mempool/txgraph: add GetConflicts for RBF conflict detection

Add GetConflicts method to detect which mempool transactions would be
replaced by a new transaction. Returns both individual conflicting
transactions and their packages to support package-based eviction.

Uses the spentBy index for O(1) conflict lookups per input.

mempool/txgraph/graph.go

@@ -556,6 +556,76 @@ func (g *TxGraph) GetOrphans(isConfirmed InputConfirmedPredicate,
 	return slices.Collect(g.IterateOrphans(isConfirmed))
 }
 
+// GetConflicts returns all transactions and packages that conflict with the
+// given transaction. A conflict occurs when the input transaction attempts to
+// spend an output that is already spent by a transaction in the mempool.
+//
+// The method uses the spentBy index for O(1) conflict detection per input.
+// For each conflicting transaction found, it includes all descendants since
+// they would become invalid if their ancestor is replaced. It also identifies
+// any packages that contain conflicting transactions.
+//
+// The returned ConflictSet provides both individual transactions (for
+// fine-grained analysis) and packages (for package-based eviction policies).
+//
+// Returns an empty ConflictSet if there are no conflicts.
+func (g *TxGraph) GetConflicts(tx *btcutil.Tx) *ConflictSet {
+	g.mu.RLock()
+	defer g.mu.RUnlock()
+
+	result := &ConflictSet{
+		Transactions: make(map[chainhash.Hash]*TxGraphNode),
+		Packages:     make(map[PackageID]*TxPackage),
+	}
+
+	// Check each input of the candidate transaction for conflicts with
+	// existing mempool transactions. The spentBy index maps each spent
+	// output to the transaction that spends it, enabling O(1) lookups.
+	for _, txIn := range tx.MsgTx().TxIn {
+		// If this outpoint is already spent by a mempool transaction,
+		// that transaction conflicts with our candidate.
+		conflictNode, exists := g.indexes.spentBy[txIn.PreviousOutPoint]
+		if !exists {
+			continue
+		}
+
+		// Add the directly conflicting transaction.
+		result.Transactions[conflictNode.TxHash] = conflictNode
+
+		// Add all descendants of the conflict. When we replace a
+		// transaction via RBF, all its descendants must also be
+		// removed because they spend outputs that will no longer
+		// exist.
+		descendants := g.GetDescendants(conflictNode.TxHash, -1)
+		for hash, node := range descendants {
+			result.Transactions[hash] = node
+		}
+	}
+
+	// Identify packages that contain any conflicting transactions. This
+	// enables package-based eviction where entire packages are treated as
+	// atomic units.
+	for hash := range result.Transactions {
+		pkgID, exists := g.indexes.nodeToPackage[hash]
+		if !exists {
+			continue
+		}
+
+		// Only add each package once even if multiple transactions
+		// from the same package are in the conflict set.
+		if _, alreadyAdded := result.Packages[pkgID]; alreadyAdded {
+			continue
+		}
+
+		pkg, exists := g.indexes.packages[pkgID]
+		if exists {
+			result.Packages[pkgID] = pkg
+		}
+	}
+
+	return result
+}
+
 // ValidatePackage validates a transaction package.
 func (g *TxGraph) ValidatePackage(pkg *TxPackage) error {
 	if pkg == nil {

mempool/txgraph/interfaces.go

@@ -372,6 +372,16 @@ type Graph interface {
 	// indicates mempool fragmentation and is useful for understanding the
 	// effectiveness of cluster-based optimizations.
 	GetClusterCount() int
+
+	// GetConflicts returns all transactions and packages that would be
+	// replaced if the given transaction were added to the mempool. A
+	// conflict occurs when a transaction input spends an output that is
+	// already spent by a transaction in the graph. The returned ConflictSet
+	// includes both directly conflicting transactions and all their
+	// descendants, since descendants become invalid when their ancestor is
+	// replaced. It also includes any packages that contain conflicting
+	// transactions, enabling package-based eviction policies.
+	GetConflicts(tx *btcutil.Tx) *ConflictSet
 }
 
 // TraversalOrder defines the traversal strategy for graph iteration.
@@ -485,6 +495,24 @@ func WithIncludeStart(include bool) IterOption {
 	}
 }
 
+// ConflictSet contains the result of a conflict check, providing both
+// individual conflicting transactions and their associated packages.
+type ConflictSet struct {
+	// Transactions contains all conflicting transactions and their
+	// descendants as a flat map. This enables direct iteration and
+	// individual transaction analysis.
+	Transactions map[chainhash.Hash]*TxGraphNode
+
+	// Packages contains all packages that include at least one conflicting
+	// transaction. This enables package-based eviction policies where entire
+	// packages are considered as atomic units.
+	//
+	// Note: Not all conflicting transactions are necessarily in packages.
+	// Standalone transactions will appear in Transactions but not in
+	// Packages.
+	Packages map[PackageID]*TxPackage
+}
+
 // InputConfirmedPredicate is a function that checks if a transaction input
 // references a confirmed UTXO. This is used to distinguish between:
 // - Orphans: transactions with unconfirmed inputs not in the mempool