Fix accelerator revenue impact always showing zero

The old approach compared coinbasevalue from two getblocktemplate
calls, but new mempool arrivals between calls masked the displacement
loss. Now uses a single template snapshot to find the marginal
(lowest fee-rate) transaction that would be displaced and reports
its fee as the revenue cost.

api/internal/accelerator/accelerator.go

@@ -43,6 +43,54 @@ type AccelerateResult struct {
 	FeeLostError    string  `json:"fee_lost_error,omitempty"`
 }
 
+// marginalFeeLost estimates the fee revenue lost by inserting a boosted
+// transaction into the template. It finds the lowest fee-rate transaction
+// in the current template (the one that would be displaced) and returns
+// the difference: displaced_fee - boosted_tx_real_fee. If the boosted tx
+// is already in the template or the template has room, returns 0.
+func marginalFeeLost(tpl *bitcoind.BlockTemplate, txid string, txVsize int64) int64 {
+	if len(tpl.Transactions) == 0 {
+		return 0
+	}
+
+	// Check if the tx is already in the template (nothing displaced).
+	for _, tx := range tpl.Transactions {
+		if tx.Txid == txid {
+			return 0
+		}
+	}
+
+	// Find the marginal transaction: lowest fee-rate in the template.
+	var marginal *bitcoind.TemplateTx
+	var marginalRate float64 = math.MaxFloat64
+	for i := range tpl.Transactions {
+		tx := &tpl.Transactions[i]
+		// Weight to vsize: ceil(weight/4)
+		vsize := (tx.Weight + 3) / 4
+		if vsize <= 0 {
+			continue
+		}
+		rate := float64(tx.Fee) / float64(vsize)
+		if rate < marginalRate {
+			marginalRate = rate
+			marginal = tx
+		}
+	}
+
+	if marginal == nil {
+		return 0
+	}
+
+	// The displaced tx's fee is the revenue we lose. We don't add the
+	// boosted tx's real fee back because the pool never had it — the tx
+	// wasn't in the template before the boost.
+	lost := marginal.Fee
+	if lost < 0 {
+		lost = 0
+	}
+	return lost
+}
+
 // Accelerate boosts a transaction to the target feerate (sat/vB).
 func (s *Service) Accelerate(ctx context.Context, txid string, targetFeerateVB float64) (*AccelerateResult, error) {
 	if targetFeerateVB > MaxFeerateVB {
@@ -68,38 +116,26 @@ func (s *Service) Accelerate(ctx context.Context, txid string, targetFeerateVB f
 			targetFeerateVB, modifiedFeeSats/float64(entry.Vsize))
 	}
 
-	// Snapshot coinbasevalue BEFORE the boost to measure actual impact.
-	var coinbaseBefore int64
+	// Estimate revenue impact from the pre-boost template. The boosted tx
+	// will displace the marginal (lowest fee-rate) transaction in the
+	// template. We compute this from the single template snapshot to avoid
+	// a race with new mempool arrivals between two getblocktemplate calls.
+	var feeLost int64
 	var tplErr string
-	tplBefore, err := s.RPC.GetBlockTemplate(ctx)
+	tpl, err := s.RPC.GetBlockTemplate(ctx)
 	if err != nil {
-		tplErr = fmt.Sprintf("getblocktemplate (before): %s", bitcoind.RPCErrorMessage(err))
-		s.Log.Warn("accelerate: getblocktemplate failed (before)", "err", err)
+		tplErr = fmt.Sprintf("getblocktemplate: %s", bitcoind.RPCErrorMessage(err))
+		s.Log.Warn("accelerate: getblocktemplate failed", "err", err)
 	} else {
-		coinbaseBefore = tplBefore.CoinbaseValue
+		feeLost = marginalFeeLost(tpl, txid, entry.Vsize)
+		s.Log.Info("accelerate: fee impact estimated",
+			"marginal_fee_lost", feeLost, "boosted_tx_fee", int64(baseFeeSats))
 	}
 
 	if err := s.RPC.PrioritiseTransaction(ctx, txid, delta); err != nil {
 		return nil, fmt.Errorf("prioritisetransaction: %w", err)
 	}
 
-	// Measure coinbasevalue AFTER — the difference is the real fee lost.
-	var feeLost int64
-	if coinbaseBefore > 0 {
-		tplAfter, err := s.RPC.GetBlockTemplate(ctx)
-		if err != nil {
-			tplErr = fmt.Sprintf("getblocktemplate (after): %s", bitcoind.RPCErrorMessage(err))
-			s.Log.Warn("accelerate: getblocktemplate failed (after)", "err", err)
-		} else {
-			feeLost = coinbaseBefore - tplAfter.CoinbaseValue
-			if feeLost < 0 {
-				feeLost = 0 // template improved (new tx arrived between calls)
-			}
-			s.Log.Info("accelerate: fee impact measured",
-				"before", coinbaseBefore, "after", tplAfter.CoinbaseValue, "lost", feeLost)
-		}
-	}
-
 	rec := store.BoostedTx{
 		Txid:            txid,
 		OriginalFeerate: currentFeerate,