft limit on the number
of script fields that are allowed per request. The setting can be
changed per index using the index.max_script_fields setting.

Relates to #26390

core/src/main/java/org/elasticsearch/common/settings/IndexScopedSettings.java

@@ -111,6 +111,7 @@ public final class IndexScopedSettings extends AbstractScopedSettings {
         IndexSettings.INDEX_REFRESH_INTERVAL_SETTING,
         IndexSettings.MAX_RESULT_WINDOW_SETTING,
         IndexSettings.MAX_DOCVALUE_FIELDS_SEARCH_SETTING,
+        IndexSettings.MAX_SCRIPT_FIELDS_SETTING,
         IndexSettings.MAX_RESCORE_WINDOW_SETTING,
         IndexSettings.MAX_ADJACENCY_MATRIX_FILTERS_SETTING,
         IndexSettings.INDEX_TRANSLOG_SYNC_INTERVAL_SETTING,

core/src/main/java/org/elasticsearch/index/IndexSettings.java

@@ -102,6 +102,15 @@ public final class IndexSettings {
      */
     public static final Setting<Integer> MAX_RESULT_WINDOW_SETTING =
         Setting.intSetting("index.max_result_window", 10000, 1, Property.Dynamic, Property.IndexScope);
+
+    /**
+     * Index setting describing the maximum value of allowed `script_fields`that can be retrieved
+     * per search request. The default maximum of 32 is defensive for the reason that retrieving
+     * script fields is a costly operation.
+     */
+    public static final Setting<Integer> MAX_SCRIPT_FIELDS_SETTING =
+        Setting.intSetting("index.max_script_fields", 32, 0, Property.Dynamic, Property.IndexScope);
+
     /**
      * Index setting describing the maximum value of allowed `docvalue_fields`that can be retrieved
      * per search request. The default maximum of 100 is defensive for the reason that retrieving
@@ -232,6 +241,7 @@ public final class IndexSettings {
     private volatile int maxAdjacencyMatrixFilters;
     private volatile int maxRescoreWindow;
     private volatile int maxDocvalueFields;
+    private volatile int maxScriptFields;
     private volatile boolean TTLPurgeDisabled;
     /**
      * The maximum number of refresh listeners allows on this shard.
@@ -329,6 +339,7 @@ public IndexSettings(final IndexMetaData indexMetaData, final Settings nodeSetti
         maxAdjacencyMatrixFilters = scopedSettings.get(MAX_ADJACENCY_MATRIX_FILTERS_SETTING);
         maxRescoreWindow = scopedSettings.get(MAX_RESCORE_WINDOW_SETTING);
         maxDocvalueFields = scopedSettings.get(MAX_DOCVALUE_FIELDS_SEARCH_SETTING);
+        maxScriptFields = scopedSettings.get(MAX_SCRIPT_FIELDS_SETTING);
         TTLPurgeDisabled = scopedSettings.get(INDEX_TTL_DISABLE_PURGE_SETTING);
         maxRefreshListeners = scopedSettings.get(MAX_REFRESH_LISTENERS_PER_SHARD);
         maxSlicesPerScroll = scopedSettings.get(MAX_SLICES_PER_SCROLL);
@@ -358,6 +369,7 @@ public IndexSettings(final IndexMetaData indexMetaData, final Settings nodeSetti
         scopedSettings.addSettingsUpdateConsumer(MAX_ADJACENCY_MATRIX_FILTERS_SETTING, this::setMaxAdjacencyMatrixFilters);
         scopedSettings.addSettingsUpdateConsumer(MAX_RESCORE_WINDOW_SETTING, this::setMaxRescoreWindow);
         scopedSettings.addSettingsUpdateConsumer(MAX_DOCVALUE_FIELDS_SEARCH_SETTING, this::setMaxDocvalueFields);
+        scopedSettings.addSettingsUpdateConsumer(MAX_SCRIPT_FIELDS_SETTING, this::setMaxScriptFields);
         scopedSettings.addSettingsUpdateConsumer(INDEX_WARMER_ENABLED_SETTING, this::setEnableWarmer);
         scopedSettings.addSettingsUpdateConsumer(INDEX_GC_DELETES_SETTING, this::setGCDeletes);
         scopedSettings.addSettingsUpdateConsumer(INDEX_TRANSLOG_FLUSH_THRESHOLD_SIZE_SETTING, this::setTranslogFlushThresholdSize);
@@ -613,6 +625,17 @@ private void setMaxDocvalueFields(int maxDocvalueFields) {
         this.maxDocvalueFields = maxDocvalueFields;
     }
 
+    /**
+     * Returns the maximum number of allowed script_fields to retrieve in a search request
+     */
+    public int getMaxScriptFields() {
+        return this.maxScriptFields;
+    }
+
+    private void setMaxScriptFields(int maxScriptFields) {
+        this.maxScriptFields = maxScriptFields;
+    }
+
     /**
      * Returns the GC deletes cycle in milliseconds.
      */

core/src/main/java/org/elasticsearch/search/SearchService.java

@@ -755,6 +755,13 @@ private void parseSource(DefaultSearchContext context, SearchSourceBuilder sourc
             }
         }
         if (source.scriptFields() != null) {
+            int maxAllowedScriptFields = context.mapperService().getIndexSettings().getMaxScriptFields();
+            if (source.scriptFields().size() > maxAllowedScriptFields) {
+                throw new IllegalArgumentException(
+                        "Trying to retrieve too many script_fields. Must be less than or equal to: [" + maxAllowedScriptFields
+                                + "] but was [" + source.scriptFields().size() + "]. This limit can be set by changing the ["
+                                + IndexSettings.MAX_SCRIPT_FIELDS_SETTING.getKey() + "] index level setting.");
+            }
             for (org.elasticsearch.search.builder.SearchSourceBuilder.ScriptField field : source.scriptFields()) {
                 SearchScript.Factory factory = scriptService.compile(field.script(), SearchScript.CONTEXT);
                 SearchScript.LeafFactory searchScript = factory.newFactory(field.script().getParams(), context.lookup());

core/src/test/java/org/elasticsearch/index/IndexSettingsTests.java

@@ -305,6 +305,22 @@ public void testMaxDocvalueFields() {
         assertEquals(IndexSettings.MAX_DOCVALUE_FIELDS_SEARCH_SETTING.get(Settings.EMPTY).intValue(), settings.getMaxDocvalueFields());
     }
 
+    public void testMaxScriptFields() {
+        IndexMetaData metaData = newIndexMeta("index", Settings.builder().put(IndexMetaData.SETTING_VERSION_CREATED, Version.CURRENT)
+                .put(IndexSettings.MAX_SCRIPT_FIELDS_SETTING.getKey(), 100).build());
+        IndexSettings settings = new IndexSettings(metaData, Settings.EMPTY);
+        assertEquals(100, settings.getMaxScriptFields());
+        settings.updateIndexMetaData(
+                newIndexMeta("index", Settings.builder().put(IndexSettings.MAX_SCRIPT_FIELDS_SETTING.getKey(), 20).build()));
+        assertEquals(20, settings.getMaxScriptFields());
+        settings.updateIndexMetaData(newIndexMeta("index", Settings.EMPTY));
+        assertEquals(IndexSettings.MAX_SCRIPT_FIELDS_SETTING.get(Settings.EMPTY).intValue(), settings.getMaxScriptFields());
+
+        metaData = newIndexMeta("index", Settings.builder().put(IndexMetaData.SETTING_VERSION_CREATED, Version.CURRENT).build());
+        settings = new IndexSettings(metaData, Settings.EMPTY);
+        assertEquals(IndexSettings.MAX_SCRIPT_FIELDS_SETTING.get(Settings.EMPTY).intValue(), settings.getMaxScriptFields());
+    }
+
     public void testMaxAdjacencyMatrixFiltersSetting() {
         IndexMetaData metaData = newIndexMeta("index", Settings.builder()
             .put(IndexMetaData.SETTING_VERSION_CREATED, Version.CURRENT)

core/src/test/java/org/elasticsearch/search/SearchServiceTests.java

@@ -47,6 +47,10 @@
 import org.elasticsearch.indices.IndicesService;
 import org.elasticsearch.plugins.Plugin;
 import org.elasticsearch.plugins.SearchPlugin;
+import org.elasticsearch.script.MockScriptEngine;
+import org.elasticsearch.script.MockScriptPlugin;
+import org.elasticsearch.script.Script;
+import org.elasticsearch.script.ScriptType;
 import org.elasticsearch.search.aggregations.bucket.global.GlobalAggregationBuilder;
 import org.elasticsearch.search.aggregations.bucket.terms.TermsAggregationBuilder;
 import org.elasticsearch.search.aggregations.support.ValueType;
@@ -60,11 +64,14 @@
 
 import java.io.IOException;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
+import java.util.Map;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Semaphore;
 import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.function.Function;
 
 import static java.util.Collections.singletonList;
 import static org.elasticsearch.action.support.WriteRequest.RefreshPolicy.IMMEDIATE;
@@ -83,7 +90,19 @@ protected boolean resetNodeAfterTest() {
 
     @Override
     protected Collection<Class<? extends Plugin>> getPlugins() {
-        return pluginList(FailOnRewriteQueryPlugin.class);
+        return pluginList(FailOnRewriteQueryPlugin.class, CustomScriptPlugin.class);
+    }
+
+    public static class CustomScriptPlugin extends MockScriptPlugin {
+
+        static final String DUMMY_SCRIPT = "dummyScript";
+
+        @Override
+        protected Map<String, Function<Map<String, Object>, Object>> pluginScripts() {
+            return Collections.singletonMap(DUMMY_SCRIPT, vars -> {
+                return "dummy";
+            });
+        }
     }
 
     @Override
@@ -290,6 +309,39 @@ searchSourceBuilder, new String[0], false, new AliasFilter(null, Strings.EMPTY_A
         }
     }
 
+    /**
+     * test that getting more than the allowed number of script_fields throws an exception
+     */
+    public void testMaxScriptFieldsSearch() throws IOException {
+        createIndex("index");
+        final SearchService service = getInstanceFromNode(SearchService.class);
+        final IndicesService indicesService = getInstanceFromNode(IndicesService.class);
+        final IndexService indexService = indicesService.indexServiceSafe(resolveIndex("index"));
+        final IndexShard indexShard = indexService.getShard(0);
+
+        SearchSourceBuilder searchSourceBuilder = new SearchSourceBuilder();
+        // adding the maximum allowed number of script_fields to retrieve
+        int maxScriptFields = indexService.getIndexSettings().getMaxScriptFields();
+        for (int i = 0; i < maxScriptFields; i++) {
+            searchSourceBuilder.scriptField("field" + i,
+                    new Script(ScriptType.INLINE, MockScriptEngine.NAME, CustomScriptPlugin.DUMMY_SCRIPT, Collections.emptyMap()));
+        }
+        try (SearchContext context = service.createContext(new ShardSearchLocalRequest(indexShard.shardId(), 1, SearchType.DEFAULT,
+                searchSourceBuilder, new String[0], false, new AliasFilter(null, Strings.EMPTY_ARRAY), 1.0f), null)) {
+            assertNotNull(context);
+            searchSourceBuilder.scriptField("anotherScriptField",
+                    new Script(ScriptType.INLINE, MockScriptEngine.NAME, CustomScriptPlugin.DUMMY_SCRIPT, Collections.emptyMap()));
+            IllegalArgumentException ex = expectThrows(IllegalArgumentException.class,
+                    () -> service.createContext(new ShardSearchLocalRequest(indexShard.shardId(), 1, SearchType.DEFAULT,
+                            searchSourceBuilder, new String[0], false, new AliasFilter(null, Strings.EMPTY_ARRAY), 1.0f), null));
+            assertEquals(
+                    "Trying to retrieve too many script_fields. Must be less than or equal to: [" + maxScriptFields + "] but was ["
+                            + (maxScriptFields + 1)
+                            + "]. This limit can be set by changing the [index.max_script_fields] index level setting.",
+                    ex.getMessage());
+        }
+    }
+
     public static class FailOnRewriteQueryPlugin extends Plugin implements SearchPlugin {
         @Override
         public List<QuerySpec<?>> getQueries() {

docs/reference/index-modules.asciidoc

@@ -134,6 +134,11 @@ specific index module:
     Defaults to `100`. Doc-value fields are costly since they might incur
     a per-field per-document seek.
 
+`index.max_script_fields`::
+
+    The maximum number of `script_fields` that are allowed in a query.
+    Defaults to `32`.
+
 `index.blocks.read_only`::
 
     Set to `true` to make the index and index metadata read only, `false` to

docs/reference/migration/migrate_6_0/search.asciidoc

@@ -142,6 +142,11 @@ The deprecated `fielddata_fields` have now been removed. `docvalue_fields` shoul
 `docvalue_fields` now have a default upper limit of 100 fields that can be requested.
 This limit can be overridden by using the `index.max_docvalue_fields_search` index setting.
 
+==== `script_fields`
+
+`script_fields` now have a default upper limit of 32 script fields that can be requested.
+This limit can be overridden by using the `index.max_script_fields` index setting.
+
 ==== Inner hits
 
 The source inside a hit of inner hits keeps its full path with respect to the entire source.

rest-api-spec/src/main/resources/rest-api-spec/test/search/30_limits.yml

@@ -72,3 +72,29 @@ setup:
           query:
             match_all: {}
           docvalue_fields: ["one", "two", "three"]
+
+---
+"Script_fields size limit":
+  - skip:
+      version: " - 5.99.99"
+      reason: soft limit for script_fields only available as of 6.0.0
+
+  - do:
+      indices.create:
+          index: test_3
+          body:
+            settings:
+              index.max_script_fields: 2
+
+  - do:
+      catch:      /Trying to retrieve too many script_fields\. Must be less than or equal to[:] \[2\] but was \[3\]\. This limit can be set by changing the \[index.max_script_fields\] index level setting\./
+      search:
+        index: test_3
+        body:
+          query:
+            match_all: {}
+          script_fields: {
+            "test1" : { "script" : { "lang": "painless", "source": "1" }},
+            "test2" : { "script" : { "lang": "painless", "source": "1" }},
+            "test3" : { "script" : { "lang": "painless", "source": "1" }}
+          }