Hello Greg,
Wai applications HAVE TO call a system call (stat()) to get file information (for instance, modification time) and can get a file size at the same time. So, they can add Content-Length: with one system call.
There are 2 basic kinds of WAI apps- those for serving files that might change (a file server), and those that know exactly what files exist in advance (a web application). In the case of a web application, we can get all this information just once at compile time. For a file server, we could allow a configuration parameter for whether or not to do the system call. Of course, it is possible to mix these 2 use cases in one application, but that can be handled with multiple instances of wai-app-static.
Yes, I know. The former uses ResponseFile and the latter typically uses ResponseBuilder/ResponseEnumerator. I discussed in the context of ResponseFile which uses sendfile. Sorry but I don't understand what you try to explain here. --Kazu