fix(bitcoind_rpc): FilterIter detects reorgs

Co-authored-by: valued mammal <valuedmammal@protonmail.com>

Author: LagginTimes

crates/bitcoind_rpc/src/bip158.rs

@@ -6,7 +6,7 @@
 //! [0]: https://github.com/bitcoin/bips/blob/master/bip-0157.mediawiki
 //! [1]: https://github.com/bitcoin/bips/blob/master/bip-0158.mediawiki
 
-use bdk_core::collections::BTreeMap;
+use bdk_core::collections::{BTreeMap, BTreeSet};
 use core::fmt;
 
 use bdk_core::bitcoin;
@@ -33,20 +33,29 @@ pub struct FilterIter<'c, C> {
     cp: Option<CheckPoint>,
     // blocks map
     blocks: BTreeMap<Height, BlockHash>,
+    // set of heights with filters that matched any watched SPK
+    matched: BTreeSet<Height>,
+    // initial height
+    start: Height,
     // best height counter
     height: Height,
     // stop height
     stop: Height,
 }
 
 impl<'c, C: RpcApi> FilterIter<'c, C> {
+    /// Hard cap on how far to walk back when a reorg is detected.
+    const MAX_REORG_DEPTH: u32 = 100;
+
     /// Construct [`FilterIter`] from a given `client` and start `height`.
     pub fn new_with_height(client: &'c C, height: u32) -> Self {
         Self {
             client,
             spks: vec![],
             cp: None,
             blocks: BTreeMap::new(),
+            matched: BTreeSet::new(),
+            start: height,
             height,
             stop: 0,
         }
@@ -69,57 +78,28 @@ impl<'c, C: RpcApi> FilterIter<'c, C> {
         self.spks.push(spk);
     }
 
-    /// Get the next filter and increment the current best height.
-    ///
-    /// Returns `Ok(None)` when the stop height is exceeded.
-    fn next_filter(&mut self) -> Result<Option<NextFilter>, Error> {
-        if self.height > self.stop {
-            return Ok(None);
-        }
-        let height = self.height;
-        let hash = match self.blocks.get(&height) {
-            Some(h) => *h,
-            None => self.client.get_block_hash(height as u64)?,
-        };
-        let filter_bytes = self.client.get_block_filter(&hash)?.filter;
-        let filter = BlockFilter::new(&filter_bytes);
-        self.height += 1;
-        Ok(Some((BlockId { height, hash }, filter)))
+    /// Get the block hash by `height` if it is found in the blocks map.
+    fn get_block_hash(&self, height: &Height) -> Option<BlockHash> {
+        self.blocks.get(height).copied()
     }
 
     /// Get the remote tip.
     ///
-    /// Returns `None` if the remote height is not strictly greater than the height of this
-    /// [`FilterIter`].
+    /// Returns `None` if the remote height is less than the height of this [`FilterIter`].
     pub fn get_tip(&mut self) -> Result<Option<BlockId>, Error> {
         let tip_hash = self.client.get_best_block_hash()?;
-        let mut header = self.client.get_block_header_info(&tip_hash)?;
+        let header = self.client.get_block_header_info(&tip_hash)?;
         let tip_height = header.height as u32;
-        if self.height >= tip_height {
+        // Allow returning tip if we're exactly at it. Return `None`` if we've already scanned past.
+        if self.height > tip_height {
             // nothing to do
             return Ok(None);
         }
-        self.blocks.insert(tip_height, tip_hash);
 
-        // if we have a checkpoint we use a lookback of ten blocks
-        // to ensure consistency of the local chain
+        // start scanning from point of agreement + 1
         if let Some(cp) = self.cp.as_ref() {
-            // adjust start height to point of agreement + 1
             let base = self.find_base_with(cp.clone())?;
-            self.height = base.height + 1;
-
-            for _ in 0..9 {
-                let hash = match header.previous_block_hash {
-                    Some(hash) => hash,
-                    None => break,
-                };
-                header = self.client.get_block_header_info(&hash)?;
-                let height = header.height as u32;
-                if height < self.height {
-                    break;
-                }
-                self.blocks.insert(height, hash);
-            }
+            self.height = base.height.saturating_add(1);
         }
 
         self.stop = tip_height;
@@ -131,9 +111,6 @@ impl<'c, C: RpcApi> FilterIter<'c, C> {
     }
 }
 
-/// Alias for a compact filter and associated block id.
-type NextFilter = (BlockId, BlockFilter);
-
 /// Event inner type
 #[derive(Debug, Clone)]
 pub struct EventInner {
@@ -171,27 +148,80 @@ impl<C: RpcApi> Iterator for FilterIter<'_, C> {
     type Item = Result<Event, Error>;
 
     fn next(&mut self) -> Option<Self::Item> {
-        (|| -> Result<_, Error> {
-            // if the next filter matches any of our watched spks, get the block
-            // and return it, inserting relevant block ids along the way
-            self.next_filter()?.map_or(Ok(None), |(block, filter)| {
-                let height = block.height;
-                let hash = block.hash;
-
-                if self.spks.is_empty() {
-                    Err(Error::NoScripts)
-                } else if filter
-                    .match_any(&hash, self.spks.iter().map(|script| script.as_bytes()))
-                    .map_err(Error::Bip158)?
-                {
-                    let block = self.client.get_block(&hash)?;
-                    self.blocks.insert(height, hash);
-                    let inner = EventInner { height, block };
-                    Ok(Some(Event::Block(inner)))
-                } else {
-                    Ok(Some(Event::NoMatch(height)))
+        (|| -> Result<Option<_>, Error> {
+            if self.height > self.stop {
+                return Ok(None);
+            }
+            // Fetch next header.
+            let mut height = self.height;
+            let mut hash = self.client.get_block_hash(height as _)?;
+            let mut header = self.client.get_block_header(&hash)?;
+
+            // Detect and resolve reorgs: either block at height changed, or its parent changed.
+            let stored_hash = self.blocks.get(&height).copied();
+            let prev_hash = height
+                .checked_sub(1)
+                .and_then(|height| self.blocks.get(&height).copied());
+
+            // If we've seen this height before but the hash has changed, or parent changed, trigger
+            // reorg.
+            let reorg_detected = if let Some(old_hash) = stored_hash {
+                old_hash != hash
+            } else if let Some(expected_prev) = prev_hash {
+                header.prev_blockhash != expected_prev
+            } else {
+                false
+            };
+
+            // Reorg detected, rewind to last known-good ancestor.
+            if reorg_detected {
+                let mut reorg_depth = 0;
+                loop {
+                    if reorg_depth >= Self::MAX_REORG_DEPTH || height == 0 {
+                        return Err(Error::ReorgDepthExceeded);
+                    }
+
+                    height = height.saturating_sub(1);
+                    hash = self.client.get_block_hash(height as _)?;
+                    header = self.client.get_block_header(&hash)?;
+
+                    let prev_height = height.saturating_sub(1);
+                    if let Some(prev_hash) = self.blocks.get(&prev_height) {
+                        if header.prev_blockhash == *prev_hash {
+                            break;
+                        }
+                    }
+
+                    reorg_depth += 1;
                 }
-            })
+
+                self.blocks.split_off(&height);
+                self.matched.split_off(&height);
+            }
+
+            let filter_bytes = self.client.get_block_filter(&hash)?.filter;
+            let filter = BlockFilter::new(&filter_bytes);
+
+            // record the scanned block
+            self.blocks.insert(height, hash);
+            // increment best height
+            self.height = height.saturating_add(1);
+
+            // If the filter matches any of our watched SPKs, fetch the full
+            // block, and record the matching block entry.
+            if self.spks.is_empty() {
+                Err(Error::NoScripts)
+            } else if filter
+                .match_any(&hash, self.spks.iter().map(|s| s.as_bytes()))
+                .map_err(Error::Bip158)?
+            {
+                let block = self.client.get_block(&hash)?;
+                self.matched.insert(height);
+                let inner = EventInner { height, block };
+                Ok(Some(Event::Block(inner)))
+            } else {
+                Ok(Some(Event::NoMatch(height)))
+            }
         })()
         .transpose()
     }
@@ -202,8 +232,8 @@ impl<C: RpcApi> FilterIter<'_, C> {
     fn find_base_with(&mut self, mut cp: CheckPoint) -> Result<BlockId, Error> {
         loop {
             let height = cp.height();
-            let fetched_hash = match self.blocks.get(&height) {
-                Some(hash) => *hash,
+            let fetched_hash = match self.get_block_hash(&height) {
+                Some(hash) => hash,
                 None if height == 0 => cp.hash(),
                 _ => self.client.get_block_hash(height as _)?,
             };
@@ -221,17 +251,27 @@ impl<C: RpcApi> FilterIter<'_, C> {
     /// Returns a chain update from the newly scanned blocks.
     ///
     /// Returns `None` if this [`FilterIter`] was not constructed using a [`CheckPoint`], or
-    /// if no blocks have been fetched for example by using [`get_tip`](Self::get_tip).
+    /// if not all events have been emitted (by calling `next`).
     pub fn chain_update(&mut self) -> Option<CheckPoint> {
-        if self.cp.is_none() || self.blocks.is_empty() {
+        if self.cp.is_none() || self.blocks.is_empty() || self.height <= self.stop {
             return None;
         }
 
-        // note: to connect with the local chain we must guarantee that `self.blocks.first()`
-        // is also the point of agreement with `self.cp`.
+        // We return blocks up to and including the initial height, all of the matching blocks,
+        // and blocks in the terminal range.
+        let tail_range = self.stop.saturating_sub(9)..=self.stop;
         Some(
-            CheckPoint::from_block_ids(self.blocks.iter().map(BlockId::from))
-                .expect("blocks must be in order"),
+            CheckPoint::from_block_ids(self.blocks.iter().filter_map(|(&height, &hash)| {
+                if height <= self.start
+                    || self.matched.contains(&height)
+                    || tail_range.contains(&height)
+                {
+                    Some(BlockId { height, hash })
+                } else {
+                    None
+                }
+            }))
+            .expect("blocks must be in order"),
         )
     }
 }
@@ -245,6 +285,8 @@ pub enum Error {
     NoScripts,
     /// `bitcoincore_rpc` error
     Rpc(bitcoincore_rpc::Error),
+    /// `MAX_REORG_DEPTH` exceeded
+    ReorgDepthExceeded,
 }
 
 impl From<bitcoincore_rpc::Error> for Error {
@@ -259,6 +301,7 @@ impl fmt::Display for Error {
             Self::Bip158(e) => e.fmt(f),
             Self::NoScripts => write!(f, "no script pubkeys were provided to match with"),
             Self::Rpc(e) => e.fmt(f),
+            Self::ReorgDepthExceeded => write!(f, "maximum reorg depth exceeded"),
         }
     }
 }

crates/bitcoind_rpc/tests/test_filter_iter.rs

@@ -400,7 +400,6 @@ fn filter_iter_handles_reorg() -> anyhow::Result<()> {
 
 // Test that while a reorg is detected we delay incrementing the best height
 #[test]
-#[ignore]
 fn repeat_reorgs() -> anyhow::Result<()> {
     const MINE_TO: u32 = 11;