consensus: Add a block verification service

Part of #428.

Cargo.lock

@@ -2124,6 +2124,12 @@ version = "0.1.0"
 [[package]]
 name = "zebra-consensus"
 version = "0.1.0"
+dependencies = [
+ "futures",
+ "tower",
+ "zebra-chain",
+ "zebra-state",
+]
 
 [[package]]
 name = "zebra-network"

zebra-consensus/Cargo.toml

@@ -8,3 +8,7 @@ edition = "2018"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+zebra-chain = { path = "../zebra-chain" }
+zebra-state = { path = "../zebra-state" }
+tower = "0.3.1"
+futures = "0.3.5"

zebra-consensus/src/lib.rs

@@ -1,10 +1,15 @@
+//! Consensus handling for Zebra.
+//!
+//! `verify::BlockVerifier` verifies blocks and their transactions, then adds them to
+//! `zebra_state::ZebraState`.
+//!
+//! `mempool::ZebraMempool` verifies transactions, and adds them to the mempool state.
+//!
+//! Consensus handling is provided using `tower::Service`s, to support backpressure
+//! and batch verification.
+
 #![doc(html_logo_url = "https://www.zfnd.org/images/zebra-icon.png")]
 #![doc(html_root_url = "https://doc.zebra.zfnd.org/zebra_consensus")]
+#![deny(missing_docs)]
 
-#[cfg(test)]
-mod tests {
-    #[test]
-    fn it_works() {
-        assert_eq!(2 + 2, 4);
-    }
-}
+pub mod verify;

zebra-consensus/src/verify.rs

@@ -0,0 +1,72 @@
+//! Block verification and chain state updates for Zebra.
+//!
+//! Verification occurs in multiple stages:
+//!   - getting blocks (disk- or network-bound)
+//!   - context-free verification of signatures, proofs, and scripts (CPU-bound)
+//!   - context-dependent verification of the chain state (awaits a verified parent block)
+//!
+//! Verification is provided via a `tower::Service`, to support backpressure and batch
+//! verification.
+
+use std::{
+    error::Error,
+    future::Future,
+    pin::Pin,
+    task::{Context, Poll},
+};
+use tower::{buffer::Buffer, Service};
+
+use zebra_chain::block::Block;
+
+/// Block verification service.
+///
+/// After verification, blocks and their associated transactions are added to
+/// `zebra_state::ZebraState`.
+#[derive(Default)]
+struct BlockVerifier {}
+
+/// The result type for the BlockVerifier Service.
+// TODO(teor): Response = BlockHeaderHash
+type Response = ();
+
+/// The error type for the BlockVerifier Service.
+// TODO(jlusby): Error = Report ?
+type ServiceError = Box<dyn Error + Send + Sync + 'static>;
+
+/// The BlockVerifier service implementation.
+impl Service<Block> for BlockVerifier {
+    type Response = Response;
+    type Error = ServiceError;
+    type Future =
+        Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>> + Send + 'static>>;
+
+    fn poll_ready(&mut self, context: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        // TODO(teor): is this a shared state?
+        let mut state_service = zebra_state::in_memory::init();
+        state_service.poll_ready(context)
+    }
+
+    fn call(&mut self, block: Block) -> Self::Future {
+        let mut state_service = zebra_state::in_memory::init();
+
+        // Ignore errors, they are already handled correctly.
+        // AddBlock discards invalid blocks.
+        let _ = state_service.call(zebra_state::Request::AddBlock {
+            block: block.into(),
+        });
+
+        Box::pin(async { Ok(()) })
+    }
+}
+
+/// Initialise the BlockVerifier service.
+pub fn init() -> impl Service<
+    Block,
+    Response = Response,
+    Error = ServiceError,
+    Future = impl Future<Output = Result<Response, ServiceError>>,
+> + Send
+       + Clone
+       + 'static {
+    Buffer::new(BlockVerifier::default(), 1)
+}