Add OptionalBackend wrapper

Backends can be activated at runtime depending on the configuration or
execution environment.  This patch adds an OptionalBackend wrapper that
implements Backend and ExtensionImpl but returns a RequestNotAvailable
error if the inner backend is not set.

src/backend.rs

@@ -92,3 +92,60 @@ impl TryFrom<u8> for NoId {
         Err(Error::InternalError)
     }
 }
+
+/// Helper type for optional backends.
+///
+/// If the backend is `None`, [`Error::RequestNotAvailable`][] is returned.
+#[derive(Debug)]
+pub struct OptionalBackend<T>(Option<T>);
+
+impl<T> OptionalBackend<T> {
+    /// Crates a new optional backend from an `Option<T>`.
+    pub fn new(backend: Option<T>) -> Self {
+        Self(backend)
+    }
+
+    /// Returns a mutable reference to the wrapped backend or [`Error::RequestNotAvailable`][] if
+    /// it is `None`.
+    pub fn inner(&mut self) -> Result<&mut T, Error> {
+        self.0.as_mut().ok_or(Error::RequestNotAvailable)
+    }
+
+    /// Returns the wrapped backend.
+    pub fn into_inner(self) -> Option<T> {
+        self.0
+    }
+}
+
+impl<T> Default for OptionalBackend<T> {
+    fn default() -> Self {
+        Self(None)
+    }
+}
+
+impl<T> From<T> for OptionalBackend<T> {
+    fn from(backend: T) -> Self {
+        Self(Some(backend))
+    }
+}
+
+impl<T> From<Option<T>> for OptionalBackend<T> {
+    fn from(backend: Option<T>) -> Self {
+        Self(backend)
+    }
+}
+
+impl<T: Backend> Backend for OptionalBackend<T> {
+    type Context = T::Context;
+
+    fn request<P: Platform>(
+        &mut self,
+        core_ctx: &mut CoreContext,
+        backend_ctx: &mut Self::Context,
+        request: &Request,
+        resources: &mut ServiceResources<P>,
+    ) -> Result<Reply, Error> {
+        self.inner()?
+            .request(core_ctx, backend_ctx, request, resources)
+    }
+}

src/serde_extensions.rs

@@ -17,7 +17,7 @@ use core::{marker::PhantomData, task::Poll};
 
 use crate::{
     api::{reply, request, Reply, Request},
-    backend::{Backend, CoreOnly, Dispatch, NoId},
+    backend::{Backend, CoreOnly, Dispatch, NoId, OptionalBackend},
     client::{ClientError, ClientImplementation, FutureResult, PollClient},
     error::Error,
     platform::{Platform, Syscall},
@@ -131,6 +131,19 @@ pub trait ExtensionImpl<E: Extension>: Backend {
     }
 }
 
+impl<E: Extension, I: ExtensionImpl<E>> ExtensionImpl<E> for OptionalBackend<I> {
+    fn extension_request<P: Platform>(
+        &mut self,
+        core_ctx: &mut CoreContext,
+        backend_ctx: &mut Self::Context,
+        request: &E::Request,
+        resources: &mut ServiceResources<P>,
+    ) -> Result<E::Reply, Error> {
+        self.inner()?
+            .extension_request(core_ctx, backend_ctx, request, resources)
+    }
+}
+
 /// Provides access to the extension IDs assigned by the runner.
 pub trait ExtensionId<E> {
     /// The ID type used by the runner.