Add libp2p-request-response protocol. (#1596)

* Add the libp2p-request-response protocol.

This crate provides a generic implementation for request/response
protocols, whereby each request is sent on a new substream.

* Fix OneShotHandler usage in floodsub.

* Custom ProtocolsHandler and multiple protocols.

  1. Implement a custom ProtocolsHandler instead of using
     the OneShotHandler for better control and error handling.
     In particular, all request/response sending/receiving is
     kept in the substreams upgrades and thus the background
     task of a connection.
  2. Support multiple protocols (usually protocol versions)
     with a single `RequestResponse` instance, with
     configurable inbound/outbound support.

* Small doc clarification.

* Remove unnecessary Sync bounds.

* Remove redundant Clone constraint.

* Update protocols/request-response/Cargo.toml

Co-authored-by: Toralf Wittner <tw@dtex.org>

* Update dev-dependencies.

* Update Cargo.tomls.

* Add changelog.

* Remove Sync bound from RequestResponseCodec::Protocol.

Apparently the compiler just needs some help with the scope
of borrows, which is unfortunate.

* Try async-trait.

* Allow checking whether a ResponseChannel is still open.

Also expand the commentary on `send_response` to indicate that
responses may be discard if they come in too late.

* Add `RequestResponse::is_pending`.

As an analogue of `ResponseChannel::is_open` for outbound requests.

* Revert now unnecessary changes to the OneShotHandler.

Since `libp2p-request-response` is no longer using it.

* Update CHANGELOG for libp2p-swarm.

Co-authored-by: Toralf Wittner <tw@dtex.org>

protocols/request-response/src/codec.rs

@@ -0,0 +1,66 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the "Software"),
+// to deal in the Software without restriction, including without limitation
+// the rights to use, copy, modify, merge, publish, distribute, sublicense,
+// and/or sell copies of the Software, and to permit persons to whom the
+// Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+pub use libp2p_core::ProtocolName;
+
+use async_trait::async_trait;
+use futures::prelude::*;
+use std::io;
+
+/// A `RequestResponseCodec` defines the request and response types
+/// for a [`RequestResponse`](crate::RequestResponse) protocol or
+/// protocol family and how they are encoded / decoded on an I/O stream.
+#[async_trait]
+pub trait RequestResponseCodec {
+    /// The type of protocol(s) or protocol versions being negotiated.
+    type Protocol: ProtocolName + Send + Clone;
+    /// The type of inbound and outbound requests.
+    type Request: Send;
+    /// The type of inbound and outbound responses.
+    type Response: Send;
+
+    /// Reads a request from the given I/O stream according to the
+    /// negotiated protocol.
+    async fn read_request<T>(&mut self, protocol: &Self::Protocol, io: &mut T)
+        -> io::Result<Self::Request>
+    where
+        T: AsyncRead + Unpin + Send;
+
+    /// Reads a response from the given I/O stream according to the
+    /// negotiated protocol.
+    async fn read_response<T>(&mut self, protocol: &Self::Protocol, io: &mut T)
+        -> io::Result<Self::Response>
+    where
+        T: AsyncRead + Unpin + Send;
+
+    /// Writes a request to the given I/O stream according to the
+    /// negotiated protocol.
+    async fn write_request<T>(&mut self, protocol: &Self::Protocol, io: &mut T, req: Self::Request)
+        -> io::Result<()>
+    where
+        T: AsyncWrite + Unpin + Send;
+
+    /// Writes a response to the given I/O stream according to the
+    /// negotiated protocol.
+    async fn write_response<T>(&mut self, protocol: &Self::Protocol, io: &mut T, res: Self::Response)
+        -> io::Result<()>
+    where
+        T: AsyncWrite + Unpin + Send;
+}