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/common/exec.rs

@@ -3,14 +3,28 @@ use std::sync::Arc;
 
 use futures::future::{Executor, Future};
 
-/// Either the user provides an executor for background tasks, or we use
-/// `tokio::spawn`.
+use body::Payload;
+use proto::h2::server::H2Stream;
+use server::conn::spawn_all::{NewSvcTask, Watcher};
+use service::Service;
+
+pub trait H2Exec<F, B: Payload>: Clone {
+    fn execute_h2stream(&self, fut: H2Stream<F, B>) -> ::Result<()>;
+}
+
+pub trait NewSvcExec<I, N, S: Service, E, W: Watcher<I, S, E>>: Clone {
+    fn execute_new_svc(&self, fut: NewSvcTask<I, N, S, E, W>) -> ::Result<()>;
+}
+
+// Either the user provides an executor for background tasks, or we use
+// `tokio::spawn`.
 #[derive(Clone)]
-pub(crate) enum Exec {
+pub enum Exec {
     Default,
     Executor(Arc<Executor<Box<Future<Item=(), Error=()> + Send>> + Send + Sync>),
 }
 
+// ===== impl Exec =====
 
 impl Exec {
     pub(crate) fn execute<F>(&self, fut: F) -> ::Result<()>
@@ -52,3 +66,58 @@ impl fmt::Debug for Exec {
             .finish()
     }
 }
+
+
+impl<F, B> H2Exec<F, B> for Exec
+where
+    H2Stream<F, B>: Future<Item=(), Error=()> + Send + 'static,
+    B: Payload,
+{
+    fn execute_h2stream(&self, fut: H2Stream<F, B>) -> ::Result<()> {
+        self.execute(fut)
+    }
+}
+
+impl<I, N, S, E, W> NewSvcExec<I, N, S, E, W> for Exec
+where
+    NewSvcTask<I, N, S, E, W>: Future<Item=(), Error=()> + Send + 'static,
+    S: Service,
+    W: Watcher<I, S, E>,
+{
+    fn execute_new_svc(&self, fut: NewSvcTask<I, N, S, E, W>) -> ::Result<()> {
+        self.execute(fut)
+    }
+}
+
+// ==== impl Executor =====
+
+impl<E, F, B> H2Exec<F, B> for E
+where
+    E: Executor<H2Stream<F, B>> + Clone,
+    H2Stream<F, B>: Future<Item=(), Error=()>,
+    B: Payload,
+{
+    fn execute_h2stream(&self, fut: H2Stream<F, B>) -> ::Result<()> {
+        self.execute(fut)
+            .map_err(|err| {
+                warn!("executor error: {:?}", err.kind());
+                ::Error::new_execute()
+            })
+    }
+}
+
+impl<I, N, S, E, W> NewSvcExec<I, N, S, E, W> for E
+where
+    E: Executor<NewSvcTask<I, N, S, E, W>> + Clone,
+    NewSvcTask<I, N, S, E, W>: Future<Item=(), Error=()>,
+    S: Service,
+    W: Watcher<I, S, E>,
+{
+    fn execute_new_svc(&self, fut: NewSvcTask<I, N, S, E, W>) -> ::Result<()> {
+        self.execute(fut)
+            .map_err(|err| {
+                warn!("executor error: {:?}", err.kind());
+                ::Error::new_execute()
+            })
+    }
+}