msglist: Throttle fetchOlder retries

This approach is different from how a BackoffMachine is typically used,
because the message list doesn't send and retry requests in a loop; its
caller retries rapidly on scroll changes, and we want to ignore the
excessive requests.

The test drops irrelevant requests with `connection.takeRequests`
without checking, as we are only interested in verifying that no request
was sent.

Fixes: #945

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/model/message_list.dart

@@ -1,6 +1,9 @@
+import 'dart:async';
+
 import 'package:collection/collection.dart';
 import 'package:flutter/foundation.dart';
 
+import '../api/backoff.dart';
 import '../api/model/events.dart';
 import '../api/model/model.dart';
 import '../api/route/messages.dart';
@@ -89,9 +92,32 @@ mixin _MessageSequence {
   bool _haveOldest = false;
 
   /// Whether we are currently fetching the next batch of older messages.
+  ///
+  /// When this is true, [fetchOlder] is a no-op.
+  /// That method is called frequently by Flutter's scrolling logic,
+  /// and this field helps us avoid spamming the same request just to get
+  /// the same response each time.
+  ///
+  /// See also [fetchOlderCoolingDown].
   bool get fetchingOlder => _fetchingOlder;
   bool _fetchingOlder = false;
 
+  /// Whether [fetchOlder] had a request error recently.
+  ///
+  /// When this is true, [fetchOlder] is a no-op.
+  /// That method is called frequently by Flutter's scrolling logic,
+  /// and this field mitigates spamming the same request and getting
+  /// the same error each time.
+  ///
+  /// "Recently" is decided by a [BackoffMachine] that resets
+  /// when a [fetchOlder] request succeeds.
+  ///
+  /// See also [fetchingOlder].
+  bool get fetchOlderCoolingDown => _fetchOlderCoolingDown;
+  bool _fetchOlderCoolingDown = false;
+
+  BackoffMachine? _fetchOlderCooldownBackoffMachine;
+
   /// The parsed message contents, as a list parallel to [messages].
   ///
   /// The i'th element is the result of parsing the i'th element of [messages].
@@ -107,7 +133,7 @@ mixin _MessageSequence {
   /// before, between, or after the messages.
   ///
   /// This information is completely derived from [messages] and
-  /// the flags [haveOldest] and [fetchingOlder].
+  /// the flags [haveOldest], [fetchingOlder] and [fetchOlderCoolingDown].
   /// It exists as an optimization, to memoize that computation.
   final QueueList<MessageListItem> items = QueueList();
 
@@ -241,6 +267,8 @@ mixin _MessageSequence {
     _fetched = false;
     _haveOldest = false;
     _fetchingOlder = false;
+    _fetchOlderCoolingDown = false;
+    _fetchOlderCooldownBackoffMachine = null;
     contents.clear();
     items.clear();
   }
@@ -290,10 +318,12 @@ mixin _MessageSequence {
   void _updateEndMarkers() {
     assert(fetched);
     assert(!(haveOldest && fetchingOlder));
-    final startMarker = switch ((fetchingOlder, haveOldest)) {
-      (true, _) => const MessageListLoadingItem(MessageListDirection.older),
-      (_, true) => const MessageListHistoryStartItem(),
-      (_,    _) => null,
+    assert(!(fetchingOlder && fetchOlderCoolingDown));
+    final startMarker = switch ((fetchingOlder, haveOldest, fetchOlderCoolingDown)) {
+      (true, _, _) => const MessageListLoadingItem(MessageListDirection.older),
+      (_, true, _) => const MessageListHistoryStartItem(),
+      (_, _, true) => const MessageListLoadingItem(MessageListDirection.older),
+      (_, _,    _) => null,
     };
     final hasStartMarker = switch (items.firstOrNull) {
       MessageListLoadingItem()      => true,
@@ -470,7 +500,7 @@ class MessageListView with ChangeNotifier, _MessageSequence {
   Future<void> fetchInitial() async {
     // TODO(#80): fetch from anchor firstUnread, instead of newest
     // TODO(#82): fetch from a given message ID as anchor
-    assert(!fetched && !haveOldest && !fetchingOlder);
+    assert(!fetched && !haveOldest && !fetchingOlder && !fetchOlderCoolingDown);
     assert(messages.isEmpty && contents.isEmpty);
     // TODO schedule all this in another isolate
     final generation = this.generation;
@@ -498,20 +528,28 @@ class MessageListView with ChangeNotifier, _MessageSequence {
   Future<void> fetchOlder() async {
     if (haveOldest) return;
     if (fetchingOlder) return;
+    if (fetchOlderCoolingDown) return;
     assert(fetched);
     assert(messages.isNotEmpty);
     _fetchingOlder = true;
     _updateEndMarkers();
     notifyListeners();
     final generation = this.generation;
+    bool hasFetchError = false;
     try {
-      final result = await getMessages(store.connection,
-        narrow: narrow.apiEncode(),
-        anchor: NumericAnchor(messages[0].id),
-        includeAnchor: false,
-        numBefore: kMessageListFetchBatchSize,
-        numAfter: 0,
-      );
+      final GetMessagesResult result;
+      try {
+        result = await getMessages(store.connection,
+          narrow: narrow.apiEncode(),
+          anchor: NumericAnchor(messages[0].id),
+          includeAnchor: false,
+          numBefore: kMessageListFetchBatchSize,
+          numAfter: 0,
+        );
+      } catch (e) {
+        hasFetchError = true;
+        rethrow;
+      }
       if (this.generation > generation) return;
 
       if (result.messages.isNotEmpty
@@ -529,12 +567,24 @@ class MessageListView with ChangeNotifier, _MessageSequence {
 
       _insertAllMessages(0, fetchedMessages);
       _haveOldest = result.foundOldest;
+      _fetchOlderCooldownBackoffMachine = null;
     } finally {
       if (this.generation != generation) {
         // ignore: control_flow_in_finally
         return;
       }
       _fetchingOlder = false;
+      if (hasFetchError) {
+        assert(!fetchOlderCoolingDown);
+        _fetchOlderCoolingDown = true;
+        unawaited((_fetchOlderCooldownBackoffMachine ??= BackoffMachine())
+          .wait().then((_) {
+            if (this.generation != generation) return;
+            _fetchOlderCoolingDown = false;
+            _updateEndMarkers();
+            notifyListeners();
+          }));
+      }
       _updateEndMarkers();
       notifyListeners();
     }

test/model/message_list_test.dart

@@ -3,6 +3,7 @@ import 'dart:convert';
 import 'package:checks/checks.dart';
 import 'package:http/http.dart' as http;
 import 'package:test/scaffolding.dart';
+import 'package:zulip/api/exception.dart';
 import 'package:zulip/api/model/events.dart';
 import 'package:zulip/api/model/model.dart';
 import 'package:zulip/api/model/narrow.dart';
@@ -236,6 +237,34 @@ void main() {
       ..messages.length.equals(30);
   });
 
+  test('fetchOlder nop during backoff', () => awaitFakeAsync((async) async {
+    final olderMessages = List.generate(5, (i) => eg.streamMessage());
+    final initialMessages = List.generate(5, (i) => eg.streamMessage());
+    await prepare(narrow: const CombinedFeedNarrow());
+    await prepareMessages(foundOldest: false, messages: initialMessages);
+
+    connection.prepare(httpStatus: 400, json: {
+      'result': 'error', 'code': 'BAD_REQUEST', 'msg': 'Bad request'});
+    check(async.pendingTimers).isEmpty();
+    await check(model.fetchOlder()).throws<ZulipApiException>();
+    checkNotified(count: 2);
+    check(model).fetchOlderCoolingDown.isTrue();
+
+    connection.takeRequests();
+    await model.fetchOlder();
+    checkNotNotified();
+    check(model).fetchOlderCoolingDown.isTrue();
+    check(model).fetchingOlder.isFalse();
+
+    async.flushTimers();
+    check(model).fetchOlderCoolingDown.isFalse();
+
+    connection.prepare(json: olderResult(
+      anchor: 1000, foundOldest: false, messages: olderMessages).toJson());
+    await model.fetchOlder();
+    checkNotified(count: 2);
+  }));
+
   test('fetchOlder handles servers not understanding includeAnchor', () async {
     const narrow = CombinedFeedNarrow();
     await prepare(narrow: narrow);
@@ -983,6 +1012,42 @@ void main() {
         checkNotifiedOnce();
       }));
 
+      test('fetchOlder backoff start, _reset, fetchOlder backoff ends, move fetch finishes', () => awaitFakeAsync((async) async {
+        await prepareNarrow(narrow, initialMessages);
+
+        connection.prepare(httpStatus: 400, json: {
+          'result': 'error', 'code': 'BAD_REQUEST', 'msg': 'Bad request'});
+        await check(model.fetchOlder()).throws<ZulipApiException>();
+        final backoffTimer = async.pendingTimers.single;
+        check(model).fetchOlderCoolingDown.isTrue();
+        checkHasMessages(initialMessages);
+        checkNotified(count: 2);
+
+        connection.prepare(delay: const Duration(seconds: 2), json: newestResult(
+          foundOldest: false,
+          messages: initialMessages + movedMessages,
+        ).toJson());
+        await store.handleEvent(eg.updateMessageEventMoveTo(
+          origTopic: movedMessages[0].topic,
+          origStreamId: otherStream.streamId,
+          newMessages: movedMessages,
+        ));
+        check(model).fetchOlderCoolingDown.isFalse();
+        check(async.pendingTimers).contains(backoffTimer);
+        checkHasMessages([]);
+        checkNotifiedOnce();
+
+        // The first backoff is expected to be short enough to complete.
+        async.elapse(const Duration(seconds: 1));
+        check(model).fetchOlderCoolingDown.isFalse();
+        check(async.pendingTimers).not((x) => x.contains(backoffTimer));
+        checkNotNotified();
+
+        async.elapse(const Duration(seconds: 1));
+        checkHasMessages(initialMessages + movedMessages);
+        checkNotifiedOnce();
+      }));
+
       test('fetchOlder, _reset, move fetch finishes, fetchOlder returns', () => awaitFakeAsync((async) async {
         await prepareNarrow(narrow, initialMessages);
 
@@ -1791,7 +1856,7 @@ void checkInvariants(MessageListView model) {
   if (model.haveOldest) {
     check(model.items[i++]).isA<MessageListHistoryStartItem>();
   }
-  if (model.fetchingOlder) {
+  if (model.fetchingOlder || model.fetchOlderCoolingDown) {
     check(model.items[i++]).isA<MessageListLoadingItem>();
   }
   for (int j = 0; j < model.messages.length; j++) {
@@ -1847,6 +1912,7 @@ extension MessageListViewChecks on Subject<MessageListView> {
   Subject<bool> get fetched => has((x) => x.fetched, 'fetched');
   Subject<bool> get haveOldest => has((x) => x.haveOldest, 'haveOldest');
   Subject<bool> get fetchingOlder => has((x) => x.fetchingOlder, 'fetchingOlder');
+  Subject<bool> get fetchOlderCoolingDown => has((x) => x.fetchOlderCoolingDown, 'fetchOlderCoolingDown');
 }
 
 /// A GetMessagesResult the server might return on an `anchor=newest` request.