invoices: refactor updateInvoiceAmpState to return new state

With this commit updateInvoiceAmpState becomes getUpdatedInvoiceAmpState
which will only return the new AMP state but that needs to be applied at
the call site. This is a part of a larger refactor to gather all
mutations of an invoice update to be later applied by the invoice
updater.

channeldb/invoices.go

@@ -558,6 +558,7 @@ func (d *DB) QueryInvoices(_ context.Context, q invpkg.InvoiceQuery) (
 			// At this point, we've exhausted the offset, so we'll
 			// begin collecting invoices found within the range.
 			resp.Invoices = append(resp.Invoices, invoice)
+
 			return true, nil
 		}
 
@@ -1777,13 +1778,15 @@ func updateHtlcsAmp(invoice *invpkg.Invoice,
 	htlc *invpkg.InvoiceHTLC, setID invpkg.SetID,
 	circuitKey models.CircuitKey) error {
 
-	err := updateInvoiceAmpState(
+	newAmpState, err := getUpdatedInvoiceAmpState(
 		invoice, setID, circuitKey, invpkg.HtlcStateAccepted, htlc.Amt,
 	)
 	if err != nil {
 		return err
 	}
 
+	invoice.AMPState[setID] = newAmpState
+
 	// Now that we've updated the invoice state, we'll inform the caller of
 	// the _neitre_ HTLC set they need to write for this new set ID.
 	if _, ok := updateMap[setID]; !ok {
@@ -1810,13 +1813,15 @@ func cancelHtlcsAmp(invoice *invpkg.Invoice,
 	setID := htlc.AMP.Record.SetID()
 
 	// First, we'll update the state of the entire HTLC set to cancelled.
-	err := updateInvoiceAmpState(
+	newAmpState, err := getUpdatedInvoiceAmpState(
 		invoice, setID, circuitKey, invpkg.HtlcStateCanceled, htlc.Amt,
 	)
 	if err != nil {
 		return err
 	}
 
+	invoice.AMPState[setID] = newAmpState
+
 	if _, ok := updateMap[setID]; !ok {
 		// Only HTLCs in the accepted state, can be cancelled, but we
 		// also want to merge that with HTLCs that may be canceled as
@@ -1861,13 +1866,15 @@ func settleHtlcsAmp(invoice *invpkg.Invoice,
 
 	// Next update the main AMP meta-data to indicate that this HTLC set
 	// has been fully settled.
-	err := updateInvoiceAmpState(
+	newAmpState, err := getUpdatedInvoiceAmpState(
 		invoice, setID, circuitKey, invpkg.HtlcStateSettled, 0,
 	)
 	if err != nil {
 		return err
 	}
 
+	invoice.AMPState[setID] = newAmpState
+
 	// Finally, we'll add this to the set of HTLCs that need to be updated.
 	if _, ok := updateMap[setID]; !ok {
 		mapEntry := make(map[models.CircuitKey]*invpkg.InvoiceHTLC)
@@ -2557,11 +2564,12 @@ func getUpdatedInvoiceState(invoice *invpkg.Invoice, hash *lntypes.Hash,
 	}
 }
 
-// updateInvoiceAmpState updates the AMP state of an invoice, given the new
-// state, and the amount of the HTLC that is being updated.
-func updateInvoiceAmpState(invoice *invpkg.Invoice, setID invpkg.SetID,
+// getUpdatedInvoiceAmpState returns the AMP state of an invoice (without
+// applying it), given the new state, and the amount of the HTLC that is
+// being updated.
+func getUpdatedInvoiceAmpState(invoice *invpkg.Invoice, setID invpkg.SetID,
 	circuitKey models.CircuitKey, state invpkg.HtlcState,
-	amt lnwire.MilliSatoshi) error {
+	amt lnwire.MilliSatoshi) (invpkg.InvoiceStateAMP, error) {
 
 	// Retrieve the AMP state for this set ID.
 	ampState, ok := invoice.AMPState[setID]
@@ -2570,8 +2578,9 @@ func updateInvoiceAmpState(invoice *invpkg.Invoice, setID invpkg.SetID,
 	// this set ID, otherwise we expect that the entry already exists and
 	// we can update it.
 	if !ok && state != invpkg.HtlcStateAccepted {
-		return fmt.Errorf("unable to update AMP state for setID=%x ",
-			setID)
+		return invpkg.InvoiceStateAMP{},
+			fmt.Errorf("unable to update AMP state for setID=%x ",
+				setID)
 	}
 
 	switch state {
@@ -2598,9 +2607,8 @@ func updateInvoiceAmpState(invoice *invpkg.Invoice, setID invpkg.SetID,
 	}
 
 	ampState.InvoiceKeys[circuitKey] = struct{}{}
-	invoice.AMPState[setID] = ampState
 
-	return nil
+	return ampState, nil
 }
 
 // canCancelSingleHtlc validates cancellation of a single HTLC. If nil is