Enforce Completion Context Limit (#38675) (#39075)

This change adds a limit to the number of completion contexts that a completion field can define.

Closes #32741

docs/reference/migration/migrate_7_0/mappings.asciidoc

@@ -72,3 +72,10 @@ or `quadtree`. This will ensure compatibility with previously created indexes.
 The following type parameters are deprecated for the `geo_shape` field type: `tree`,
 `precision`, `tree_levels`, `distance_error_pct`, `points_only`, and `strategy`. They
 will be removed in a future version.
+
+[float]
+==== Limiting the number of completion contexts
+
+The maximum allowed number of completion contexts in a mapping will be limited
+to 10 in the next major version. Completion fields that define more than 10
+contexts in a mapping will log a deprecation warning in this version.

server/src/main/java/org/elasticsearch/index/mapper/CompletionFieldMapper.java

@@ -18,6 +18,7 @@
  */
 package org.elasticsearch.index.mapper;
 
+import org.apache.logging.log4j.LogManager;
 import org.apache.lucene.codecs.PostingsFormat;
 import org.apache.lucene.index.IndexableField;
 import org.apache.lucene.index.Term;
@@ -31,8 +32,10 @@
 import org.apache.lucene.search.suggest.document.RegexCompletionQuery;
 import org.apache.lucene.search.suggest.document.SuggestField;
 import org.elasticsearch.Version;
+import org.elasticsearch.cluster.metadata.IndexMetaData;
 import org.elasticsearch.common.ParseField;
 import org.elasticsearch.common.ParsingException;
+import org.elasticsearch.common.logging.DeprecationLogger;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.unit.Fuzziness;
 import org.elasticsearch.common.util.set.Sets;
@@ -85,6 +88,11 @@
 public class CompletionFieldMapper extends FieldMapper implements ArrayValueMapperParser {
     public static final String CONTENT_TYPE = "completion";
 
+    /**
+     * Maximum allowed number of completion contexts in a mapping.
+     */
+    static final int COMPLETION_CONTEXTS_LIMIT = 10;
+
     public static class Defaults {
         public static final MappedFieldType FIELD_TYPE = new CompletionFieldType();
         static {
@@ -354,6 +362,8 @@ public static class Builder extends FieldMapper.Builder<Builder, CompletionField
         private boolean preserveSeparators = Defaults.DEFAULT_PRESERVE_SEPARATORS;
         private boolean preservePositionIncrements = Defaults.DEFAULT_POSITION_INCREMENTS;
 
+        private static final DeprecationLogger deprecationLogger = new DeprecationLogger(LogManager.getLogger(Builder.class));
+
         /**
          * @param name of the completion field to build
          */
@@ -397,6 +407,7 @@ public Builder preservePositionIncrements(boolean preservePositionIncrements) {
 
         @Override
         public CompletionFieldMapper build(BuilderContext context) {
+            checkCompletionContextsLimit(context);
             setupFieldType(context);
             CompletionFieldType completionFieldType = (CompletionFieldType) this.fieldType;
             completionFieldType.setContextMappings(contextMappings);
@@ -405,6 +416,15 @@ public CompletionFieldMapper build(BuilderContext context) {
             return new CompletionFieldMapper(name, this.fieldType, context.indexSettings(),
                 multiFieldsBuilder.build(this, context), copyTo, maxInputLength);
         }
+
+        private void checkCompletionContextsLimit(BuilderContext context) {
+            if (this.contextMappings != null && this.contextMappings.size() > COMPLETION_CONTEXTS_LIMIT) {
+                deprecationLogger.deprecated("You have defined more than [" + COMPLETION_CONTEXTS_LIMIT + "] completion contexts" +
+                    " in the mapping for index [" + context.indexSettings().get(IndexMetaData.SETTING_INDEX_PROVIDED_NAME) + "]. " +
+                    "The maximum allowed number of completion contexts in a mapping will be limited to " +
+                    "[" + COMPLETION_CONTEXTS_LIMIT + "] starting in version [8.0].");
+            }
+        }
     }
 
     private int maxInputLength;

server/src/test/java/org/elasticsearch/index/mapper/CompletionFieldMapperTests.java

@@ -54,6 +54,7 @@
 import java.util.function.Function;
 
 import static org.elasticsearch.common.xcontent.XContentFactory.jsonBuilder;
+import static org.elasticsearch.index.mapper.CompletionFieldMapper.COMPLETION_CONTEXTS_LIMIT;
 import static org.hamcrest.Matchers.arrayContainingInAnyOrder;
 import static org.hamcrest.Matchers.arrayWithSize;
 import static org.hamcrest.Matchers.containsString;
@@ -908,6 +909,27 @@ public void testEmptyName() throws IOException {
         assertThat(e.getMessage(), containsString("name cannot be empty string"));
     }
 
+    public void testLimitOfContextMappings() throws Throwable {
+        final String index = "test";
+        XContentBuilder mappingBuilder = XContentFactory.jsonBuilder().startObject().startObject("properties")
+            .startObject("suggest").field("type", "completion").startArray("contexts");
+        for (int i = 0; i < COMPLETION_CONTEXTS_LIMIT + 1; i++) {
+            mappingBuilder.startObject();
+            mappingBuilder.field("name", Integer.toString(i));
+            mappingBuilder.field("type", "category");
+            mappingBuilder.endObject();
+        }
+
+        mappingBuilder.endArray().endObject().endObject().endObject();
+        String mappings = Strings.toString(mappingBuilder);
+
+        DocumentMapper mapper = createIndex(index).mapperService().documentMapperParser()
+            .parse("type1", new CompressedXContent(mappings));
+        assertWarnings("You have defined more than [" + COMPLETION_CONTEXTS_LIMIT + "] completion contexts" +
+            " in the mapping for index [test]. The maximum allowed number of completion contexts in a mapping will be limited to " +
+                "[" + COMPLETION_CONTEXTS_LIMIT + "] starting in version [8.0].");
+    }
+
     private Matcher<IndexableField> suggestField(String value) {
         return Matchers.allOf(hasProperty(IndexableField::stringValue, equalTo(value)),
             Matchers.instanceOf(SuggestField.class));