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.
This adds a "combinator" method to `Server`, which accepts a user's
future to "select" on. All connections received by the `Server` will
be tracked, and if the user's future finishes, graceful shutdown will
begin.
- The listener will be closed immediately.
- The currently active connections will all be notified to start a
graceful shutdown. For HTTP/1, that means finishing the existing
response and using `connection: clone`. For HTTP/2, the graceful
`GOAWAY` process is started.
- Once all active connections have terminated, the graceful future
will return.
Closes#1575
