Merge #917: Fix panic in compile_tr_private_experimental for non-expandable policies

f728d78 fix(policy): fix panic in compile_tr_private_experimental for non-expandable policies (Yeji Han)

Pull request description:

  Closes #902.
  
  - Keep `pol_prob_map` in sync with `tapleaf_prob_vec` by removing entries from both data structures together. 
  - Previously, removing a node from `tapleaf_prob_vec` without removing it from `pol_prob_map` caused a panic when `expand_fn` returned the same policy (e.g. for `pk(A)` or `and(pk(A),pk(B))`).
  - Adds regression tests for both cases.


ACKs for top commit:
  apoelstra:
    ACK f728d78; successfully ran local tests


Tree-SHA512: da733d9d60329ef909c50fac87675542056fa441cf3bc1bb101581ec89f885dc23cdeafe7786c6414fcb50b0560f1aff17da3dca91604d1acbac117e8607857f

Author: apoelstra

src/policy/concrete.rs

@@ -614,10 +614,12 @@ impl<Pk: MiniscriptKey> Policy<Pk> {
 
             // Remove current node
             assert!(tapleaf_prob_vec.remove(&(prob, curr_policy.clone())));
+            pol_prob_map.remove(&curr_policy);
 
             // OPTIMIZATION - Move marked nodes into final vector
             for (p, pol) in to_del {
                 assert!(tapleaf_prob_vec.remove(&(Reverse(OrdF64(p)), pol.clone())));
+                pol_prob_map.remove(&pol);
                 ret.push((p, pol.clone()));
             }
 
@@ -1276,6 +1278,21 @@ mod compiler_tests {
         // pk(A) promoted to the internal key, leaving the script tree empty
         assert_eq!(desc.to_string(), "tr(A)#xyg3grex");
     }
+
+    #[test]
+    fn compile_tr_private_non_expandable() {
+        // Single key: extract_key promotes it, leaving Unsatisfiable
+        let pol = Policy::<String>::from_str("pk(A)").unwrap();
+        let desc = pol.compile_tr_private_experimental(None).unwrap();
+        assert_eq!(desc.to_string(), "tr(A)#xyg3grex");
+
+        // Two keys: one promoted to internal key, other becomes single leaf
+        let pol = Policy::<String>::from_str("and(pk(A),pk(B))").unwrap();
+        let desc = pol
+            .compile_tr_private_experimental(Some("UNSPENDABLE".to_string()))
+            .unwrap();
+        assert_eq!(desc.to_string(), "tr(UNSPENDABLE,and_v(v:pk(A),pk(B)))#787dpsca");
+    }
 }
 
 #[cfg(test)]