Add blockstore skipped api (#14145)

* Add blockstore api to determine if a slot was skipped

* Return custom rpc error if slot is skipped

ledger/src/blockstore.rs

@@ -2649,6 +2649,21 @@ impl Blockstore {
         matches!(self.db.get::<cf::Root>(slot), Ok(Some(true)))
     }
 
+    /// Returns true if a slot is between the rooted slot bounds of the ledger, but has not itself
+    /// been rooted. This is either because the slot was skipped, or due to a gap in ledger data,
+    /// as when booting from a newer snapshot.
+    pub fn is_skipped(&self, slot: Slot) -> bool {
+        let lowest_root = self
+            .rooted_slot_iterator(0)
+            .ok()
+            .and_then(|mut iter| iter.next())
+            .unwrap_or_default();
+        match self.db.get::<cf::Root>(slot).ok().flatten() {
+            Some(_) => false,
+            None => slot < self.max_root() && slot > lowest_root,
+        }
+    }
+
     pub fn set_roots(&self, rooted_slots: &[u64]) -> Result<()> {
         let mut write_batch = self.db.batch()?;
         for slot in rooted_slots {
@@ -5523,6 +5538,25 @@ pub mod tests {
         Blockstore::destroy(&blockstore_path).expect("Expected successful database destruction");
     }
 
+    #[test]
+    fn test_is_skipped() {
+        let blockstore_path = get_tmp_ledger_path!();
+        let blockstore = Blockstore::open(&blockstore_path).unwrap();
+        let roots = vec![2, 4, 7, 12, 15];
+        blockstore.set_roots(&roots).unwrap();
+
+        for i in 0..20 {
+            if i < 2 || roots.contains(&i) || i > 15 {
+                assert!(!blockstore.is_skipped(i));
+            } else {
+                assert!(blockstore.is_skipped(i));
+            }
+        }
+
+        drop(blockstore);
+        Blockstore::destroy(&blockstore_path).expect("Expected successful database destruction");
+    }
+
     #[test]
     fn test_iter_bounds() {
         let blockstore_path = get_tmp_ledger_path!();