Enforce Completion Context Limit

* Enforcing a maximum number of completion contexts as reqested in #32741
* Closes #32741

Author: TommyWind

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

@@ -108,6 +108,9 @@ public enum MergeReason {
         Setting.boolSetting("index.mapper.dynamic", INDEX_MAPPER_DYNAMIC_DEFAULT,
                 Property.Dynamic, Property.IndexScope, Property.Deprecated);
 
+    // Maximum allowed number of completion contexts in a mapping
+    public static final int COMPLETION_CONTEXTS_LIMIT = 10;
+
     //TODO this needs to be cleaned up: _timestamp and _ttl are not supported anymore, _field_names, _seq_no, _version and _source are
     //also missing, not sure if on purpose. See IndicesModule#getMetadataMappers
     private static final String[] SORTED_META_FIELDS = new String[]{
@@ -497,6 +500,7 @@ private synchronized Map<String, DocumentMapper> internalMerge(@Nullable Documen
             ContextMapping.validateContextPaths(indexSettings.getIndexVersionCreated(), fieldMappers, fieldTypes::get);
 
             if (reason == MergeReason.MAPPING_UPDATE) {
+                checkCompletionContextsLimit(fieldMappers);
                 // this check will only be performed on the master node when there is
                 // a call to the update mapping API. For all other cases like
                 // the master node restoring mappings from disk or data nodes
@@ -557,6 +561,18 @@ private synchronized Map<String, DocumentMapper> internalMerge(@Nullable Documen
         return results;
     }
 
+    private void checkCompletionContextsLimit(List<FieldMapper> fieldMappers) {
+        for (FieldMapper fieldMapper : fieldMappers) {
+            if (CompletionFieldMapper.CONTENT_TYPE.equals(fieldMapper.typeName())) {
+                CompletionFieldMapper.CompletionFieldType fieldType = ((CompletionFieldMapper) fieldMapper).fieldType();
+                if (fieldType.hasContextMappings() && fieldType.getContextMappings().size() > COMPLETION_CONTEXTS_LIMIT) {
+                    throw new IllegalArgumentException("Limit of contexts [" + COMPLETION_CONTEXTS_LIMIT + "] in index ["
+                        + index().getName() + "] has been exceeded");
+                }
+            }
+        }
+    }
+
     private boolean assertMappersShareSameFieldType() {
         if (mapper != null) {
             List<FieldMapper> fieldMappers = new ArrayList<>();

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

@@ -323,4 +323,25 @@ public void testDefaultMappingIsRejectedOn7() throws IOException {
             + " that indices can have at most one type.", e.getMessage());
     }
 
+    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 < MapperService.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);
+
+        IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> {
+            createIndex(index).mapperService().merge("type1", new CompressedXContent(mappings), MergeReason.MAPPING_UPDATE);
+        });
+        assertTrue(e.getMessage(),
+            e.getMessage().contains("Limit of contexts [" + MapperService.COMPLETION_CONTEXTS_LIMIT +
+                "] in index [" + index + "] has been exceeded"));
+    }
 }