Add a mempool_spenders method to the Bitcoin interface

src/bitcoin/d/mod.rs

@@ -1140,6 +1140,29 @@ impl BitcoinD {
         }
     }
 
+    /// Get the list of txids spending those outpoints in mempool.
+    pub fn mempool_txs_spending_prevouts(
+        &self,
+        outpoints: &[bitcoin::OutPoint],
+    ) -> Vec<bitcoin::Txid> {
+        let prevouts: Json = outpoints
+            .iter()
+            .map(|op| serde_json::json!({"txid": op.txid.to_string(), "vout": op.vout}))
+            .collect();
+        self.make_node_request("gettxspendingprevout", &params!(prevouts))
+            .as_array()
+            .expect("Always returns an array")
+            .iter()
+            .filter_map(|e| {
+                e.get("spendingtxid").map(|e| {
+                    e.as_str()
+                        .and_then(|s| bitcoin::Txid::from_str(s).ok())
+                        .expect("Must be a valid txid if present")
+                })
+            })
+            .collect()
+    }
+
     /// Stop bitcoind.
     pub fn stop(&self) {
         self.make_node_request("stop", &[]);

src/bitcoin/mod.rs

@@ -117,6 +117,9 @@ pub trait BitcoinInterface: Send {
     /// Get mempool entry of the given transaction.
     /// Returns `None` if it is not in the mempool.
     fn mempool_entry(&self, txid: &bitcoin::Txid) -> Option<MempoolEntry>;
+
+    /// Get the details of unconfirmed transactions spending these outpoints, if any.
+    fn mempool_spenders(&self, outpoints: &[bitcoin::OutPoint]) -> Vec<MempoolEntry>;
 }
 
 impl BitcoinInterface for d::BitcoinD {
@@ -364,6 +367,13 @@ impl BitcoinInterface for d::BitcoinD {
     fn mempool_entry(&self, txid: &bitcoin::Txid) -> Option<MempoolEntry> {
         self.mempool_entry(txid)
     }
+
+    fn mempool_spenders(&self, outpoints: &[bitcoin::OutPoint]) -> Vec<MempoolEntry> {
+        self.mempool_txs_spending_prevouts(outpoints)
+            .into_iter()
+            .filter_map(|txid| self.mempool_entry(&txid))
+            .collect()
+    }
 }
 
 // FIXME: do we need to repeat the entire trait implemenation? Isn't there a nicer way?
@@ -454,6 +464,10 @@ impl BitcoinInterface for sync::Arc<sync::Mutex<dyn BitcoinInterface + 'static>>
     fn mempool_entry(&self, txid: &bitcoin::Txid) -> Option<MempoolEntry> {
         self.lock().unwrap().mempool_entry(txid)
     }
+
+    fn mempool_spenders(&self, outpoints: &[bitcoin::OutPoint]) -> Vec<MempoolEntry> {
+        self.lock().unwrap().mempool_spenders(outpoints)
+    }
 }
 
 // FIXME: We could avoid this type (and all the conversions entailing allocations) if bitcoind

src/testutils.rs

@@ -137,6 +137,10 @@ impl BitcoinInterface for DummyBitcoind {
             }
         })
     }
+
+    fn mempool_spenders(&self, _: &[bitcoin::OutPoint]) -> Vec<MempoolEntry> {
+        Vec::new()
+    }
 }
 
 struct DummyDbState {