Merge pull request #2700 from SatoshiPortal/fix/urqr-fountain-seqnum

fix(qr): accept fountain parts with seqnum above sequence count

Author: ethicnology

lib/core/urqr/urqr.dart

@@ -138,8 +138,12 @@ class UrQrReader {
 
     final sequenceNumber = int.parse(match.group(1)!);
     final sequenceCount = int.parse(match.group(2)!);
+    // Note: sequenceNumber may legitimately exceed sequenceCount. Animated
+    // URs are fountain-coded (BCR-2020-005): once the pure fragments 1..N
+    // have played, the stream keeps emitting mixed parts N+1, N+2, ... so a
+    // decoder that missed frames can still recover. Rejecting those parts
+    // breaks scanning of any stream the camera joins mid-animation.
     if (sequenceNumber < 1 ||
-        sequenceNumber > sequenceCount ||
         sequenceCount > maxMultipartParts ||
         data.length > maxMultipartMessageSize) {
       throw UrSequenceLimitExceeded();

test/core_test/urqr/ur_qr_reader_test.dart

@@ -0,0 +1,34 @@
+import 'dart:convert';
+import 'dart:typed_data';
+
+import 'package:bb_mobile/core/urqr/urqr.dart';
+import 'package:cbor/cbor.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:ur/ur.dart';
+import 'package:ur/ur_encoder.dart';
+
+void main() {
+  group('UrQrReader', () {
+    // Animated URs are fountain-coded (BCR-2020-005): once the pure
+    // fragments 1..N have played, the stream keeps emitting mixed parts
+    // N+1, N+2, ... The reader must accept them, or any scan that joins
+    // the animation mid-stream fails immediately.
+    test('decodes a stream the camera joins mid-animation', () {
+      final payload = cbor.encode(CborBytes(utf8.encode('x' * 100)));
+      final encoder = UREncoder(UR('bytes', Uint8List.fromList(payload)), 20);
+      final pureParts = <String>[];
+      while (!encoder.isComplete) {
+        pureParts.add(encoder.nextPart());
+      }
+      final mixedPart = encoder.nextPart(); // seqNum N+1 of N
+
+      final reader = UrQrReader();
+      reader.receive(mixedPart); // first captured frame is a mixed part
+      for (final part in pureParts) {
+        reader.receive(part);
+      }
+
+      expect(reader.isComplete, isTrue);
+    });
+  });
+}