reindex: Guard against user disabling fields

modules/reindex/src/main/java/org/elasticsearch/index/reindex/AbstractAsyncBulkIndexByScrollAction.java

@@ -87,6 +87,11 @@ public abstract class AbstractAsyncBulkIndexByScrollAction<
 
         for (SearchHit doc : docs) {
             if (doc.isSourceEmpty()) {
+                /*
+                 * Either the document didn't store _source or we didn't fetch it for some reason. Since we don't allow the user to
+                 * change the "fields" part of the search request it is unlikely that we got here because we didn't fetch _source.
+                 * Thus the error message assumes that it wasn't stored.
+                 */
                 throw new IllegalArgumentException("[" + doc.index() + "][" + doc.type() + "][" + doc.id() + "] didn't store _source");
             }
             IndexRequest index = buildIndexRequest(doc);

modules/reindex/src/main/java/org/elasticsearch/index/reindex/AbstractBulkByScrollRequest.java

@@ -117,6 +117,9 @@ public abstract class AbstractBulkByScrollRequest<Self extends AbstractBulkByScr
         if (searchRequest.source().from() != -1) {
             e = addValidationError("from is not supported in this context", e);
         }
+        if (searchRequest.source().fields() != null) {
+            e = addValidationError("fields is not supported in this context", e);
+        }
         if (maxRetries < 0) {
             e = addValidationError("retries cannnot be negative", e);
         }

modules/reindex/src/test/resources/rest-api-spec/test/reindex/20_validation.yaml

@@ -188,3 +188,15 @@
             index: test
           dest:
             index: dest
+
+---
+"source fields may not be modified":
+  - do:
+      catch: /fields is not supported in this context/
+      reindex:
+        body:
+          source:
+            index: test
+            fields: [_id]
+          dest:
+            index: dest

modules/reindex/src/test/resources/rest-api-spec/test/update_by_query/20_validation.yaml

@@ -77,3 +77,12 @@
       catch: /\[test\]\[test\]\[1\] didn't store _source/
       update_by_query:
         index: test
+
+---
+"source fields may not be modified":
+  - do:
+      catch: /fields is not supported in this context/
+      update_by_query:
+        index: test
+        body:
+          fields: [_id]