transfer-contracts: add sync method

Resolves #2240

contracts/transfer/src/lib.rs

@@ -94,6 +94,13 @@ unsafe fn leaves_from_pos(arg_len: u32) -> u32 {
     rusk_abi::wrap_call(arg_len, |pos| STATE.leaves_from_pos(pos))
 }
 
+#[no_mangle]
+unsafe fn sync(arg_len: u32) -> u32 {
+    rusk_abi::wrap_call(arg_len, |(from, count_limint)| {
+        STATE.sync(from, count_limint)
+    })
+}
+
 // "Management" transactions
 
 #[no_mangle]

contracts/transfer/src/state.rs

@@ -557,8 +557,24 @@ impl TransferState {
     /// Feeds the host with the leaves in the tree, starting from the given
     /// position.
     pub fn leaves_from_pos(&self, pos: u64) {
-        for leaf in self.tree.leaves_pos(pos) {
-            rusk_abi::feed(leaf.clone());
+        self.sync(pos, 0)
+    }
+
+    /// Feeds the host with the leaves in the tree (up to `count_limit`
+    /// occurrences), starting from the given `from` position.
+    ///
+    /// If `count_limit` is 0 there is no occurrences limit`
+    pub fn sync(&self, from: u64, count_limit: u64) {
+        let iter = self.tree.leaves_pos(from);
+
+        if count_limit == 0 {
+            for leaf in iter {
+                rusk_abi::feed(leaf.clone());
+            }
+        } else {
+            for leaf in iter.take(count_limit as usize) {
+                rusk_abi::feed(leaf.clone());
+            }
         }
     }