light-client: Fix verification of blocks between two trusted heights

light-client/src/components/scheduler.rs

@@ -20,7 +20,7 @@ pub trait Scheduler: Send + Sync {
     ///
     /// ## Postcondition
     /// - The resulting height must be valid according to `valid_schedule`. [LCV-SCHEDULE-POST.1]
-    #[requires(light_store.highest_trusted_or_verified().is_some())]
+    #[requires(light_store.highest_trusted_or_verified_before(target_height).is_some())]
     #[ensures(valid_schedule(ret, target_height, current_height, light_store))]
     fn schedule(
         &self,
@@ -61,7 +61,7 @@ pub fn basic_bisecting_schedule(
     target_height: Height,
 ) -> Height {
     let trusted_height = light_store
-        .highest_trusted_or_verified()
+        .highest_trusted_or_verified_before(target_height)
         .map(|lb| lb.height())
         .unwrap();
 
@@ -105,7 +105,7 @@ pub fn valid_schedule(
     light_store: &dyn LightStore,
 ) -> bool {
     let latest_trusted_height = light_store
-        .highest_trusted_or_verified()
+        .highest_trusted_or_verified_before(target_height)
         .map(|lb| lb.height())
         .unwrap();
 

light-client/src/light_client.rs

@@ -161,7 +161,8 @@ impl LightClient {
         // Get the highest trusted state
         let highest = state
             .light_store
-            .highest_trusted_or_verified()
+            .highest_trusted_or_verified_before(target_height)
+            .or_else(|| state.light_store.lowest_trusted_or_verified())
             .ok_or_else(Error::no_initial_trusted_state)?;
 
         if target_height >= highest.height() {
@@ -187,7 +188,7 @@ impl LightClient {
             // Get the latest trusted state
             let trusted_block = state
                 .light_store
-                .highest_trusted_or_verified()
+                .highest_trusted_or_verified_before(target_height)
                 .ok_or_else(Error::no_initial_trusted_state)?;
 
             if target_height < trusted_block.height() {
@@ -267,7 +268,8 @@ impl LightClient {
     ) -> Result<LightBlock, Error> {
         let trusted_state = state
             .light_store
-            .highest_trusted_or_verified()
+            .highest_trusted_or_verified_before(target_height)
+            .or_else(|| state.light_store.lowest_trusted_or_verified())
             .ok_or_else(Error::no_initial_trusted_state)?;
 
         Err(Error::target_lower_than_trusted_state(
@@ -304,7 +306,8 @@ impl LightClient {
 
         let root = state
             .light_store
-            .highest_trusted_or_verified()
+            .highest_trusted_or_verified_before(target_height)
+            .or_else(|| state.light_store.lowest_trusted_or_verified())
             .ok_or_else(Error::no_initial_trusted_state)?;
 
         assert!(root.height() >= target_height);

light-client/src/store.rs

@@ -43,6 +43,9 @@ pub trait LightStore: Debug + Send + Sync {
     /// Get the light block of greatest height with the given status.
     fn highest(&self, status: Status) -> Option<LightBlock>;
 
+    /// Get the light block of greatest hight before the given height with the given status.
+    fn highest_before(&self, height: Height, status: Status) -> Option<LightBlock>;
+
     /// Get the light block of lowest height with the given status.
     fn lowest(&self, status: Status) -> Option<LightBlock>;
 
@@ -76,6 +79,16 @@ pub trait LightStore: Debug + Send + Sync {
         })
     }
 
+    /// Get the first light block before the given height with the trusted or verified status.
+    fn highest_trusted_or_verified_before(&self, height: Height) -> Option<LightBlock> {
+        let highest_trusted = self.highest_before(height, Status::Trusted);
+        let highest_verified = self.highest_before(height, Status::Verified);
+
+        std_ext::option::select(highest_trusted, highest_verified, |t, v| {
+            std_ext::cmp::max_by_key(t, v, |lb| lb.height())
+        })
+    }
+
     /// Get the light block of lowest height with the trusted or verified status.
     fn lowest_trusted_or_verified(&self) -> Option<LightBlock> {
         let lowest_trusted = self.lowest(Status::Trusted);

light-client/src/store/memory.rs

@@ -72,6 +72,15 @@ impl LightStore for MemoryStore {
             .map(|(_, e)| e.light_block.clone())
     }
 
+    fn highest_before(&self, height: Height, status: Status) -> Option<LightBlock> {
+        self.store
+            .iter()
+            .filter(|(_, e)| e.status == status)
+            .filter(|(h, _)| h <= &&height)
+            .max_by_key(|(&height, _)| height)
+            .map(|(_, e)| e.light_block.clone())
+    }
+
     fn lowest(&self, status: Status) -> Option<LightBlock> {
         self.store
             .iter()

light-client/src/store/sled.rs

@@ -79,6 +79,11 @@ impl LightStore for SledStore {
         self.db(status).iter().next_back()
     }
 
+    fn highest_before(&self, height: Height, status: Status) -> Option<LightBlock> {
+        // TODO: Should use .range(..=key_bytes(height)).next_back() instead for more efficiency.
+        self.db(status).iter().rfind(|lb| lb.height() <= height)
+    }
+
     fn lowest(&self, status: Status) -> Option<LightBlock> {
         self.db(status).iter().next()
     }
@@ -109,6 +114,16 @@ mod tests {
         })
     }
 
+    #[test]
+    fn highest_before_returns_correct_block() {
+        with_blocks(10, |mut db, blocks| {
+            for block in blocks {
+                db.insert(block.clone(), Status::Verified);
+                assert_eq!(db.highest_before(block.height(), Status::Verified), Some(block));
+            }
+        })
+    }
+
     #[test]
     fn lowest_returns_earliest_block() {
         with_blocks(10, |mut db, blocks| {