Remove services that are never called from Inbound

Uses the `ServiceExt::oneshot` design pattern from ZcashFoundation#1593.

zebrad/src/components/inbound.rs

@@ -53,7 +53,7 @@ pub type SetupData = (Outbound, Arc<Mutex<AddressBook>>);
 /// responding to block gossip by attempting to download and validate advertised
 /// blocks.
 pub struct Inbound {
-    // invariant: address_book, outbound, downloads are Some if network_setup is None
+    // invariant: address_book and downloads are Some if network_setup or verifier are None
     //
     // why not use an enum for the inbound state? because it would mean
     // match-wrapping the body of Service::call rather than just expect()ing
@@ -62,28 +62,29 @@ pub struct Inbound {
     // Setup
     /// A oneshot channel used to receive the address_book and outbound services
     /// after the network is set up.
+    ///
+    /// `None` after the network is set up.
     network_setup: Option<oneshot::Receiver<SetupData>>,
 
+    /// A service that verifies downloaded blocks. Given to `downloads`
+    /// after the network is set up.
+    ///
+    /// `None` after the network is set up and `downloads` is created.
+    verifier: Option<Verifier>,
+
     // Services
-    /// A list of peer addresses.
+    /// A service that maintains a list of peer addresses.
+    ///
+    /// `None` until the network is set up.
     address_book: Option<Arc<Mutex<zn::AddressBook>>>,
 
-    /// A service that downloads and verifies gossipped blocks.
-    downloads: Option<Pin<Box<Downloads<Timeout<Outbound>, Timeout<Verifier>, State>>>>,
-
-    /// A service that forwards requests to connected peers, and returns their
-    /// responses.
+    /// A stream that downloads and verifies gossipped blocks.
     ///
-    /// Only used for readiness checks, and via `downloads`.
-    outbound: Option<Outbound>,
+    /// `None` until the network is set up.
+    downloads: Option<Pin<Box<Downloads<Timeout<Outbound>, Timeout<Verifier>, State>>>>,
 
     /// A service that manages cached blockchain state.
     state: State,
-
-    /// A service that verifies downloaded blocks.
-    ///
-    /// Only used for readiness checks, and via `downloads`.
-    verifier: Verifier,
 }
 
 impl Inbound {
@@ -94,11 +95,10 @@ impl Inbound {
     ) -> Self {
         Self {
             network_setup: Some(network_setup),
+            verifier: Some(verifier),
             address_book: None,
             downloads: None,
-            outbound: None,
             state,
-            verifier,
         }
     }
 }
@@ -119,14 +119,18 @@ impl Service<zn::Request> for Inbound {
             use oneshot::error::TryRecvError;
             match rx.try_recv() {
                 Ok((outbound, address_book)) => {
-                    self.outbound = Some(outbound.clone());
                     self.address_book = Some(address_book);
-                    self.network_setup = None;
                     self.downloads = Some(Box::pin(Downloads::new(
                         Timeout::new(outbound, BLOCK_DOWNLOAD_TIMEOUT),
-                        Timeout::new(self.verifier.clone(), BLOCK_VERIFY_TIMEOUT),
+                        Timeout::new(
+                            self.verifier
+                                .take()
+                                .expect("verifier is Some when network_setup is Some"),
+                            BLOCK_VERIFY_TIMEOUT,
+                        ),
                         self.state.clone(),
                     )));
+                    self.network_setup = None;
                 }
                 Err(TryRecvError::Empty) => {
                     self.network_setup = Some(rx);
@@ -141,23 +145,21 @@ impl Service<zn::Request> for Inbound {
             };
         }
 
-        // Clean up completed download tasks
+        // Clean up completed download tasks, ignoring their results
         if let Some(downloads) = self.downloads.as_mut() {
             while let Poll::Ready(Some(_)) = downloads.as_mut().poll_next(cx) {}
         }
 
         // Now report readiness based on readiness of the inner services, if they're available.
-        // XXX do we want to propagate backpressure from the network here?
-        match (
-            self.state.poll_ready(cx),
-            self.outbound
-                .as_mut()
-                .map(|svc| svc.poll_ready(cx))
-                .unwrap_or(Poll::Ready(Ok(()))),
-        ) {
-            (Poll::Ready(Err(e)), _) | (_, Poll::Ready(Err(e))) => Poll::Ready(Err(e)),
-            (Poll::Pending, _) | (_, Poll::Pending) => Poll::Pending,
-            (Poll::Ready(Ok(())), Poll::Ready(Ok(()))) => Poll::Ready(Ok(())),
+        //
+        // TODO: do we want to propagate backpressure from the download queue or its outbound network here?
+        //       currently, the download queue waits for the outbound network in the download future, and
+        //       drops new requests after it reaches a hard-coded limit. This is the "load shed directly"
+        //       pattern from #1618.
+        match self.state.poll_ready(cx) {
+            Poll::Ready(Err(e)) => Poll::Ready(Err(e)),
+            Poll::Pending => Poll::Pending,
+            Poll::Ready(Ok(())) => Poll::Ready(Ok(())),
         }
     }