mempool: reject a replacement that spends its conflict's output

changelog.d/mempool-spends-conflicting-tx.md

@@ -0,0 +1,7 @@
+Fixed
+
+- **The mempool rejects a replacement that spends an output of a tx it
+  replaces** (`bad-txns-spends-conflicting-tx`, as in Core). It used to be
+  admitted with an input that left with the replaced tx, so
+  `getblocktemplate` offered a block that fails
+  `bad-txns-inputs-missingorspent`.

crates/rbitcoin-mempool/src/accept.rs

@@ -1316,11 +1316,7 @@ impl ActiveMempool {
         } else {
             BTreeSet::new()
         };
-        let parent_txids: BTreeSet<Txid> = scan
-            .parent_txids
-            .into_iter()
-            .filter(|p| !conflict_set.contains(p))
-            .collect();
+        let parent_txids = scan.parent_txids;
         // Cluster limits count raw weight (Libre): at 20 B/sigop an adjusted
         // cap would reject any tx above ~20,200 sigop cost.
         let (n_members, base_w) = self
@@ -1335,6 +1331,11 @@ impl ActiveMempool {
                 weight: combined_w,
             });
         }
+        // Core `bad-txns-spends-conflicting-tx`. A parent in the conflict set
+        // is evicted by this accept, and its outputs with it.
+        if parent_txids.iter().any(|p| conflict_set.contains(p)) {
+            return Err(AcceptError::Policy("bad-txns-spends-conflicting-tx"));
+        }
 
         Ok((conflict_set, fee_sat, weight))
     }

crates/rbitcoin-test/tests/cross_surface.rs

@@ -1800,6 +1800,44 @@
         "too-low RBF must leave the original: {mem}"
     );
 
+    // Core `bad-txns-spends-conflicting-tx`: a replacement cannot spend an
+    // output of the tx it evicts; that output leaves with it.
+    let mut spends_conflict = acs_spend(
+        rpc_cb,
+        50_0000_0000 + 50_0000_0000 - 1_000,
+        100_000,
+        ScriptBuf::from_bytes(vec![0x58]),
+    );
+    let mut conflict_out = spends_conflict.input[0].clone();
+    conflict_out.previous_output = OutPoint {
+        txid: rpc_spend.compute_txid(),
+        vout: 0,
+    };
+    spends_conflict.input.push(conflict_out);
+    let spends_conflict_hex = encode_tx(&spends_conflict);
+    let tma = jsonrpc(
+        rpc_addr,
+        "testmempoolaccept",
+        json!([[spends_conflict_hex.clone()]]),
+    )
+    .await;
+    assert_eq!(tma["result"][0]["allowed"], false, "{tma}");
+    assert_eq!(
+        tma["result"][0]["reject-reason"], "bad-txns-spends-conflicting-tx",
+        "{tma}"
+    );
+    let rejected = jsonrpc(rpc_addr, "sendrawtransaction", json!([spends_conflict_hex])).await;
+    assert_eq!(rejected["error"]["code"], -26, "{rejected}");
+    assert_eq!(
+        rejected["error"]["message"], "bad-txns-spends-conflicting-tx",
+        "{rejected}"
+    );
+    let mem = jsonrpc(rpc_addr, "getrawmempool", json!([])).await;
+    assert!(
+        mempool_has(&mem, &rpc_txid),
+        "a replacement spending its conflict must leave the original: {mem}"
+    );
+
     let high = acs_spend(
         rpc_cb,
         50_0000_0000,