channeldb: don't cancel other HTLCs w/ diff setID once once is settled

With this change, we allow multiple users to concurrently pay the same
AMP invoice with distinct set IDs.

channeldb/invoice_test.go

@@ -1669,7 +1669,9 @@ func testUpdateHTLCPreimages(t *testing.T, test updateHTLCPreimageTestCase) {
 	// Update the invoice with an accepted HTLC that also accepts the
 	// invoice.
 	ref := InvoiceRefByAddr(invoice.Terms.PaymentAddr)
-	dbInvoice, err := db.UpdateInvoice(ref, updateAcceptAMPHtlc(0, amt, setID, true))
+	dbInvoice, err := db.UpdateInvoice(
+		ref, updateAcceptAMPHtlc(0, amt, setID, true),
+	)
 	require.Nil(t, err)
 
 	htlcPreimages := make(map[CircuitKey]lntypes.Preimage)
@@ -2062,6 +2064,10 @@ func TestUpdateHTLC(t *testing.T) {
 			expErr: nil,
 		},
 		{
+			// With the newer AMP logic, this is now valid, as we
+			// want to be able to accept multiple settle attempts
+			// to a given pay_addr. In this case, the HTLC should
+			// remain in the accepted state.
 			name: "AMP settle valid preimage different htlc set",
 			input: InvoiceHTLC{
 				Amt:           5000,
@@ -2085,9 +2091,9 @@ func TestUpdateHTLC(t *testing.T) {
 				MppTotalAmt:   5000,
 				AcceptHeight:  100,
 				AcceptTime:    testNow,
-				ResolveTime:   testNow,
+				ResolveTime:   time.Time{},
 				Expiry:        40,
-				State:         HtlcStateCanceled,
+				State:         HtlcStateAccepted,
 				CustomRecords: make(record.CustomSet),
 				AMP: &InvoiceHtlcAMPData{
 					Record:   *ampRecord,

channeldb/invoices.go

@@ -2710,13 +2710,12 @@ func updateHtlc(resolveTime time.Time, htlc *InvoiceHTLC,
 			}
 
 			htlcState = HtlcStateSettled
-		} else {
-			htlcState = HtlcStateCanceled
 		}
 
 		// Only persist the changes if the invoice is moving to the
-		// settled state.
-		if persist {
+		// settled state, and we're actually updating the state to
+		// settled.
+		if persist && htlcState == HtlcStateSettled {
 			htlc.State = htlcState
 			htlc.ResolveTime = resolveTime
 		}