consensus: Add a transaction verification stub

Part of #428.

zebra-consensus/src/verify.rs

@@ -18,6 +18,8 @@ use tower::{buffer::Buffer, Service};
 
 use zebra_chain::block::Block;
 
+mod transaction;
+
 /// Block verification service.
 ///
 /// After verification, blocks and their associated transactions are added to

zebra-consensus/src/verify/transaction.rs

@@ -0,0 +1,22 @@
+//! Transaction verification for Zebra.
+//!
+//! Verification occurs in multiple stages:
+//!   - getting transactions from blocks or the mempool (disk- or network-bound)
+//!   - context-free verification of signatures, proofs, and scripts (CPU-bound)
+//!   - context-dependent verification of transactions against the chain state
+//!     (awaits an up-to-date chain)
+//!
+//! Verification is provided via a `tower::Service`, to support backpressure and batch
+//! verification.
+//!
+//! This is an internal module. Use `verify::BlockVerifier` for blocks and their
+//! transactions, or `mempool::MempoolTransactionVerifier` for mempool transactions.
+
+/// Internal transaction verification service.
+///
+/// After verification, the transaction future completes. State changes are handled by
+/// `BlockVerifier` or `MempoolTransactionVerifier`.
+///
+/// `TransactionVerifier` is not yet implemented.
+#[derive(Default)]
+pub(crate) struct TransactionVerifier {}