Limit the number of tap-leaves output by the tr_compiler

Author: SarcasticNastik

src/policy/concrete.rs

@@ -278,6 +278,7 @@ impl<Pk: MiniscriptKey> Policy<Pk> {
             )),
             _ => {
                 let (internal_key, policy) = self.clone().extract_key(unspendable_key)?;
+                policy.check_tapleaf()?;
                 let tree = Descriptor::new_tr(
                     internal_key,
                     match policy {
@@ -501,6 +502,26 @@ impl<Pk: MiniscriptKey> Policy<Pk> {
         }
     }
 
+    /// Get the number of [TapLeaf][`TapTree::Leaf`] considering exhaustive root-level [OR][`Policy::Or`]
+    /// and [Thresh][`Policy::Threshold`] disjunctions for the TapTree.
+    #[cfg(feature = "compiler")]
+    fn num_tap_leaves(&self) -> usize {
+        match self {
+            Policy::Or(subs) => subs.iter().map(|(_prob, pol)| pol.num_tap_leaves()).sum(),
+            Policy::Threshold(k, subs) if *k == 1 => combination(subs.len(), *k),
+            _ => 1,
+        }
+    }
+
+    /// Check on the number of TapLeaves
+    #[cfg(feature = "compiler")]
+     fn check_tapleaf(&self) -> Result<(), Error> {
+        if self.num_tap_leaves() > 100_000 {
+            return Err(errstr("Too many Tapleaves"));
+        }
+        Ok(())
+    }
+
     /// Check whether the policy contains duplicate public keys
     pub fn check_duplicate_keys(&self) -> Result<(), PolicyError> {
         let pks = self.keys();
@@ -918,5 +939,12 @@ fn with_huffman_tree<Pk: MiniscriptKey>(
         .pop()
         .expect("huffman tree algorithm is broken")
         .1;
-    Ok(node)
+
+#[cfg(feature = "compiler")]
+fn combination(n: usize, k: usize) -> usize {
+    if k == 0 {
+        1
+    } else {
+        n * combination(n - 1, k - 1) / k
+    }
 }