mock-consensus: 🔀 test node can fast_forward

fixes #3933.

this introduces a `fast_forward` method to a test node, allowing tests
using the mock consensus engine (#3588) to fast forward a certain number
of blocks.

the existing `mock_consensus_can_send_a_sequence_of_empty_blocks` test
is rewritten to make use of this method.

this is done in service of #3995.

---

* #3588
* #3933
* #3995

crates/core/app/tests/mock_consensus.rs

@@ -20,7 +20,7 @@ use {
     },
     rand_core::OsRng,
     tap::Tap,
-    tracing::{error_span, info, Instrument},
+    tracing::info,
 };
 
 /// Exercises that a test node can be instantiated using the consensus service.
@@ -83,29 +83,12 @@ async fn mock_consensus_can_send_a_sequence_of_empty_blocks() -> anyhow::Result<
     let storage = TempStorage::new().await?;
     let mut test_node = common::start_test_node(&storage).await?;
 
-    // Check that we begin at height 0, before any blocks have been generated.
-    assert_eq!(
-        storage.latest_snapshot().get_block_height().await?,
-        0,
-        "height should begin at 0"
-    );
-
-    for expected in 1..=8 {
-        // Generate an empty block.
-        test_node
-            .block()
-            .with_data(vec![])
-            .execute()
-            .instrument(error_span!("executing block", %expected))
-            .await?;
-
-        // Check that the latest snapshot has the expected block height.
-        let height = storage.latest_snapshot().get_block_height().await?;
-        assert_eq!(
-            height, expected,
-            "height should continue to incrementally grow"
-        );
-    }
+    let height = || async { storage.latest_snapshot().get_block_height().await };
+
+    // Fast forward eight blocks, and show that the height is 8 after doing so.
+    assert_eq!(height().await?, 0, "height should begin at 0");
+    test_node.fast_forward(8).await?;
+    assert_eq!(height().await?, 8_u64, "height should grow");
 
     // Free our temporary storage.
     drop(storage);

crates/test/mock-consensus/src/lib.rs

@@ -40,3 +40,42 @@ impl<C> TestNode<C> {
         format!("{:02X?}", self.last_app_hash)
     }
 }
+
+impl<C> TestNode<C>
+where
+    C: tower::Service<
+            tendermint::v0_37::abci::ConsensusRequest,
+            Response = tendermint::v0_37::abci::ConsensusResponse,
+            Error = tower::BoxError,
+        > + Send
+        + Clone
+        + 'static,
+    C::Future: Send + 'static,
+    C::Error: Sized,
+{
+    /// Fast forwards a number of blocks.
+    #[tracing::instrument(
+        skip(self),
+        fields(
+            height = %self.height,
+            fast_forward.blocks = %blocks,
+        )
+    )]
+    pub async fn fast_forward(&mut self, blocks: usize) -> anyhow::Result<()> {
+        use {
+            tap::Tap,
+            tracing::{info, trace},
+        };
+
+        for i in 0..blocks {
+            self.block()
+                .with_data(vec![])
+                .execute()
+                .tap(|_| trace!(%i, "executing empty block"))
+                .await
+                .tap(|_| trace!(%i, "finished executing empty block"))?;
+        }
+
+        Ok(()).tap(|_| info!("finished fast forward"))
+    }
+}