Add forking related comments

pkg/protocol/engines.go

@@ -97,7 +97,10 @@ func (e *Engines) ForkAtSlot(slot iotago.SlotIndex) (*engine.Engine, error) {
 	if err = newStorage.Commitments().Rollback(slot, latestCommitment.Slot()); err != nil {
 		return nil, ierrors.Wrap(err, "failed to rollback commitments")
 	}
-	// create temporary components and rollback their permanent state, which will be reflected on disk.
+
+	// some components are automatically rolled back by deleting their data on disk (e.g. slot based storage).
+	// some other components need to be rolled back manually, like the UTXO ledger for example.
+	// we need to create temporary components to rollback their permanent state, which will be reflected on disk.
 	evictionState := eviction.NewState(newStorage.Settings(), newStorage.RootBlocks)
 	evictionState.Initialize(latestCommitment.Slot())
 

pkg/retainer/txretainer/tx_retainer.go

@@ -249,6 +249,12 @@ func NewProvider(opts ...options.Option[TransactionRetainer]) module.Provider[*e
 
 // Reset resets the component to a clean state as if it was created at the last commitment.
 func (r *TransactionRetainer) Reset(targetSlot iotago.SlotIndex) {
+	// In the TransactionRetainer, we rely on the fact that "Reset" is always called
+	// when a new engine is initialized (even during chain switching).
+	// This is the cleanest way to rollback the state on disc to the
+	// last committed slot without the need to initialize temporary
+	// components in the "ForkAtSlot" method.
+	//
 	// we need to rollback the transaction retainer to the target slot
 	// to delete all transactions that were committed after the target slot.
 	if err := r.txRetainerDatabase.Rollback(targetSlot); err != nil {