channeldb/invoices: add IsAMP and IsKeysend helpers

The only way to know if an invoice is AMP, Keysend, etc is to look at
its shape/characteristics. This commit adds a couple of helper functions
to encapsulate the logic of these checks.

If all these types cannot intersect: an invoice cannot be AMP and
Keysend or Keysend and Bolt12, etc it could be useful to add an extra
field to store this information instead of relying on checking how the
invoice looks like.

channeldb/invoices.go

@@ -907,10 +907,7 @@ func serializeInvoice(w io.Writer, i *invpkg.Invoice) error {
 
 	// Only if this is a _non_ AMP invoice do we serialize the HTLCs
 	// in-line with the rest of the invoice.
-	ampInvoice := i.Terms.Features.HasFeature(
-		lnwire.AMPOptional,
-	)
-	if ampInvoice {
+	if i.IsAMP() {
 		return nil
 	}
 
@@ -1148,40 +1145,51 @@ func fetchInvoice(invoiceNum []byte, invoices kvdb.RBucket,
 		return invpkg.Invoice{}, err
 	}
 
-	// If this is an AMP invoice, then we'll also attempt to read out the
-	// set of HTLCs that were paid to prior set IDs. However, we'll only do
-	// this is the invoice didn't already have HTLCs stored in-line.
-	invoiceIsAMP := invoice.Terms.Features.HasFeature(
-		lnwire.AMPOptional,
-	)
-	switch {
-	case !invoiceIsAMP:
-		return invoice, nil
-
-	// For AMP invoice that already have HTLCs populated (created before
-	// recurring invoices), then we don't need to read from the prefix
-	// keyed section of the bucket.
-	case invoiceIsAMP && len(invoice.Htlcs) != 0:
-		return invoice, nil
-
-	// If the "zero" setID was specified, then this means that no HTLC data
-	// should be returned alongside of it.
-	case invoiceIsAMP && len(setIDs) != 0 && setIDs[0] != nil &&
-		*setIDs[0] == invpkg.BlankPayAddr:
-
+	// If this is an AMP invoice we'll also attempt to read out the set of
+	// HTLCs that were paid to prior set IDs, if needed.
+	if !invoice.IsAMP() {
 		return invoice, nil
 	}
 
-	invoice.Htlcs, err = fetchAmpSubInvoices(
-		invoices, invoiceNum, setIDs...,
-	)
-	if err != nil {
-		return invoice, nil
+	if shouldFetchAMPHTLCs(invoice, setIDs) {
+		invoice.Htlcs, err = fetchAmpSubInvoices(
+			invoices, invoiceNum, setIDs...,
+		)
+		// TODO(positiveblue): we should fail when we are not able to
+		// fetch all the HTLCs for an AMP invoice. Multiple tests in
+		// the invoice and channeldb package break if we return this
+		// error. We need to update them when we migrate this logic to
+		// the sql implementation.
+		if err != nil {
+			log.Errorf("unable to fetch amp htlcs for inv "+
+				"%v and setIDs %v: %w", invoiceNum, setIDs, err)
+		}
 	}
 
 	return invoice, nil
 }
 
+// shouldFetchAMPHTLCs returns true if we need to fetch the set of HTLCs that
+// were paid to the relevant set IDs.
+func shouldFetchAMPHTLCs(invoice invpkg.Invoice, setIDs []*invpkg.SetID) bool {
+	// For AMP invoice that already have HTLCs populated (created before
+	// recurring invoices), then we don't need to read from the prefix
+	// keyed section of the bucket.
+	if len(invoice.Htlcs) != 0 {
+		return false
+	}
+
+	// If the "zero" setID was specified, then this means that no HTLC data
+	// should be returned alongside of it.
+	if len(setIDs) != 0 && setIDs[0] != nil &&
+		*setIDs[0] == invpkg.BlankPayAddr {
+
+		return false
+	}
+
+	return true
+}
+
 // fetchInvoiceStateAMP retrieves the state of all the relevant sub-invoice for
 // an AMP invoice. This methods only decode the relevant state vs the entire
 // invoice.
@@ -1905,9 +1913,7 @@ func (d *DB) updateInvoice(hash *lntypes.Hash, refSetID *invpkg.SetID, invoices,
 
 	now := d.clock.Now()
 
-	invoiceIsAMP := invoiceCopy.Terms.Features.HasFeature(
-		lnwire.AMPOptional,
-	)
+	invoiceIsAMP := invoiceCopy.IsAMP()
 
 	// Process add actions from update descriptor.
 	htlcsAmpUpdate := make(map[invpkg.SetID]map[models.CircuitKey]*invpkg.InvoiceHTLC) //nolint:lll