invoices: treat replayed HTLCs beforehand.

We make sure that HTLCs which have already been decided upon
are resolved before before allowing the external interceptor to
potentially cancel them back. This makes the implementation for
the external HTLC interceptor more streamlined.

invoices/update.go

@@ -109,41 +109,52 @@ func (i invoiceUpdateCtx) acceptRes(
 	return newAcceptResolution(i.circuitKey, outcome)
 }
 
+// resolveReplayedHtlc returns the HTLC resolution for a replayed HTLC. The
+// returned boolean indicates whether the HTLC was replayed or not.
+func resolveReplayedHtlc(ctx *invoiceUpdateCtx, inv *Invoice) (bool,
+	HtlcResolution, error) {
+
+	// Don't update the invoice when this is a replayed htlc.
+	htlc, replayedHTLC := inv.Htlcs[ctx.circuitKey]
+	if !replayedHTLC {
+		return false, nil, nil
+	}
+
+	switch htlc.State {
+	case HtlcStateCanceled:
+		return true, ctx.failRes(ResultReplayToCanceled), nil
+
+	case HtlcStateAccepted:
+		return true, ctx.acceptRes(resultReplayToAccepted), nil
+
+	case HtlcStateSettled:
+		pre := inv.Terms.PaymentPreimage
+
+		// Terms.PaymentPreimage will be nil for AMP invoices.
+		// Set it to the HTLCs AMP Preimage instead.
+		if pre == nil {
+			pre = htlc.AMP.Preimage
+		}
+
+		return true, ctx.settleRes(
+			*pre,
+			ResultReplayToSettled,
+		), nil
+
+	default:
+		return true, nil, errors.New("unknown htlc state")
+	}
+}
+
 // updateInvoice is a callback for DB.UpdateInvoice that contains the invoice
 // settlement logic. It returns a HTLC resolution that indicates what the
 // outcome of the update was.
+//
+// NOTE: Make sure replayed HTLCs are always considered before calling this
+// function.
 func updateInvoice(ctx *invoiceUpdateCtx, inv *Invoice) (
 	*InvoiceUpdateDesc, HtlcResolution, error) {
 
-	// Don't update the invoice when this is a replayed htlc.
-	htlc, ok := inv.Htlcs[ctx.circuitKey]
-	if ok {
-		switch htlc.State {
-		case HtlcStateCanceled:
-			return nil, ctx.failRes(ResultReplayToCanceled), nil
-
-		case HtlcStateAccepted:
-			return nil, ctx.acceptRes(resultReplayToAccepted), nil
-
-		case HtlcStateSettled:
-			pre := inv.Terms.PaymentPreimage
-
-			// Terms.PaymentPreimage will be nil for AMP invoices.
-			// Set it to the HTLCs AMP Preimage instead.
-			if pre == nil {
-				pre = htlc.AMP.Preimage
-			}
-
-			return nil, ctx.settleRes(
-				*pre,
-				ResultReplayToSettled,
-			), nil
-
-		default:
-			return nil, nil, errors.New("unknown htlc state")
-		}
-	}
-
 	// If no MPP payload was provided, then we expect this to be a keysend,
 	// or a payment to an invoice created before we started to require the
 	// MPP payload.