protocols/rendezvous: Implement protocol (#2107)

Implement the libp2p rendezvous protocol.

> A lightweight mechanism for generalized peer discovery. It can be used for
bootstrap purposes, real time peer discovery, application specific routing, and
so on.

Co-authored-by: rishflab <rishflab@hotmail.com>
Co-authored-by: Daniel Karzel <daniel@comit.network>

protocols/rendezvous/src/handler.rs

@@ -0,0 +1,48 @@
+// Copyright 2021 COMIT Network.
+//
+// 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.
+
+use crate::codec;
+use crate::codec::Message;
+use void::Void;
+
+const PROTOCOL_IDENT: &[u8] = b"/rendezvous/1.0.0";
+
+pub mod inbound;
+pub mod outbound;
+
+/// Errors that can occur while interacting with a substream.
+#[derive(Debug, thiserror::Error)]
+pub enum Error {
+    #[error("Reading message {0:?} at this stage is a protocol violation")]
+    BadMessage(Message),
+    #[error("Failed to write message to substream")]
+    WriteMessage(#[source] codec::Error),
+    #[error("Failed to read message from substream")]
+    ReadMessage(#[source] codec::Error),
+    #[error("Substream ended unexpectedly mid-protocol")]
+    UnexpectedEndOfStream,
+}
+
+pub type OutboundInEvent = crate::substream_handler::InEvent<outbound::OpenInfo, Void, Void>;
+pub type OutboundOutEvent =
+    crate::substream_handler::OutEvent<Void, outbound::OutEvent, Void, Error>;
+
+pub type InboundInEvent = crate::substream_handler::InEvent<(), inbound::InEvent, Void>;
+pub type InboundOutEvent = crate::substream_handler::OutEvent<inbound::OutEvent, Void, Error, Void>;