Capnproto APIs: make mapping to legacy protocols more direct.

[zenhack]

This is sortof a meta issue regarding design API philosophy. I'll pick on WebSession to make the problem concrete, but similar arguments apply to other APIs that wrap existing protocols, e.g. the stuff in email.capnp.

Right now, implementing a sandstorm app that doesn't use sandstorm-http-bridge is uncompelling, even in a language with good capnproto support, because:

• Most languages folks would want to use to build web apps have good support for HTTP, standard interfaces for working with it within the language, and lots of libraries and frameworks built atop those interfaces that make building apps easier.
• Using the capnproto APIs directly, without any kind of compatibility shim, means forgoing the use of much of that language's web ecosystem.
• A possible solution would be to implement the language's standard HTTP interface on top of the WebSession API. However, the complexity of the WebSession API and the fact that its mapping to HTTP is not terribly straightforward means that this is rather a lot of work. I did a good chunk of this for Go, though the implementation is incomplete, and I can't in good conscience recommend that anyone else take this approach.

It seems like the current APIs are designed to capture a higher-level view of the protocols' semantics, but ironically the end result is that developers wishing to use these APIs would have to work at a lower level of abstraction than would normally be the case.

I'd propose replacing the web session interface with something simpler, which maps more directly to HTTP.

Advantages of this approach:

• This would make wrapping the capnproto API with language-standard interfaces easier, and we then could easily have libraries that take e.g. a standard python wsgi app, a Go http.Handler, Haskell's WAI interface, etc. and serve it as a Sandstorm app.
• It would mean not having to also write shims around everything else Sandstorm exposes via capnproto. It would be nice not to have to deal with issues like Provide a mechanism for apps to export capabilities while using sandstorm-http-bridge #3027.

Spitballing a possible alternative:

interface HttpSession {
  request @0 (method :Text, headers :List(Util.KeyValue), responder :Responder)
    -> (requestBody :Util.ByteStream)
}

interface Responder {
  respond @0 (status :UInt16, headers :List(Util.KeyValue))
    -> (responseBody :Utill.ByteStream);
}

In terms of upgrade path, we could have Sandstorm try the existing WebSession methods, and if they throw Unimplemented, try calling HttpSession.request instead.

Thoughts?

[kentonv]

Check this out, which we use in Cloudflare Workers:

https://github.com/capnproto/capnproto/blob/master/c++/src/capnp/compat/http-over-capnp.capnp

The big reason I didn't do it that way in Sandstorm, though, is because I wanted to map out HTTP features semantically in order to decide how to treat each feature for sandboxing purposes. For one random example, it would be very bad if an app could send Public-Key-Pins.

I could possibly be convinced that it would be safe to expose a more "raw" version of HTTP provided that we limit headers to some sort of whitelist.

[zenhack]

To be clear, I'm not proposing we change what is accepted/filtered by Sandstorm, just that apps see an interface that looks close to normal HTTP.

Re: the schema you linked, my one concern is the WebSocket interface; it looks like it operates at the level of frames? While this is nice conceptually, when working on apps that used websockets, it was actually really useful to have Sandstorm's WebSocketStream.sendBytes, since it meant I could just use Go's existing WebSocket support. But it looks like the server has control over whether it uses that or just streams raw bytes, so as long as we implement the Sandstorm side such that it's still possible to use startResponse to handle the websocket framing in the app, I think reusing that schema as-is would be fine.

[zenhack]

From an implementation perspective: we could limit attack service by doing a translation between WebSession and HttpService inside the sandbox, so the privileged part of Sandstorm is still speaking the strongly-typed version.

[zenhack]

...for that matter, we could modify sandstorm-http-bridge to have a mode where it just proxies the grain's supervisor, and does this translation there -- so launches the child process with a socket on fd 3, and exposes all the same interfaces but wraps the app's UiSessions to translate websession calls into HttpService calls. It would probably be straightforward to adapt most of the existing code, since it's already doing the HTTP translation, so there's no semantic gap, just a marshalling change.

[kentonv]

But it looks like the server has control over whether it uses that or just streams raw bytes, so as long as we implement the Sandstorm side such that it's still possible to use startResponse to handle the websocket framing in the app, I think reusing that schema as-is would be fine.

Doesn't quite work. The WebSocket protocol is not a subset of standard HTTP; it diverges from HTTP as soon as the initial handshake completes. So, it needs special handling.

From an implementation perspective: we could limit attack service by doing a translation between WebSession and HttpService inside the sandbox, so the privileged part of Sandstorm is still speaking the strongly-typed version.

Hmm, but how is that better than what we have now, where we translate from WebSession to actual HTTP inside the sandbox?

[zenhack]

Hmm, but how is that better than what we have now, where we translate from WebSession to actual HTTP inside the sandbox?

I guess the main thing is it lets us keep things in-band on the RPC socket. The app needing to connect to "/tmp/sandstorm-api" and start a separate rpc session makes things a bit awkward, in that the bridge is forced to handle all of the expected Api, not just the http bits. This includes MainView.drop/restore, getViewInfo()...

This is part of the reason for #3027 (though it's also adding some smarts re: http apis). Fixing it involves some fiddly questions regarding how the bridge should find something to delegate to, and depending on the details questions regarding synchronization, waiting for another service to start up... I'm sure these can be solved, but it seems like it gets a lot simpler if you can just have everything on the initial RPC socket.

[zenhack]

Quoting Kenton Varda (2019-12-17 19:02:33)

Doesn't quite work. The WebSocket protocol is not a subset of standard HTTP; it diverges from HTTP as soon as the initial handshake completes. So, it needs special handling.

If we go with the approach of making this a wrapper over websession, we could just shell out to sendBytes() in this case, and it will still work.