fix(chain): introduce keychain-variant-ranking to KeychainTxOutIndex

This fixes the bug with changesets not being monotone. Previously, the
result of applying changesets individually v.s. applying the aggregate
of changesets may result in different `KeychainTxOutIndex` states.

The nature of the changeset allows different keychain types to share the
same descriptor. However, the previous design did not take this into
account. To do this properly, we should keep track of all keychains
currently associated with a given descriptor. However, the API only
allows returning one keychain per spk/txout/outpoint (which is a good
API).

Therefore, we rank keychain variants by `Ord`. Earlier keychain variants
have a higher rank, and the first keychain will be returned.

Author: evanlinjin

crates/chain/src/keychain/txout_index.rs

@@ -160,6 +160,20 @@ const DEFAULT_LOOKAHEAD: u32 = 25;
 /// let new_spk_for_user = txout_index.reveal_next_spk(&MyKeychain::MyAppUser{ user_id: 42 });
 /// ```
 ///
+/// # Re-assigning descriptors
+///
+/// Under the hood, the [`KeychainTxOutIndex`] uses a [`SpkTxOutIndex`] that keeps track of spks,
+/// indexed by descriptors. Users then assign or unassign keychains to those descriptors. It's
+/// important to note that descriptors, once added, never get removed from the [`SpkTxOutIndex`];
+/// this is useful in case a user unassigns a keychain from a descriptor and after some time
+/// assigns it again.
+///
+/// Additionally, although a keychain can only be assigned to one descriptor, different keychains
+/// can be assigned to the same descriptor. When a method returns spks/outpoints that is associated
+/// with a descriptor, it may be associated with multiple keychain variants. The keychain variant
+/// with the higher rank will be returned. Rank is determined by the [`Ord`] implementation of the
+/// keychain type. Earlier keychain variants have higher rank.
+///
 /// [`Ord`]: core::cmp::Ord
 /// [`SpkTxOutIndex`]: crate::spk_txout_index::SpkTxOutIndex
 /// [`Descriptor`]: crate::miniscript::Descriptor
@@ -172,17 +186,17 @@ const DEFAULT_LOOKAHEAD: u32 = 25;
 /// [`new`]: KeychainTxOutIndex::new
 /// [`unbounded_spk_iter`]: KeychainTxOutIndex::unbounded_spk_iter
 /// [`all_unbounded_spk_iters`]: KeychainTxOutIndex::all_unbounded_spk_iters
-// Under the hood, the KeychainTxOutIndex uses a SpkTxOutIndex that keeps track of spks, indexed by
-// descriptors. Users then assign or unassign keychains to those descriptors. It's important to
-// note that descriptors, once added, never get removed from the SpkTxOutIndex; this is useful in
-// case a user unassigns a keychain from a descriptor and after some time assigns it again.
 #[derive(Clone, Debug)]
 pub struct KeychainTxOutIndex<K> {
     inner: SpkTxOutIndex<(DescriptorId, u32)>,
     // keychain -> (descriptor, descriptor id) map
     keychains_to_descriptors: BTreeMap<K, (DescriptorId, Descriptor<DescriptorPublicKey>)>,
-    // descriptor id -> keychain map
-    descriptor_ids_to_keychain: BTreeMap<DescriptorId, (K, Descriptor<DescriptorPublicKey>)>,
+    // descriptor id -> keychain set
+    // Because different keychains can have the same descriptor, we rank keychains by `Ord` so that
+    // that the first keychain variant (according to `Ord`) has the highest rank. When associated
+    // data (such as spks, outpoints) are returned with a keychain, we return the highest-ranked
+    // keychain with it.
+    descriptor_ids_to_keychain_set: HashMap<DescriptorId, BTreeSet<K>>,
     // descriptor_id -> descriptor map
     // This is a "monotone" map, meaning that its size keeps growing, i.e., we never delete
     // descriptors from it. This is useful for revealing spks for descriptors that don't have
@@ -208,11 +222,9 @@ impl<K: Clone + Ord + Debug> Indexer for KeychainTxOutIndex<K> {
             Some((descriptor_id, index)) => {
                 // We want to reveal spks for descriptors that aren't tracked by any keychain, and
                 // so we call reveal with descriptor_id
-                if let Some((_, changeset)) = self.reveal_to_target_with_id(descriptor_id, index) {
-                    changeset
-                } else {
-                    super::ChangeSet::default()
-                }
+                let (_, changeset) = self.reveal_to_target_with_id(descriptor_id, index)
+                    .expect("descriptors are added in a monotone manner, there cannot be a descriptor id with no corresponding descriptor");
+                changeset
             }
             None => super::ChangeSet::default(),
         }
@@ -259,9 +271,9 @@ impl<K> KeychainTxOutIndex<K> {
     pub fn new(lookahead: u32) -> Self {
         Self {
             inner: SpkTxOutIndex::default(),
-            descriptor_ids_to_keychain: BTreeMap::new(),
-            descriptor_ids_to_descriptors: BTreeMap::new(),
             keychains_to_descriptors: BTreeMap::new(),
+            descriptor_ids_to_keychain_set: HashMap::new(),
+            descriptor_ids_to_descriptors: BTreeMap::new(),
             last_revealed: BTreeMap::new(),
             lookahead,
         }
@@ -270,6 +282,12 @@ impl<K> KeychainTxOutIndex<K> {
 
 /// Methods that are *re-exposed* from the internal [`SpkTxOutIndex`].
 impl<K: Clone + Ord + Debug> KeychainTxOutIndex<K> {
+    /// Get the highest-ranked keychain that is currently associated with the given `desc_id`.
+    fn keychain_of_desc_id(&self, desc_id: &DescriptorId) -> Option<&K> {
+        let keychains = self.descriptor_ids_to_keychain_set.get(desc_id)?;
+        keychains.iter().next()
+    }
+
     /// Return a reference to the internal [`SpkTxOutIndex`].
     ///
     /// **WARNING:** The internal index will contain lookahead spks. Refer to
@@ -284,18 +302,16 @@ impl<K: Clone + Ord + Debug> KeychainTxOutIndex<K> {
             .outpoints()
             .iter()
             .filter_map(|((desc_id, index), op)| {
-                self.descriptor_ids_to_keychain
-                    .get(desc_id)
-                    .map(|(k, _)| ((k.clone(), *index), *op))
+                let keychain = self.keychain_of_desc_id(desc_id)?;
+                Some(((keychain.clone(), *index), *op))
             })
     }
 
     /// Iterate over known txouts that spend to tracked script pubkeys.
     pub fn txouts(&self) -> impl DoubleEndedIterator<Item = (K, u32, OutPoint, &TxOut)> + '_ {
         self.inner.txouts().filter_map(|((desc_id, i), op, txo)| {
-            self.descriptor_ids_to_keychain
-                .get(desc_id)
-                .map(|(k, _)| (k.clone(), *i, op, txo))
+            let keychain = self.keychain_of_desc_id(desc_id)?;
+            Some((keychain.clone(), *i, op, txo))
         })
     }
 
@@ -307,9 +323,8 @@ impl<K: Clone + Ord + Debug> KeychainTxOutIndex<K> {
         self.inner
             .txouts_in_tx(txid)
             .filter_map(|((desc_id, i), op, txo)| {
-                self.descriptor_ids_to_keychain
-                    .get(desc_id)
-                    .map(|(k, _)| (k.clone(), *i, op, txo))
+                let keychain = self.keychain_of_desc_id(desc_id)?;
+                Some((keychain.clone(), *i, op, txo))
             })
     }
 
@@ -321,7 +336,7 @@ impl<K: Clone + Ord + Debug> KeychainTxOutIndex<K> {
     /// This calls [`SpkTxOutIndex::txout`] internally.
     pub fn txout(&self, outpoint: OutPoint) -> Option<(K, u32, &TxOut)> {
         let ((descriptor_id, index), txo) = self.inner.txout(outpoint)?;
-        let (keychain, _) = self.descriptor_ids_to_keychain.get(descriptor_id)?;
+        let keychain = self.keychain_of_desc_id(descriptor_id)?;
         Some((keychain.clone(), *index, txo))
     }
 
@@ -338,9 +353,8 @@ impl<K: Clone + Ord + Debug> KeychainTxOutIndex<K> {
     /// This calls [`SpkTxOutIndex::index_of_spk`] internally.
     pub fn index_of_spk(&self, script: &Script) -> Option<(K, u32)> {
         let (desc_id, last_index) = self.inner.index_of_spk(script)?;
-        self.descriptor_ids_to_keychain
-            .get(desc_id)
-            .map(|(k, _)| (k.clone(), *last_index))
+        let keychain = self.keychain_of_desc_id(desc_id)?;
+        Some((keychain.clone(), *last_index))
     }
 
     /// Returns whether the spk under the `keychain`'s `index` has been used.
@@ -448,45 +462,42 @@ impl<K: Clone + Ord + Debug> KeychainTxOutIndex<K> {
         keychain: K,
         descriptor: Descriptor<DescriptorPublicKey>,
     ) -> super::ChangeSet<K> {
-        let descriptor_id = descriptor.descriptor_id();
-        // First, we fill the keychain -> (desc_id, descriptor) map
-        let old_descriptor_opt = self
+        let mut changeset = super::ChangeSet::<K>::default();
+        let desc_id = descriptor.descriptor_id();
+
+        let old_desc = self
             .keychains_to_descriptors
-            .insert(keychain.clone(), (descriptor_id, descriptor.clone()));
-
-        // Then, we fill the descriptor_id -> (keychain, descriptor) map
-        let old_keychain_opt = self
-            .descriptor_ids_to_keychain
-            .insert(descriptor_id, (keychain.clone(), descriptor.clone()));
-
-        // If `keychain` already had a `descriptor` associated, different from the `descriptor`
-        // passed in, we remove it from the descriptor -> keychain map
-        if let Some((old_desc_id, _)) = old_descriptor_opt {
-            if old_desc_id != descriptor_id {
-                self.descriptor_ids_to_keychain.remove(&old_desc_id);
+            .insert(keychain.clone(), (desc_id, descriptor.clone()));
+
+        if let Some((old_desc_id, _)) = old_desc {
+            // nothing needs to be done if caller reinsterted the same descriptor under the same
+            // keychain
+            if old_desc_id == desc_id {
+                return changeset;
             }
+            // we should remove old descriptor that is associated with this keychain as the index
+            // is designed to track one descriptor per keychain (however different keychains can
+            // share the same descriptor)
+            let _is_keychain_removed = self
+                .descriptor_ids_to_keychain_set
+                .get_mut(&old_desc_id)
+                .expect("we must have already inserted this descriptor")
+                .remove(&keychain);
+            debug_assert!(_is_keychain_removed);
         }
 
-        // Lastly, we fill the desc_id -> desc map
+        self.descriptor_ids_to_keychain_set
+            .entry(desc_id)
+            .or_default()
+            .insert(keychain.clone());
         self.descriptor_ids_to_descriptors
-            .insert(descriptor_id, descriptor.clone());
-
+            .insert(desc_id, descriptor.clone());
         self.replenish_lookahead(&keychain, self.lookahead);
 
-        // If both the keychain and descriptor were already inserted and associated, the
-        // keychains_added changeset must be empty
-        let keychains_added = if old_keychain_opt.map(|(k, _)| k) == Some(keychain.clone())
-            && old_descriptor_opt.map(|(_, d)| d) == Some(descriptor.clone())
-        {
-            [].into()
-        } else {
-            [(keychain, descriptor)].into()
-        };
-
-        super::ChangeSet {
-            keychains_added,
-            last_revealed: [].into(),
-        }
+        changeset
+            .keychains_added
+            .insert(keychain.clone(), descriptor.clone());
+        changeset
     }
 
     /// Gets the descriptor associated with the keychain. Returns `None` if the keychain doesn't
@@ -584,11 +595,8 @@ impl<K: Clone + Ord + Debug> KeychainTxOutIndex<K> {
                     .range((start, end))
                     .map(|((descriptor_id, i), spk)| {
                         (
-                            &self
-                                .descriptor_ids_to_keychain
-                                .get(descriptor_id)
-                                .expect("Must be here")
-                                .0,
+                            self.keychain_of_desc_id(descriptor_id)
+                                .expect("must have keychain"),
                             *i,
                             spk.as_script(),
                         )
@@ -673,10 +681,9 @@ impl<K: Clone + Ord + Debug> KeychainTxOutIndex<K> {
     pub fn last_revealed_indices(&self) -> BTreeMap<K, u32> {
         self.last_revealed
             .iter()
-            .filter_map(|(descriptor_id, index)| {
-                self.descriptor_ids_to_keychain
-                    .get(descriptor_id)
-                    .map(|(k, _)| (k.clone(), *index))
+            .filter_map(|(desc_id, index)| {
+                let keychain = self.keychain_of_desc_id(desc_id)?;
+                Some((keychain.clone(), *index))
             })
             .collect()
     }
@@ -870,16 +877,11 @@ impl<K: Clone + Ord + Debug> KeychainTxOutIndex<K> {
         let bounds = self.map_to_inner_bounds(range);
         self.inner
             .outputs_in_range(bounds)
-            .map(move |((descriptor_id, i), op)| {
-                (
-                    &self
-                        .descriptor_ids_to_keychain
-                        .get(descriptor_id)
-                        .expect("must be here")
-                        .0,
-                    *i,
-                    op,
-                )
+            .map(move |((desc_id, i), op)| {
+                let keychain = self
+                    .keychain_of_desc_id(desc_id)
+                    .expect("keychain must exist");
+                (keychain, *i, op)
             })
     }
 
@@ -941,10 +943,9 @@ impl<K: Clone + Ord + Debug> KeychainTxOutIndex<K> {
         }
         let last_revealed = last_revealed
             .into_iter()
-            .filter_map(|(descriptor_id, index)| {
-                self.descriptor_ids_to_keychain
-                    .get(&descriptor_id)
-                    .map(|(k, _)| (k.clone(), index))
+            .filter_map(|(desc_id, index)| {
+                let keychain = self.keychain_of_desc_id(&desc_id)?;
+                Some((keychain.clone(), index))
             })
             .collect();
         let _ = self.reveal_to_target_multi(&last_revealed);