msglist: Follow /with/ links through message moves

Fixes: zulip#1028

lib/api/model/narrow.dart

@@ -14,20 +14,33 @@ typedef ApiNarrow = List<ApiNarrowElement>;
 /// reasonably be omitted will be omitted.
 ApiNarrow resolveApiNarrowForServer(ApiNarrow narrow, int zulipFeatureLevel) {
   final supportsOperatorDm = zulipFeatureLevel >= 177; // TODO(server-7)
+  final supportsOperatorWith = zulipFeatureLevel >= 271; // TODO(server-9)
 
   bool hasDmElement = false;
+  bool hasWithElement = false;
   for (final element in narrow) {
     switch (element) {
-      case ApiNarrowDm(): hasDmElement = true;
+      case ApiNarrowDm():   hasDmElement = true;
+      case ApiNarrowWith(): hasWithElement = true;
       default:
     }
   }
-  if (!hasDmElement) return narrow;
+  if (!hasDmElement && (!hasWithElement || supportsOperatorWith)) {
+    return narrow;
+  }
 
-  return narrow.map((element) => switch (element) {
-    ApiNarrowDm() => element.resolve(legacy: !supportsOperatorDm),
-    _             => element,
-  }).toList();
+  final result = <ApiNarrowElement>[];
+  for (final element in narrow) {
+    switch (element) {
+      case ApiNarrowDm():
+        result.add(element.resolve(legacy: !supportsOperatorDm));
+      case ApiNarrowWith() when !supportsOperatorWith:
+        break; // drop unsupported element
+      default:
+        result.add(element);
+    }
+  }
+  return result;
 }
 
 /// An element in the list representing a narrow in the Zulip API.
@@ -160,6 +173,22 @@ class ApiNarrowPmWith extends ApiNarrowDm {
   ApiNarrowPmWith._(super.operand, {super.negated});
 }
 
+/// An [ApiNarrowElement] with the 'with' operator.
+///
+/// If part of [ApiNarrow] use [resolveApiNarrowElements].
+class ApiNarrowWith extends ApiNarrowElement {
+  @override String get operator => 'with';
+
+  @override final int operand;
+
+  ApiNarrowWith(this.operand, {super.negated});
+
+  factory ApiNarrowWith.fromJson(Map<String, dynamic> json) => ApiNarrowWith(
+    json['operand'] as int,
+    negated: json['negated'] as bool? ?? false,
+  );
+}
+
 class ApiNarrowIs extends ApiNarrowElement {
   @override String get operator => 'is';
 

lib/model/internal_link.dart

@@ -78,6 +78,8 @@ Uri narrowLink(PerAccountStore store, Narrow narrow, {int? nearMessageId}) {
         fragment.write('$streamId-$slugifiedName');
       case ApiNarrowTopic():
         fragment.write(_encodeHashComponent(element.operand.apiName));
+      case ApiNarrowWith():
+        fragment.write(element.operand.toString());
       case ApiNarrowDmModern():
         final suffix = element.operand.length >= 3 ? 'group' : 'dm';
         fragment.write('${element.operand.join(',')}-$suffix');
@@ -160,6 +162,7 @@ Narrow? _interpretNarrowSegments(List<String> segments, PerAccountStore store) {
   ApiNarrowStream? streamElement;
   ApiNarrowTopic? topicElement;
   ApiNarrowDm? dmElement;
+  ApiNarrowWith? withElement;
   Set<IsOperand> isElementOperands = {};
 
   for (var i = 0; i < segments.length; i += 2) {
@@ -188,12 +191,17 @@ Narrow? _interpretNarrowSegments(List<String> segments, PerAccountStore store) {
         if (dmIds == null) return null;
         dmElement = ApiNarrowDm(dmIds, negated: negated);
 
+      case _NarrowOperator.with_:
+        if (withElement != null) return null;
+        final messageId = int.tryParse(operand, radix: 10);
+        if (messageId == null) return null;
+        withElement = ApiNarrowWith(messageId);
+
       case _NarrowOperator.is_:
         // It is fine to have duplicates of the same [IsOperand].
         isElementOperands.add(IsOperand.fromRawString(operand));
 
       case _NarrowOperator.near: // TODO(#82): support for near
-      case _NarrowOperator.with_: // TODO(#683): support for with
         continue;
 
       case _NarrowOperator.unknown:
@@ -202,7 +210,9 @@ Narrow? _interpretNarrowSegments(List<String> segments, PerAccountStore store) {
   }
 
   if (isElementOperands.isNotEmpty) {
-    if (streamElement != null || topicElement != null || dmElement != null) return null;
+    if (streamElement != null || topicElement != null || dmElement != null || withElement != null) {
+      return null;
+    }
     if (isElementOperands.length > 1) return null;
     switch (isElementOperands.single) {
       case IsOperand.mentioned:
@@ -219,13 +229,14 @@ Narrow? _interpretNarrowSegments(List<String> segments, PerAccountStore store) {
         return null;
     }
   } else if (dmElement != null) {
-    if (streamElement != null || topicElement != null) return null;
+    if (streamElement != null || topicElement != null || withElement != null) return null;
     return DmNarrow.withUsers(dmElement.operand, selfUserId: store.selfUserId);
   } else if (streamElement != null) {
     final streamId = streamElement.operand;
     if (topicElement != null) {
-      return TopicNarrow(streamId, topicElement.operand);
+      return TopicNarrow(streamId, topicElement.operand, with_: withElement?.operand);
     } else {
+      if (withElement != null) return null;
       return ChannelNarrow(streamId);
     }
   }

lib/model/message_list.dart

@@ -511,6 +511,7 @@ class MessageListView with ChangeNotifier, _MessageSequence {
       numAfter: 0,
     );
     if (this.generation > generation) return;
+    _adjustNarrowForTopicPermalink(result.messages.firstOrNull);
     store.reconcileMessages(result.messages);
     store.recentSenders.handleMessages(result.messages); // TODO(#824)
     for (final message in result.messages) {
@@ -524,12 +525,47 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     notifyListeners();
   }
 
+  /// Update [narrow] for the result of a "with" narrow (topic permalink) fetch.
+  ///
+  /// To avoid an extra round trip, the server handles [ApiNarrowWith]
+  /// by returning results from the indicated message's current stream/topic
+  /// (if the user has access),
+  /// even if that differs from the narrow's stream/topic filters
+  /// because the message was moved.
+  ///
+  /// If such a "redirect" happened, this helper updates the stream and topic
+  /// in [narrow] to match the message's current conversation.
+  /// It also removes the "with" component from [narrow]
+  /// whether or not a redirect happened.
+  ///
+  /// See API doc:
+  ///   https://zulip.com/api/construct-narrow#message-ids
+  void _adjustNarrowForTopicPermalink(Message? someFetchedMessageOrNull) {
+    final narrow = this.narrow;
+    if (narrow is! TopicNarrow || narrow.with_ == null) return;
+
+    switch (someFetchedMessageOrNull) {
+      case null:
+        // This can't be a redirect; a redirect can't produce an empty result.
+        // (The server only redirects if the message is accessible to the user,
+        // and if it is, it'll appear in the result, making it non-empty.)
+        this.narrow = narrow.sansWith();
+      case StreamMessage():
+        this.narrow = TopicNarrow.ofMessage(someFetchedMessageOrNull);
+      case DmMessage(): // TODO(log)
+        assert(false);
+    }
+  }
+
   /// Fetch the next batch of older messages, if applicable.
   Future<void> fetchOlder() async {
     if (haveOldest) return;
     if (fetchingOlder) return;
     if (fetchOlderCoolingDown) return;
     assert(fetched);
+    assert(narrow is! TopicNarrow
+      // We only intend to send "with" in [fetchInitial]; see there.
+      || (narrow as TopicNarrow).with_ == null);
     assert(messages.isNotEmpty);
     _fetchingOlder = true;
     _updateEndMarkers();

lib/model/narrow.dart

@@ -92,14 +92,17 @@ class ChannelNarrow extends Narrow {
 }
 
 class TopicNarrow extends Narrow implements SendableNarrow {
-  const TopicNarrow(this.streamId, this.topic);
+  const TopicNarrow(this.streamId, this.topic, {this.with_});
 
   factory TopicNarrow.ofMessage(StreamMessage message) {
     return TopicNarrow(message.streamId, message.topic);
   }
 
   final int streamId;
   final TopicName topic;
+  final int? with_;
+
+  TopicNarrow sansWith() => TopicNarrow(streamId, topic);
 
   @override
   bool containsMessage(Message message) {
@@ -108,22 +111,33 @@ class TopicNarrow extends Narrow implements SendableNarrow {
   }
 
   @override
-  ApiNarrow apiEncode() => [ApiNarrowStream(streamId), ApiNarrowTopic(topic)];
+  ApiNarrow apiEncode() => [
+    ApiNarrowStream(streamId),
+    ApiNarrowTopic(topic),
+    if (with_ != null) ApiNarrowWith(with_!),
+  ];
 
   @override
   StreamDestination get destination => StreamDestination(streamId, topic);
 
   @override
-  String toString() => 'TopicNarrow($streamId, ${topic.displayName})';
+  String toString() {
+    final fields = [
+      streamId.toString(),
+      topic.displayName,
+      if (with_ != null) 'with: ${with_!}',
+    ];
+    return 'TopicNarrow(${fields.join(', ')})';
+  }
 
   @override
   bool operator ==(Object other) {
     if (other is! TopicNarrow) return false;
-    return other.streamId == streamId && other.topic == topic;
+    return other.streamId == streamId && other.topic == topic && other.with_ == with_;
   }
 
   @override
-  int get hashCode => Object.hash('TopicNarrow', streamId, topic);
+  int get hashCode => Object.hash('TopicNarrow', streamId, topic, with_);
 }
 
 /// The narrow for a direct-message conversation.

lib/widgets/message_list.dart

@@ -503,8 +503,11 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
 
   void _modelChanged() {
     if (model!.narrow != widget.narrow) {
-      // A message move event occurred, where propagate mode is
-      // [PropagateMode.changeAll] or [PropagateMode.changeLater].
+      // Either:
+      // - A message move event occurred, where propagate mode is
+      //   [PropagateMode.changeAll] or [PropagateMode.changeLater]. Or:
+      // - We fetched a "with" / topic-permalink narrow, and the response
+      //   redirected us to the new location of the operand message ID.
       widget.onNarrowChanged(model!.narrow);
     }
     setState(() {

test/api/model/narrow_test.dart

@@ -0,0 +1,69 @@
+import 'package:checks/checks.dart';
+import 'package:test/scaffolding.dart';
+import 'package:zulip/api/model/events.dart';
+import 'package:zulip/api/model/initial_snapshot.dart';
+import 'package:zulip/api/model/model.dart';
+import 'package:zulip/api/model/narrow.dart';
+
+import '../../example_data.dart' as eg;
+import '../../stdlib_checks.dart';
+import 'events_checks.dart';
+import 'model_checks.dart';
+
+void main() {
+  group('resolveApiNarrowForServer', () {
+    void doTest(
+      String description,
+      ApiNarrow input,
+      ApiNarrow expected, {
+      int zulipFeatureLevel = eg.recentZulipFeatureLevel
+    }) {
+      test('$description, FL $zulipFeatureLevel', () {
+        final actual = resolveApiNarrowForServer(input, zulipFeatureLevel);
+        check(actual).jsonEquals(expected);
+      });
+    }
+
+    final stream = ApiNarrowStream(eg.stream().streamId);
+    final topic = ApiNarrowTopic(eg.t('topic'));
+    final dm = ApiNarrowDm([eg.user().userId]);
+    final with_ = ApiNarrowWith(eg.streamMessage().id);
+
+    group('recent FL', () {
+      doTest('dm (modern)', [dm], [dm.resolve(legacy: false)]);
+      doTest('topic, not permalink', [stream, topic], [stream, topic]);
+      doTest('topic permalink (modern)', [stream, topic, with_], [stream, topic, with_]);
+
+      // Unlikely to occur in the wild but should still be handled correctly
+      doTest('dm + with', [dm, with_], [dm.resolve(legacy: false), with_]);
+    });
+
+    // TODO(server-7)
+    group('FL 176', () {
+      final zulipFeatureLevel = 176;
+      doTest('dm (legacy)', zulipFeatureLevel: zulipFeatureLevel,
+        [dm], [dm.resolve(legacy: true)]);
+      doTest('topic, not permalink', [stream, topic], [stream, topic]);
+      doTest('topic permalink (legacy)', zulipFeatureLevel: zulipFeatureLevel,
+        [stream, topic, with_], [stream, topic]);
+
+      // Unlikely to occur in the wild but should still be handled correctly
+      doTest('dm + with', zulipFeatureLevel: zulipFeatureLevel,
+        [dm, with_], [dm.resolve(legacy: true)]);
+    });
+
+    // TODO(server-9)
+    group('FL 270', () {
+      final zulipFeatureLevel = 270;
+      doTest('topic permalink (legacy)', zulipFeatureLevel: zulipFeatureLevel,
+        [stream, topic, with_], [stream, topic]);
+      doTest('topic, not permalink', [stream, topic], [stream, topic]);
+      doTest('dm (modern)', zulipFeatureLevel: zulipFeatureLevel,
+        [dm], [dm.resolve(legacy: false)]);
+
+      // Unlikely to occur in the wild but should still be handled correctly
+      doTest('dm + with', zulipFeatureLevel: zulipFeatureLevel,
+        [dm, with_], [dm.resolve(legacy: false)]);
+    });
+  });
+}

test/api/route/messages_test.dart

@@ -188,6 +188,11 @@ void main() {
         {'operator': 'stream', 'operand': 12},
         {'operator': 'topic', 'operand': 'stuff'},
       ]));
+      checkNarrow(eg.topicNarrow(12, 'stuff', with_: 1).apiEncode(), jsonEncode([
+        {'operator': 'stream', 'operand': 12},
+        {'operator': 'topic', 'operand': 'stuff'},
+        {'operator': 'with', 'operand': 1},
+      ]));
       checkNarrow(const MentionsNarrow().apiEncode(), jsonEncode([
         {'operator': 'is', 'operand': 'mentioned'},
       ]));
@@ -203,6 +208,11 @@ void main() {
       checkNarrow([ApiNarrowDm([123, 234])], jsonEncode([
         {'operator': 'pm-with', 'operand': [123, 234]},
       ]));
+      connection.zulipFeatureLevel = 270;
+      checkNarrow(eg.topicNarrow(12, 'stuff', with_: 1).apiEncode(), jsonEncode([
+        {'operator': 'stream', 'operand': 12},
+        {'operator': 'topic', 'operand': 'stuff'},
+      ]));
       connection.zulipFeatureLevel = eg.futureZulipFeatureLevel;
     });
   });

test/example_data.dart

@@ -289,8 +289,8 @@ Subscription subscription(
 /// Useful in test code that mentions a lot of topics in a compact format.
 TopicName t(String apiName) => TopicName(apiName);
 
-TopicNarrow topicNarrow(int channelId, String topicName) {
-  return TopicNarrow(channelId, TopicName(topicName));
+TopicNarrow topicNarrow(int channelId, String topicName, {int? with_}) {
+  return TopicNarrow(channelId, TopicName(topicName), with_: with_);
 }
 
 UserTopicItem userTopicItem(