Add an OptionalTransport type (#1116)

* Add OptionalTransport

* Fix copyright

* Some documentation

* Apply suggestions from code review

Co-Authored-By: Toralf Wittner <tw@dtex.org>

core/src/transport/mod.rs

@@ -42,8 +42,11 @@ pub mod memory;
 pub mod timeout;
 pub mod upgrade;
 
+mod optional;
+
 pub use self::choice::OrTransport;
 pub use self::memory::MemoryTransport;
+pub use self::optional::OptionalTransport;
 pub use self::upgrade::Upgrade;
 
 /// A transport provides connection-oriented communication between two peers

core/src/transport/optional.rs

@@ -0,0 +1,77 @@
+// Copyright 2019 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.
+
+use crate::transport::{Transport, TransportError};
+use multiaddr::Multiaddr;
+
+/// Transport that is possibly disabled.
+///
+/// An `OptionalTransport<T>` is a wrapper around an `Option<T>`. If it is disabled (read: contains
+/// `None`), then any attempt to dial or listen will return `MultiaddrNotSupported`. If it is
+/// enabled (read: contains `Some`), then dialing and listening will be handled by the inner
+/// transport.
+#[derive(Debug, Copy, Clone)]
+pub struct OptionalTransport<T>(Option<T>);
+
+impl<T> OptionalTransport<T> {
+    /// Builds an `OptionalTransport` with the given transport in an enabled
+    /// state.
+    pub fn some(inner: T) -> OptionalTransport<T> {
+        OptionalTransport(Some(inner))
+    }
+
+    /// Builds a disabled `OptionalTransport`.
+    pub fn none() -> OptionalTransport<T> {
+        OptionalTransport(None)
+    }
+}
+
+impl<T> From<T> for OptionalTransport<T> {
+    fn from(inner: T) -> Self {
+        OptionalTransport(Some(inner))
+    }
+}
+
+impl<T> Transport for OptionalTransport<T>
+where
+    T: Transport,
+{
+    type Output = T::Output;
+    type Error = T::Error;
+    type Listener = T::Listener;
+    type ListenerUpgrade = T::ListenerUpgrade;
+    type Dial = T::Dial;
+
+    fn listen_on(self, addr: Multiaddr) -> Result<Self::Listener, TransportError<Self::Error>> {
+        if let Some(inner) = self.0 {
+            inner.listen_on(addr)
+        } else {
+            Err(TransportError::MultiaddrNotSupported(addr))
+        }
+    }
+
+    fn dial(self, addr: Multiaddr) -> Result<Self::Dial, TransportError<Self::Error>> {
+        if let Some(inner) = self.0 {
+            inner.dial(addr)
+        } else {
+            Err(TransportError::MultiaddrNotSupported(addr))
+        }
+    }
+}