feat(server): allow !Send Servers

Until this commit, servers have required that `Service` and their
`Future` to be `Send`, since the server needs to spawn some internal
tasks to an executor, and by default, that is `tokio::spawn`, which
could be spawning to a threadpool. This was true even if the user were
certain there was no threadpool involved, and was instead using a
different single-threaded runtime, like
`tokio::runtime::current_thread`.

This changes makes all the server pieces generic over an `E`, which is
essentially `Executor<PrivateTypes<Server::Future>>`. There's a new set
of internal traits, `H2Exec` and `NewSvcExec`, which allow for the type
signature to only show the generics that the user is providing. The
traits cannot be implemented explicitly, but there are blanket
implementations for `E: Executor<SpecificType>`. If the user provides
their own executor, it simply needs to have a generic `impl<F>
Executor<F> for MyExec`. That impl can have bounds deciding whether to
require `F: Send`. If the executor does require `Send`, and the
`Service` futures are `!Send`, there will be compiler errors.

To prevent a breaking change, all the types that gained the `E` generic
have a default type set, which is the original `tokio::spawn` executor.

src/proto/h2/server.rs

@@ -5,7 +5,7 @@ use tokio_io::{AsyncRead, AsyncWrite};
 
 use ::headers::content_length_parse_all;
 use ::body::Payload;
-use ::common::Exec;
+use ::common::exec::H2Exec;
 use ::headers;
 use ::service::Service;
 use ::proto::Dispatched;
@@ -13,12 +13,12 @@ use super::{PipeToSendStream, SendBuf};
 
 use ::{Body, Response};
 
-pub(crate) struct Server<T, S, B>
+pub(crate) struct Server<T, S, B, E>
 where
     S: Service,
     B: Payload,
 {
-    exec: Exec,
+    exec: E,
     service: S,
     state: State<T, B>,
 }
@@ -40,15 +40,16 @@ where
 }
 
 
-impl<T, S, B> Server<T, S, B>
+impl<T, S, B, E> Server<T, S, B, E>
 where
     T: AsyncRead + AsyncWrite,
     S: Service<ReqBody=Body, ResBody=B>,
     S::Error: Into<Box<::std::error::Error + Send + Sync>>,
-    S::Future: Send + 'static,
+    //S::Future: Send + 'static,
     B: Payload,
+    E: H2Exec<S::Future, B>,
 {
-    pub(crate) fn new(io: T, service: S, exec: Exec) -> Server<T, S, B> {
+    pub(crate) fn new(io: T, service: S, exec: E) -> Server<T, S, B, E> {
         let handshake = Builder::new()
             .handshake(io);
         Server {
@@ -76,13 +77,14 @@ where
     }
 }
 
-impl<T, S, B> Future for Server<T, S, B>
+impl<T, S, B, E> Future for Server<T, S, B, E>
 where
     T: AsyncRead + AsyncWrite,
     S: Service<ReqBody=Body, ResBody=B>,
     S::Error: Into<Box<::std::error::Error + Send + Sync>>,
-    S::Future: Send + 'static,
+    //S::Future: Send + 'static,
     B: Payload,
+    E: H2Exec<S::Future, B>,
 {
     type Item = Dispatched;
     type Error = ::Error;
@@ -116,14 +118,14 @@ where
     T: AsyncRead + AsyncWrite,
     B: Payload,
 {
-    fn poll_server<S>(&mut self, service: &mut S, exec: &Exec) -> Poll<(), ::Error>
+    fn poll_server<S, E>(&mut self, service: &mut S, exec: &E) -> Poll<(), ::Error>
     where
         S: Service<
             ReqBody=Body,
             ResBody=B,
         >,
         S::Error: Into<Box<::std::error::Error + Send + Sync>>,
-        S::Future: Send + 'static,
+        E: H2Exec<S::Future, B>,
     {
         while let Some((req, respond)) = try_ready!(self.conn.poll().map_err(::Error::new_h2)) {
             trace!("incoming request");
@@ -132,7 +134,7 @@ where
                 ::Body::h2(stream, content_length)
             });
             let fut = H2Stream::new(service.call(req), respond);
-            exec.execute(fut)?;
+            exec.execute_h2stream(fut)?;
         }
 
         // no more incoming streams...
@@ -141,7 +143,8 @@ where
     }
 }
 
-struct H2Stream<F, B>
+#[allow(missing_debug_implementations)]
+pub struct H2Stream<F, B>
 where
     B: Payload,
 {