When we encounter a composite request, we authorize at first without looking at the indices, to see whether the action can be executed at all. We then rely on the action to delegate to an inner action per sub-request, which will be authorized based on the indices it refers to. The first step works great for the simulate mode of search template, as it doesn't involve any index. The second step will make sure that when search template involves a search, it will be authorized as a normal search request would, based on the indices it reads from.
Note that the wildcard expansion happens now on the search side, it doesn't have to happen when executing the first authorization step, hence SearchTemplateRequest doesn't have to implement IndicesRequest, only SearchRequest has to (which it does already).
Closeselastic/elasticsearch#4171
Original commit: elastic/x-pack-elasticsearch@d586bd90cb
Ali Beyad
Nik Everett
javanna
Luca Cavanna
Ryan Ernst
Jay Modi