refactor: Add IsRBFOptInEmptyMempool

Co-authored-by: John Newbery <jonnynewbs@gmail.com>

src/interfaces/chain.cpp

@@ -276,8 +276,9 @@ public:
     }
     RBFTransactionState isRBFOptIn(const CTransaction& tx) override
     {
-        LOCK(::mempool.cs);
-        return IsRBFOptIn(tx, ::mempool);
+        if (!m_node.mempool) return IsRBFOptInEmptyMempool(tx);
+        LOCK(m_node.mempool->cs);
+        return IsRBFOptIn(tx, *m_node.mempool);
     }
     bool hasDescendantsInMempool(const uint256& txid) override
     {

src/policy/rbf.cpp

@@ -36,3 +36,9 @@ RBFTransactionState IsRBFOptIn(const CTransaction& tx, const CTxMemPool& pool)
     }
     return RBFTransactionState::FINAL;
 }
+
+RBFTransactionState IsRBFOptInEmptyMempool(const CTransaction& tx)
+{
+    // If we don't have a local mempool we can only check the transaction itself.
+    return SignalsOptInRBF(tx) ? RBFTransactionState::REPLACEABLE_BIP125 : RBFTransactionState::UNKNOWN;
+}

src/policy/rbf.h

@@ -18,5 +18,6 @@ enum class RBFTransactionState {
 // This involves checking sequence numbers of the transaction, as well
 // as the sequence numbers of all in-mempool ancestors.
 RBFTransactionState IsRBFOptIn(const CTransaction& tx, const CTxMemPool& pool) EXCLUSIVE_LOCKS_REQUIRED(pool.cs);
+RBFTransactionState IsRBFOptInEmptyMempool(const CTransaction& tx);
 
 #endif // BITCOIN_POLICY_RBF_H