policy: bip125-replaceable status can't be determined w/o mempool

The number of original transactions to be replaced and their
descendant transactions must not exceed \`MAX_BIP125_REPLACEMENT_CANDIDATES\`.
This is according to BIP125 RBF (Rule bitcoin#5). Without an entry in the mempool
(whether it's because we don't have a local mempool or we don't have \`tx\` in
our mempool) this check can't be done, and therefore the bip125-replaceable
status will be unknown regardless of how \`tx\` itself signals.

src/node/interfaces.cpp

@@ -570,7 +570,7 @@ class ChainImpl : public Chain
     }
     RBFTransactionState isRBFOptIn(const CTransaction& tx) override
     {
-        if (!m_node.mempool) return IsRBFOptInEmptyMempool(tx);
+        if (!m_node.mempool) return RBFTransactionState::UNKNOWN;
         LOCK(m_node.mempool->cs);
         return IsRBFOptIn(tx, *m_node.mempool);
     }

src/policy/rbf.cpp

@@ -15,17 +15,17 @@ RBFTransactionState IsRBFOptIn(const CTransaction& tx, const CTxMemPool& pool)
 
     CTxMemPool::setEntries ancestors;
 
-    // First check the transaction itself.
-    if (SignalsOptInRBF(tx)) {
-        return RBFTransactionState::REPLACEABLE_BIP125;
-    }
-
     // If this transaction is not in our mempool, then we can't be sure
     // we will know about all its inputs.
     if (!pool.exists(GenTxid::Txid(tx.GetHash()))) {
         return RBFTransactionState::UNKNOWN;
     }
 
+    // First check the transaction itself.
+    if (SignalsOptInRBF(tx)) {
+        return RBFTransactionState::REPLACEABLE_BIP125;
+    }
+
     // If all the inputs have nSequence >= maxint-1, it still might be
     // signaled for RBF if any unconfirmed parents have signaled.
     uint64_t noLimit = std::numeric_limits<uint64_t>::max();
@@ -41,12 +41,6 @@ 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;
-}
-
 std::optional<std::string> GetEntriesForConflicts(const CTransaction& tx,
                                                   CTxMemPool& pool,
                                                   const CTxMemPool::setEntries& iters_conflicting,

src/policy/rbf.h

@@ -38,7 +38,6 @@ enum class RBFTransactionState {
  * @return     The rbf state
  */
 RBFTransactionState IsRBFOptIn(const CTransaction& tx, const CTxMemPool& pool) EXCLUSIVE_LOCKS_REQUIRED(pool.cs);
-RBFTransactionState IsRBFOptInEmptyMempool(const CTransaction& tx);
 
 /** Get all descendants of iters_conflicting. Also enforce BIP125 Rule #5, "The number of original
  * transactions to be replaced and their descendant transactions which will be evicted from the

test/functional/wallet_listtransactions.py

@@ -198,7 +198,7 @@ def get_unconfirmed_utxo_entry(node, txid_to_match):
         for n in self.nodes[0:2]:
             assert_equal(n.gettransaction(txid_1)["bip125-replaceable"], "no")
             assert_equal(n.gettransaction(txid_2)["bip125-replaceable"], "no")
-            assert_equal(n.gettransaction(txid_3)["bip125-replaceable"], "yes")
+            assert_equal(n.gettransaction(txid_3)["bip125-replaceable"], "unknown")
             assert_equal(n.gettransaction(txid_3b)["bip125-replaceable"], "yes")
             assert_equal(n.gettransaction(txid_4)["bip125-replaceable"], "unknown")
 
@@ -207,7 +207,7 @@ def get_unconfirmed_utxo_entry(node, txid_to_match):
             txs = {tx['txid']: tx['bip125-replaceable'] for tx in n.listsinceblock()['transactions']}
             assert_equal(txs[txid_1], "no")
             assert_equal(txs[txid_2], "no")
-            assert_equal(txs[txid_3], "yes")
+            assert_equal(txs[txid_3], "unknown")
             assert_equal(txs[txid_3b], "yes")
             assert_equal(txs[txid_4], "unknown")