[node] interface to get bump fees

Author: glozow

src/interfaces/chain.h

@@ -216,6 +216,43 @@ class Chain
     //! Calculate mempool ancestor and descendant counts for the given transaction.
     virtual void getTransactionAncestry(const uint256& txid, size_t& ancestors, size_t& descendants, size_t* ancestorsize = nullptr, CAmount* ancestorfees = nullptr) = 0;
 
+    //! For each outpoint, calculate the fee-bumping cost to spend this outpoint at the specified
+    //  feerate, including bumping its ancestors. For example, if the target feerate is 10sat/vbyte
+    //  and this outpoint refers to a mempool transaction at 3sat/vbyte, the bump fee includes the
+    //  cost to bump the mempool transaction to 10sat/vbyte (i.e. 7 * mempooltx.vsize). If that
+    //  transaction also has, say, an unconfirmed parent with a feerate of 1sat/vbyte, the bump fee
+    //  includes the cost to bump the parent (i.e. 9 * parentmempooltx.vsize).
+    //
+    //  If the outpoint comes from an unconfirmed transaction that is already above the target
+    //  feerate or bumped by its descendant(s) already, it does not need to be bumped. Its bump fee
+    //  is 0. Likewise, if any of the transaction's ancestors are already bumped by a transaction
+    //  in our mempool, they are not included in the transaction's bump fee.
+    //
+    //  Also supported is bump-fee calculation in the case of replacements. If an outpoint
+    //  conflicts with another transaction in the mempool, it is assumed that the goal is to replace
+    //  that transaction. As such, the calculation will exclude the to-be-replaced transaction, but
+    //  will include the fee-bumping cost. If bump fees of descendants of the to-be-replaced
+    //  transaction are requested, the value will be 0. Fee-related RBF rules are not included as
+    //  they are logically distinct.
+    //
+    //  Any outpoints that are otherwise unavailable from the mempool (e.g. UTXOs from confirmed
+    //  transactions or transactions not yet broadcast by the wallet) are given a bump fee of 0.
+    //
+    //  If multiple outpoints come from the same transaction (which would be very rare because
+    //  it means that one transaction has multiple change outputs or paid the same wallet using multiple
+    //  outputs in the same transaction) or have shared ancestry, the bump fees are calculated
+    //  independently, i.e. as if only one of them is spent. This may result in double-fee-bumping. This
+    //  caveat can be rectified per use of the sister-function CalculateCombinedBumpFee(…).
+    virtual std::map<COutPoint, CAmount> CalculateIndividualBumpFees(const std::vector<COutPoint>& outpoints, const CFeeRate& target_feerate) = 0;
+
+    //! Calculate the combined bump fee for an input set per the same strategy
+    //  as in CalculateIndividualBumpFees(…).
+    //  Unlike CalculateIndividualBumpFees(…), this does not return individual
+    //  bump fees per outpoint, but a single bump fee for the shared ancestry.
+    //  The combined bump fee may be used to correct overestimation due to
+    //  shared ancestry by multiple UTXOs after coin selection.
+    virtual std::optional<CAmount> CalculateCombinedBumpFee(const std::vector<COutPoint>& outpoints, const CFeeRate& target_feerate) = 0;
+
     //! Get the node's package limits.
     //! Currently only returns the ancestor and descendant count limits, but could be enhanced to
     //! return more policy settings.

src/node/interfaces.cpp

@@ -28,6 +28,7 @@
 #include <node/coin.h>
 #include <node/context.h>
 #include <node/interface_ui.h>
+#include <node/mini_miner.h>
 #include <node/transaction.h>
 #include <policy/feerate.h>
 #include <policy/fees.h>
@@ -665,6 +666,26 @@ class ChainImpl : public Chain
         if (!m_node.mempool) return;
         m_node.mempool->GetTransactionAncestry(txid, ancestors, descendants, ancestorsize, ancestorfees);
     }
+
+    std::map<COutPoint, CAmount> CalculateIndividualBumpFees(const std::vector<COutPoint>& outpoints, const CFeeRate& target_feerate) override
+    {
+        if (!m_node.mempool) {
+            std::map<COutPoint, CAmount> bump_fees;
+            for (const auto& outpoint : outpoints) {
+                bump_fees.emplace(std::make_pair(outpoint, 0));
+            }
+            return bump_fees;
+        }
+        return MiniMiner(*m_node.mempool, outpoints).CalculateBumpFees(target_feerate);
+    }
+
+    std::optional<CAmount> CalculateCombinedBumpFee(const std::vector<COutPoint>& outpoints, const CFeeRate& target_feerate) override
+    {
+        if (!m_node.mempool) {
+            return 0;
+        }
+        return MiniMiner(*m_node.mempool, outpoints).CalculateTotalBumpFees(target_feerate);
+    }
     void getPackageLimits(unsigned int& limit_ancestor_count, unsigned int& limit_descendant_count) override
     {
         const CTxMemPool::Limits default_limits{};