Fix poll_ready usage in ChainVerifier (#1700)

* change `poll_ready()` and `call()` of `ChainVerifier`
* add bound, move max_checkpoint_height
* add buffers to the checkpoint and block verifiers

And rename the chain verifier fields so `block` means `Arc<Block>`, and
`block_verifier` means `Buffer<BlockVerifier, ...>`.

* Fix the error types
* Use `ServiceExt::oneshot` in `ChainVerifier::call`

And:
* make the code look like the `main` branch as much as possible
* document the `poll_ready`/`call` invariant

* Use `ServiceExt::oneshot` in `chain::init`

Co-authored-by: teor <teor@riseup.net>

zebra-consensus/src/chain.rs

@@ -22,11 +22,23 @@ use zebra_state as zs;
 
 use crate::{
     block::BlockVerifier,
-    block::VerifyBlockError,
-    checkpoint::{CheckpointList, CheckpointVerifier, VerifyCheckpointError},
+    checkpoint::{CheckpointList, CheckpointVerifier},
     BoxError, Config,
 };
 
+/// The bound for each verifier's buffer.
+///
+/// We choose the verifier buffer bound based on the maximum number of
+/// concurrent verifier users, to avoid contention:
+///   - the `ChainSync` component
+///   - the `Inbound` service
+///   - a miner component, which we might add in future, and
+///   - 1 extra slot to avoid contention.
+///
+/// We deliberately add extra slots, because they only cost a small amount of
+/// memory, but missing slots can significantly slow down Zebra.
+const VERIFIER_BUFFER_BOUND: usize = 4;
+
 /// The chain verifier routes requests to either the checkpoint verifier or the
 /// block verifier, depending on the maximum checkpoint height.
 struct ChainVerifier<S>
@@ -34,17 +46,20 @@ where
     S: Service<zs::Request, Response = zs::Response, Error = BoxError> + Send + Clone + 'static,
     S::Future: Send + 'static,
 {
-    block: BlockVerifier<S>,
-    checkpoint: CheckpointVerifier<S>,
+    // Normally, we erase the types on buffer-wrapped services.
+    // But if we did that here, the block and checkpoint services would be
+    // type-indistinguishable, risking future substitution errors.
+    block_verifier: Buffer<BlockVerifier<S>, Arc<block::Block>>,
+    checkpoint_verifier: Buffer<CheckpointVerifier<S>, Arc<block::Block>>,
     max_checkpoint_height: block::Height,
 }
 
 #[derive(Debug, Display, Error)]
 pub enum VerifyChainError {
     /// block could not be checkpointed
-    Checkpoint(#[source] VerifyCheckpointError),
+    Checkpoint(#[source] BoxError),
     /// block could not be verified
-    Block(#[source] VerifyBlockError),
+    Block(#[source] BoxError),
 }
 
 impl<S> Service<Arc<Block>> for ChainVerifier<S>
@@ -57,30 +72,34 @@ where
     type Future =
         Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>> + Send + 'static>>;
 
-    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
-        match (self.checkpoint.poll_ready(cx), self.block.poll_ready(cx)) {
-            // First, fail if either service fails.
-            (Poll::Ready(Err(e)), _) => Poll::Ready(Err(VerifyChainError::Checkpoint(e))),
-            (_, Poll::Ready(Err(e))) => Poll::Ready(Err(VerifyChainError::Block(e))),
-            // Second, we're unready if either service is unready.
-            (Poll::Pending, _) | (_, Poll::Pending) => Poll::Pending,
-            // Finally, we're ready if both services are ready and OK.
-            (Poll::Ready(Ok(())), Poll::Ready(Ok(()))) => Poll::Ready(Ok(())),
-        }
+    fn poll_ready(&mut self, _cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        // Correctness:
+        //
+        // We can't call `poll_ready` on the block and checkpoint verifiers here,
+        // because each `poll_ready` must be followed by a `call`, and we don't
+        // know which verifier we're going to choose yet.
+        // See #1593 for details.
+        Poll::Ready(Ok(()))
     }
 
     fn call(&mut self, block: Arc<Block>) -> Self::Future {
         match block.coinbase_height() {
+            // Correctness:
+            //
+            // We use `ServiceExt::oneshot` to make sure every `poll_ready` has
+            // a matching `call`. See #1593 for details.
             Some(height) if height <= self.max_checkpoint_height => self
-                .checkpoint
-                .call(block)
+                .checkpoint_verifier
+                .clone()
+                .oneshot(block)
                 .map_err(VerifyChainError::Checkpoint)
                 .boxed(),
             // This also covers blocks with no height, which the block verifier
             // will reject immediately.
             _ => self
-                .block
-                .call(block)
+                .block_verifier
+                .clone()
+                .oneshot(block)
                 .map_err(VerifyChainError::Block)
                 .boxed(),
         }
@@ -103,7 +122,7 @@ where
 pub async fn init<S>(
     config: Config,
     network: Network,
-    mut state_service: S,
+    state_service: S,
 ) -> Buffer<BoxService<Arc<Block>, block::Hash, VerifyChainError>, Arc<Block>>
 where
     S: Service<zs::Request, Response = zs::Response, Error = BoxError> + Send + Clone + 'static,
@@ -118,11 +137,13 @@ where
             .expect("hardcoded checkpoint list extends past sapling activation")
     };
 
+    // Correctness:
+    //
+    // We use `ServiceExt::oneshot` to make sure every `poll_ready` has a
+    // matching `call`. See #1593 for details.
     let tip = match state_service
-        .ready_and()
-        .await
-        .unwrap()
-        .call(zs::Request::Tip)
+        .clone()
+        .oneshot(zs::Request::Tip)
         .await
         .unwrap()
     {
@@ -131,15 +152,18 @@ where
     };
     tracing::info!(?tip, ?max_checkpoint_height, "initializing chain verifier");
 
-    let block = BlockVerifier::new(network, state_service.clone());
-    let checkpoint = CheckpointVerifier::from_checkpoint_list(list, tip, state_service);
+    let block_verifier = BlockVerifier::new(network, state_service.clone());
+    let checkpoint_verifier = CheckpointVerifier::from_checkpoint_list(list, tip, state_service);
+
+    let block_verifier = Buffer::new(block_verifier, VERIFIER_BUFFER_BOUND);
+    let checkpoint_verifier = Buffer::new(checkpoint_verifier, VERIFIER_BUFFER_BOUND);
 
     Buffer::new(
         BoxService::new(ChainVerifier {
-            block,
-            checkpoint,
+            block_verifier,
+            checkpoint_verifier,
             max_checkpoint_height,
         }),
-        3,
+        VERIFIER_BUFFER_BOUND,
     )
 }