bitcoin: interface to get the common block in our and the backend's chains

This will be used to get the common ancestor in the chain upon detecting
a block chain reorganization.

src/bitcoin/d/mod.rs

@@ -651,6 +651,34 @@ impl BitcoinD {
 
         None
     }
+
+    pub fn get_block_stats(&self, blockhash: bitcoin::BlockHash) -> BlockStats {
+        let res = self.make_node_request(
+            "getblockheader",
+            &params!(Json::String(blockhash.to_string()),),
+        );
+        let confirmations = res
+            .get("confirmations")
+            .and_then(Json::as_i64)
+            .expect("Invalid confirmations in `getblockheader` response: not an i64")
+            as i32;
+        let previous_blockhash = res
+            .get("previousblockhash")
+            .and_then(Json::as_str)
+            .and_then(|s| bitcoin::BlockHash::from_str(s).ok())
+            .expect("Invalid previousblockhash in `getblockheader` response");
+        let height = res
+            .get("height")
+            .and_then(Json::as_i64)
+            .expect("Invalid height in `getblockheader` response: not an u32")
+            as i32;
+        BlockStats {
+            confirmations,
+            previous_blockhash,
+            height,
+            blockhash,
+        }
+    }
 }
 // Bitcoind uses a guess for the value of verificationprogress. It will eventually get to
 // be 1, and we want to be less conservative.
@@ -779,3 +807,11 @@ impl From<Json> for GetTxRes {
         }
     }
 }
+
+#[derive(Debug, Clone)]
+pub struct BlockStats {
+    pub confirmations: i32,
+    pub previous_blockhash: bitcoin::BlockHash,
+    pub blockhash: bitcoin::BlockHash,
+    pub height: i32,
+}

src/bitcoin/mod.rs

@@ -52,6 +52,9 @@ pub trait BitcoinInterface: Send {
         &self,
         outpoints: &[(bitcoin::OutPoint, bitcoin::Txid)],
     ) -> Vec<(bitcoin::OutPoint, bitcoin::Txid, u32)>;
+
+    /// Get the common ancestor between the Bitcoin backend's tip and the given tip.
+    fn common_ancestor(&self, tip: &BlockChainTip) -> BlockChainTip;
 }
 
 impl BitcoinInterface for d::BitcoinD {
@@ -207,6 +210,21 @@ impl BitcoinInterface for d::BitcoinD {
 
         spent
     }
+
+    fn common_ancestor(&self, tip: &BlockChainTip) -> BlockChainTip {
+        let mut stats = self.get_block_stats(tip.hash);
+        let mut ancestor = *tip;
+
+        while stats.confirmations == -1 {
+            stats = self.get_block_stats(stats.previous_blockhash);
+            ancestor = BlockChainTip {
+                hash: stats.blockhash,
+                height: stats.height,
+            };
+        }
+
+        ancestor
+    }
 }
 
 // FIXME: do we need to repeat the entire trait implemenation? Isn't there a nicer way?
@@ -251,6 +269,10 @@ impl BitcoinInterface for sync::Arc<sync::Mutex<dyn BitcoinInterface + 'static>>
     ) -> Vec<(bitcoin::OutPoint, bitcoin::Txid, u32)> {
         self.lock().unwrap().spent_coins(outpoints)
     }
+
+    fn common_ancestor(&self, tip: &BlockChainTip) -> BlockChainTip {
+        self.lock().unwrap().common_ancestor(tip)
+    }
 }
 
 // FIXME: We could avoid this type (and all the conversions entailing allocations) if bitcoind

src/testutils.rs

@@ -62,6 +62,10 @@ impl BitcoinInterface for DummyBitcoind {
     ) -> Vec<(bitcoin::OutPoint, bitcoin::Txid, u32)> {
         Vec::new()
     }
+
+    fn common_ancestor(&self, _: &BlockChainTip) -> BlockChainTip {
+        todo!()
+    }
 }
 
 pub struct DummyDb {