NullPointer fix for field capabilities API

modules/runtime-fields-common/src/yamlRestTest/resources/rest-api-spec/test/runtime_fields/110_composite.yml

@@ -102,3 +102,19 @@ query:
   - match: {aggregations.response.buckets.0.doc_count: 5 }
   - match: {aggregations.response.buckets.1.key: 304 }
   - match: {aggregations.response.buckets.1.doc_count: 1 }
+
+---
+"Field caps with composite runtime mappings section. Issue 76742":
+
+  - skip:
+      version: " - 7.14.99"
+      reason: Composite Runtime mappings support was added in 7.15
+
+  - do:
+      field_caps:
+        index: http_logs
+        fields: "*"
+
+  - match: {indices:                             ["http_logs"]}
+  - match: {fields.http\.response.long.type:       long}
+  - match: {fields.http\.clientip.ip.type:         ip}

server/src/main/java/org/elasticsearch/action/fieldcaps/TransportFieldCapabilitiesAction.java

@@ -371,10 +371,13 @@ private FieldCapabilitiesIndexResponse shardOperation(final FieldCapabilitiesInd
                         if (searchExecutionContext.getFieldType(parentField) == null) {
                             // no field type, it must be an object field
                             ObjectMapper mapper = searchExecutionContext.getObjectMapper(parentField);
-                            String type = mapper.isNested() ? "nested" : "object";
-                            IndexFieldCapabilities fieldCap = new IndexFieldCapabilities(parentField, type,
+                            // Composite runtime fields do not have a mapped type for the root - check for null
+                            if (mapper != null) {
+                                String type = mapper.isNested() ? "nested" : "object";
+                                IndexFieldCapabilities fieldCap = new IndexFieldCapabilities(parentField, type,
                                     false, false, false, Collections.emptyMap());
-                            responseMap.put(parentField, fieldCap);
+                                responseMap.put(parentField, fieldCap);
+                            }
                         }
                         dotIndex = parentField.lastIndexOf('.');
                     }

server/src/main/java/org/elasticsearch/index/query/SearchExecutionContext.java

@@ -27,6 +27,7 @@
 import org.elasticsearch.common.xcontent.NamedXContentRegistry;
 import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.core.CheckedFunction;
+import org.elasticsearch.core.Nullable;
 import org.elasticsearch.index.Index;
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.index.IndexSortConfig;
@@ -365,6 +366,12 @@ private MappedFieldType fieldType(String name) {
         return fieldType == null ? mappingLookup.getFieldType(name) : fieldType;
     }
 
+    /**
+     * 
+     * @param name name of the object
+     * @return can be null e.g. if field is root of a composite runtime field
+     */
+    @Nullable
     public ObjectMapper getObjectMapper(String name) {
         return mappingLookup.objectMappers().get(name);
     }