Implement best tip block hashes for ChainTip receivers (#2677)

* Always prefer the non-finalized tip in ChainTipSender

This significantly simplifies the internal implementation of ChainTipSender.

Also make the methods and types a bit more generic.

* Update ChainTipSender with blocks, not heights

Also fix a bug where queued non-finalized blocks would clear the chain tip.

* Provide a best tip hash in ChainTip receivers

* Skip finalized blocks once the non-finalized state is active

* Add tip hash and NoChainTip tests

* Remove a redundant finalized tip update

* Skip `None` updates to the finalized tip

The finalized and non-finalized tips never update to `None`
once they have added at least one block.

* Stop committing finalized queued blocks if there is an error

Also return the highest committed queued block.

Co-authored-by: Janito Vaqueiro Ferreira Filho <janito.vff@gmail.com>

zebra-chain/src/chain_tip.rs

@@ -10,6 +10,9 @@ use crate::block;
 pub trait ChainTip {
     /// Return the height of the best chain tip.
     fn best_tip_height(&self) -> Option<block::Height>;
+
+    /// Return the block hash of the best chain tip.
+    fn best_tip_hash(&self) -> Option<block::Hash>;
 }
 
 /// A chain tip that is always empty.
@@ -20,4 +23,8 @@ impl ChainTip for NoChainTip {
     fn best_tip_height(&self) -> Option<block::Height> {
         None
     }
+
+    fn best_tip_hash(&self) -> Option<block::Hash> {
+        None
+    }
 }

zebra-state/src/service.rs

@@ -76,12 +76,8 @@ impl StateService {
     const PRUNE_INTERVAL: Duration = Duration::from_secs(30);
 
     pub fn new(config: Config, network: Network) -> (Self, ChainTipReceiver) {
-        let (mut chain_tip_sender, chain_tip_receiver) = ChainTipSender::new();
         let disk = FinalizedState::new(&config, network);
-
-        if let Some(finalized_height) = disk.finalized_tip_height() {
-            chain_tip_sender.set_finalized_height(finalized_height);
-        }
+        let (chain_tip_sender, chain_tip_receiver) = ChainTipSender::new(disk.tip_block());
 
         let mem = NonFinalizedState::new(network);
         let queued_blocks = QueuedBlocks::default();
@@ -132,10 +128,10 @@ impl StateService {
         let (rsp_tx, rsp_rx) = oneshot::channel();
 
         self.disk.queue_and_commit_finalized((finalized, rsp_tx));
-
-        if let Some(finalized_height) = self.disk.finalized_tip_height() {
-            self.chain_tip_sender.set_finalized_height(finalized_height);
-        }
+        // TODO: move into the finalized state,
+        //       so we can clone committed `Arc<Block>`s before they get dropped
+        self.chain_tip_sender
+            .set_finalized_tip(self.disk.tip_block());
 
         rsp_rx
     }
@@ -197,14 +193,10 @@ impl StateService {
         let finalized_tip_height = self.disk.finalized_tip_height().expect(
             "Finalized state must have at least one block before committing non-finalized state",
         );
-        let non_finalized_tip_height = self.mem.best_tip().map(|(height, _hash)| height);
-
         self.queued_blocks.prune_by_height(finalized_tip_height);
 
         self.chain_tip_sender
-            .set_finalized_height(finalized_tip_height);
-        self.chain_tip_sender
-            .set_best_non_finalized_height(non_finalized_tip_height);
+            .set_best_non_finalized_tip(self.mem.best_tip_block());
 
         tracing::trace!("finished processing queued block");
         rsp_rx

zebra-state/src/service/chain_tip.rs

@@ -1,66 +1,89 @@
+use std::sync::Arc;
+
 use tokio::sync::watch;
 
-use zebra_chain::{block, chain_tip::ChainTip};
+use zebra_chain::{
+    block::{self, Block},
+    chain_tip::ChainTip,
+};
 
 #[cfg(test)]
 mod tests;
 
+/// The internal watch channel data type for [`ChainTipSender`] and [`ChainTipReceiver`].
+type ChainTipData = Option<Arc<Block>>;
+
 /// A sender for recent changes to the non-finalized and finalized chain tips.
 #[derive(Debug)]
 pub struct ChainTipSender {
-    finalized: Option<block::Height>,
-    non_finalized: Option<block::Height>,
-    sender: watch::Sender<Option<block::Height>>,
+    /// Have we got any chain tips from the non-finalized state?
+    ///
+    /// Once this flag is set, we ignore the finalized state.
+    /// `None` tips don't set this flag.
+    non_finalized_tip: bool,
+
+    /// The sender channel for chain tip data.
+    sender: watch::Sender<ChainTipData>,
+
+    /// A copy of the data in `sender`.
     // TODO: Replace with calls to `watch::Sender::borrow` once Tokio is updated to 1.0.0 (#2573)
-    active_value: Option<block::Height>,
+    active_value: ChainTipData,
 }
 
 impl ChainTipSender {
-    /// Create new linked instances of [`ChainTipSender`] and [`ChainTipReceiver`].
-    pub fn new() -> (Self, ChainTipReceiver) {
+    /// Create new linked instances of [`ChainTipSender`] and [`ChainTipReceiver`],
+    /// using `initial_tip` as the tip.
+    pub fn new(initial_tip: impl Into<Option<Arc<Block>>>) -> (Self, ChainTipReceiver) {
         let (sender, receiver) = watch::channel(None);
+        let mut sender = ChainTipSender {
+            non_finalized_tip: false,
+            sender,
+            active_value: None,
+        };
+        let receiver = ChainTipReceiver::new(receiver);
 
-        (
-            ChainTipSender {
-                finalized: None,
-                non_finalized: None,
-                sender,
-                active_value: None,
-            },
-            ChainTipReceiver::new(receiver),
-        )
+        sender.update(initial_tip);
+
+        (sender, receiver)
     }
 
-    /// Update the current finalized block height.
+    /// Update the current finalized tip.
     ///
-    /// May trigger an update to best tip height.
-    pub fn set_finalized_height(&mut self, new_height: block::Height) {
-        if self.finalized != Some(new_height) {
-            self.finalized = Some(new_height);
-            self.update();
+    /// May trigger an update to the best tip.
+    pub fn set_finalized_tip(&mut self, new_tip: impl Into<Option<Arc<Block>>>) {
+        if !self.non_finalized_tip {
+            self.update(new_tip);
         }
     }
 
-    /// Update the current non-finalized block height.
+    /// Update the current non-finalized tip.
     ///
-    /// May trigger an update to the best tip height.
-    pub fn set_best_non_finalized_height(&mut self, new_height: Option<block::Height>) {
-        if self.non_finalized != new_height {
-            self.non_finalized = new_height;
-            self.update();
+    /// May trigger an update to the best tip.
+    pub fn set_best_non_finalized_tip(&mut self, new_tip: impl Into<Option<Arc<Block>>>) {
+        let new_tip = new_tip.into();
+
+        // once the non-finalized state becomes active, it is always populated
+        // but ignoring `None`s makes the tests easier
+        if new_tip.is_some() {
+            self.non_finalized_tip = true;
+            self.update(new_tip)
         }
     }
 
     /// Possibly send an update to listeners.
     ///
-    /// An update is only sent if the current best tip height is different from the last best tip
-    /// height that was sent.
-    fn update(&mut self) {
-        let new_value = self.non_finalized.max(self.finalized);
+    /// An update is only sent if the current best tip is different from the last best tip
+    /// that was sent.
+    fn update(&mut self, new_tip: impl Into<Option<Arc<Block>>>) {
+        let new_tip = new_tip.into();
 
-        if new_value != self.active_value {
-            let _ = self.sender.send(new_value);
-            self.active_value = new_value;
+        if new_tip.is_none() {
+            return;
+        }
+
+        if new_tip != self.active_value {
+            let _ = self.sender.send(new_tip.clone());
+            self.active_value = new_tip;
         }
     }
 }
@@ -68,25 +91,35 @@ impl ChainTipSender {
 /// A receiver for recent changes to the non-finalized and finalized chain tips.
 ///
 /// The latest changes are available from all cloned instances of this type.
+///
+/// The chain tip data is based on:
+/// * the best non-finalized chain tip, if available, or
+/// * the finalized tip.
 #[derive(Clone, Debug)]
 pub struct ChainTipReceiver {
-    receiver: watch::Receiver<Option<block::Height>>,
+    receiver: watch::Receiver<ChainTipData>,
 }
 
 impl ChainTipReceiver {
     /// Create a new chain tip receiver from a watch channel receiver.
-    fn new(receiver: watch::Receiver<Option<block::Height>>) -> Self {
+    fn new(receiver: watch::Receiver<ChainTipData>) -> Self {
         Self { receiver }
     }
 }
 
 impl ChainTip for ChainTipReceiver {
     /// Return the height of the best chain tip.
-    ///
-    /// The returned block height comes from:
-    /// * the best non-finalized chain tip, if available, or
-    /// * the finalized tip.
     fn best_tip_height(&self) -> Option<block::Height> {
-        *self.receiver.borrow()
+        self.receiver
+            .borrow()
+            .as_ref()
+            .and_then(|block| block.coinbase_height())
+    }
+
+    /// Return the block hash of the best chain tip.
+    fn best_tip_hash(&self) -> Option<block::Hash> {
+        // TODO: get the hash from the state and store it in the sender,
+        //       so we don't have to recalculate it every time
+        self.receiver.borrow().as_ref().map(|block| block.hash())
     }
 }

zebra-state/src/service/chain_tip/tests/prop.rs

@@ -1,47 +1,68 @@
+use std::{env, sync::Arc};
+
 use proptest::prelude::*;
 use proptest_derive::Arbitrary;
 
-use zebra_chain::{block, chain_tip::ChainTip};
+use zebra_chain::{block::Block, chain_tip::ChainTip};
 
 use super::super::ChainTipSender;
 
+const DEFAULT_BLOCK_VEC_PROPTEST_CASES: u32 = 4;
+
 proptest! {
+    #![proptest_config(
+        proptest::test_runner::Config::with_cases(env::var("PROPTEST_CASES")
+            .ok()
+            .and_then(|v| v.parse().ok())
+            .unwrap_or(DEFAULT_BLOCK_VEC_PROPTEST_CASES))
+    )]
+
+    /// Check that the best tip uses the non-finalized tip if available,
+    /// or otherwise the finalized tip.
     #[test]
-    fn best_tip_is_highest_of_latest_finalized_and_non_finalized_heights(
-        height_updates in any::<Vec<HeightUpdate>>(),
+    fn best_tip_is_latest_non_finalized_then_latest_finalized(
+        tip_updates in any::<Vec<BlockUpdate>>(),
     ) {
-        let (mut chain_tip_sender, chain_tip_receiver) = ChainTipSender::new();
+        let (mut chain_tip_sender, chain_tip_receiver) = ChainTipSender::new(None);
 
-        let mut latest_finalized_height = None;
-        let mut latest_non_finalized_height = None;
+        let mut latest_finalized_tip = None;
+        let mut latest_non_finalized_tip = None;
+        let mut seen_non_finalized_tip = false;
 
-        for update in height_updates {
+        for update in tip_updates {
             match update {
-                HeightUpdate::Finalized(height) => {
-                    chain_tip_sender.set_finalized_height(height);
-                    latest_finalized_height = Some(height);
+                BlockUpdate::Finalized(block) => {
+                    chain_tip_sender.set_finalized_tip(block.clone());
+                    if block.is_some() {
+                        latest_finalized_tip = block;
+                    }
                 }
-                HeightUpdate::NonFinalized(height) => {
-                    chain_tip_sender.set_best_non_finalized_height(height);
-                    latest_non_finalized_height = height;
+                BlockUpdate::NonFinalized(block) => {
+                    chain_tip_sender.set_best_non_finalized_tip(block.clone());
+                    if block.is_some() {
+                        latest_non_finalized_tip = block;
+                        seen_non_finalized_tip = true;
+                    }
                 }
             }
         }
 
-        let expected_height = match (latest_finalized_height, latest_non_finalized_height) {
-            (Some(finalized_height), Some(non_finalized_height)) => {
-                Some(finalized_height.max(non_finalized_height))
-            }
-            (finalized_height, None) => finalized_height,
-            (None, non_finalized_height) => non_finalized_height,
+        let expected_tip = if seen_non_finalized_tip {
+            latest_non_finalized_tip
+        } else {
+            latest_finalized_tip
         };
 
+        let expected_height = expected_tip.as_ref().and_then(|block| block.coinbase_height());
         prop_assert_eq!(chain_tip_receiver.best_tip_height(), expected_height);
+
+        let expected_hash = expected_tip.as_ref().map(|block| block.hash());
+        prop_assert_eq!(chain_tip_receiver.best_tip_hash(), expected_hash);
     }
 }
 
-#[derive(Arbitrary, Clone, Copy, Debug)]
-enum HeightUpdate {
-    Finalized(block::Height),
-    NonFinalized(Option<block::Height>),
+#[derive(Arbitrary, Clone, Debug)]
+enum BlockUpdate {
+    Finalized(Option<Arc<Block>>),
+    NonFinalized(Option<Arc<Block>>),
 }

zebra-state/src/service/chain_tip/tests/vectors.rs

@@ -1,10 +1,19 @@
-use zebra_chain::chain_tip::ChainTip;
+use zebra_chain::chain_tip::{ChainTip, NoChainTip};
 
 use super::super::ChainTipSender;
 
 #[test]
-fn best_tip_height_is_initially_empty() {
-    let (_chain_tip_sender, chain_tip_receiver) = ChainTipSender::new();
+fn best_tip_is_initially_empty() {
+    let (_chain_tip_sender, chain_tip_receiver) = ChainTipSender::new(None);
 
     assert_eq!(chain_tip_receiver.best_tip_height(), None);
+    assert_eq!(chain_tip_receiver.best_tip_hash(), None);
+}
+
+#[test]
+fn empty_chain_tip_is_empty() {
+    let chain_tip_receiver = NoChainTip;
+
+    assert_eq!(chain_tip_receiver.best_tip_height(), None);
+    assert_eq!(chain_tip_receiver.best_tip_hash(), None);
 }

zebra-state/src/service/finalized_state.rs

@@ -146,15 +146,27 @@ impl FinalizedState {
     ///
     /// After queueing a finalized block, this method checks whether the newly
     /// queued block (and any of its descendants) can be committed to the state.
-    pub fn queue_and_commit_finalized(&mut self, queued: QueuedFinalized) {
+    ///
+    /// Returns the highest finalized tip block committed from the queue,
+    /// or `None` if no blocks were committed in this call.
+    /// (Use [`tip_block`] to get the finalized tip, regardless of when it was committed.)
+    pub fn queue_and_commit_finalized(
+        &mut self,
+        queued: QueuedFinalized,
+    ) -> Option<FinalizedBlock> {
+        let mut highest_queue_commit = None;
+
         let prev_hash = queued.0.block.header.previous_block_hash;
         let height = queued.0.height;
         self.queued_by_prev_hash.insert(prev_hash, queued);
 
         while let Some(queued_block) = self.queued_by_prev_hash.remove(&self.finalized_tip_hash()) {
-            self.commit_finalized(queued_block);
-            metrics::counter!("state.finalized.committed.block.count", 1);
-            metrics::gauge!("state.finalized.committed.block.height", height.0 as _);
+            if let Ok(finalized) = self.commit_finalized(queued_block) {
+                highest_queue_commit = Some(finalized);
+            } else {
+                // the last block in the queue failed, so we can't commit the next block
+                break;
+            }
         }
 
         if self.queued_by_prev_hash.is_empty() {
@@ -173,6 +185,8 @@ impl FinalizedState {
             "state.finalized.queued.block.count",
             self.queued_by_prev_hash.len() as f64
         );
+
+        highest_queue_commit
     }
 
     /// Returns the hash of the current finalized tip block.
@@ -453,10 +467,32 @@ impl FinalizedState {
     /// order. This function is called by [`queue`], which ensures order.
     /// It is intentionally not exposed as part of the public API of the
     /// [`FinalizedState`].
-    fn commit_finalized(&mut self, queued_block: QueuedFinalized) {
+    fn commit_finalized(&mut self, queued_block: QueuedFinalized) -> Result<FinalizedBlock, ()> {
         let (finalized, rsp_tx) = queued_block;
-        let result = self.commit_finalized_direct(finalized, "CommitFinalized request");
+        let result = self.commit_finalized_direct(finalized.clone(), "CommitFinalized request");
+
+        let block_result;
+        if result.is_ok() {
+            metrics::counter!("state.finalized.committed.block.count", 1);
+            metrics::gauge!(
+                "state.finalized.committed.block.height",
+                finalized.height.0 as _
+            );
+
+            block_result = Ok(finalized);
+        } else {
+            metrics::counter!("state.finalized.error.block.count", 1);
+            metrics::gauge!(
+                "state.finalized.error.block.height",
+                finalized.height.0 as _
+            );
+
+            block_result = Err(());
+        }
+
         let _ = rsp_tx.send(result.map_err(Into::into));
+
+        block_result
     }
 
     /// Returns the tip height and hash if there is one.
@@ -473,6 +509,12 @@ impl FinalizedState {
             })
     }
 
+    /// Returns the tip block, if there is one.
+    pub fn tip_block(&self) -> Option<Arc<Block>> {
+        let (height, _hash) = self.tip()?;
+        self.block(height.into())
+    }
+
     /// Returns the height of the given block if it exists.
     pub fn height(&self, hash: block::Hash) -> Option<block::Height> {
         let height_by_hash = self.db.cf_handle("height_by_hash").unwrap();

zebra-state/src/service/non_finalized_state.rs

@@ -318,6 +318,12 @@ impl NonFinalizedState {
         Some((height, hash))
     }
 
+    /// Returns the block at the tip of the best chain.
+    pub fn best_tip_block(&self) -> Option<Arc<Block>> {
+        let (height, _hash) = self.best_tip()?;
+        self.best_block(height.into())
+    }
+
     /// Returns the height of `hash` in the best chain.
     pub fn best_height_by_hash(&self, hash: block::Hash) -> Option<block::Height> {
         let best_chain = self.best_chain()?;