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[web] Font fallback download failure causes infinite loop. #184449

Description

@caijw

Steps to reproduce

[web canvaskit] Font fallback download failure causes infinite loop via sendFontChangeMessage → relayout → startDownloads

Steps to reproduce

  1. change the fonts host 'https://fonts.gstatic.com/s/' to 'https://test.test.com/'
  2. Run a Flutter web application that renders text containing code points not covered by the default font (e.g., CJK characters, emoji, or other scripts that require Noto fallback fonts).
  3. Observe the browser's network tab and console output.

The cycle:

startDownloads()
  → loadFallbackFont() throws (network error)
  → catch: removes font from pendingFonts, but does NOT add it to downloadedFonts
  → after Future.wait, pendingFonts is now empty
  → calls sendFontChangeMessage()
    → triggers text relayout
      → ensureFontsSupportText()
        → addMissingCodePoints()
          → findFontsForMissingCodePoints()
            → _downloadQueue.add(font)  // the same failed font is re-added!
              → startDownloads()  // infinite loop

Expected results

When a fallback font download fails due to network issues, the engine should:

  • Retry a limited number of times with backoff, then give up.
  • NOT continuously re-trigger the same download in an infinite loop.
  • Mark persistently failing fonts so they are not retried indefinitely.

Actual results

The engine enters an infinite loop of download attempts, consuming CPU and network resources indefinitely. The browser console is flooded with Failed to load font ... warnings.

Code sample

Code sample
import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: '',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // TRY THIS: Try running your application with "flutter run". You'll see
        // the application has a purple toolbar. Then, without quitting the app,
        // try changing the seedColor in the colorScheme below to Colors.green
        // and then invoke "hot reload" (save your changes or press the "hot
        // reload" button in a Flutter-supported IDE, or press "r" if you used
        // the command line to start the app).
        //
        // Notice that the counter didn't reset back to zero; the application
        // state is not lost during the reload. To reset the state, use hot
        // restart instead.
        //
        // This works for code too, not just values: Most code changes can be
        // tested with just a hot reload.
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
      ),
      home: const MyHomePage(title: ''),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // TRY THIS: Try changing the color here to a specific color (to
        // Colors.amber, perhaps?) and trigger a hot reload to see the AppBar
        // change color while the other colors stay the same.
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          //
          // TRY THIS: Invoke "debug painting" (choose the "Toggle Debug Paint"
          // action in the IDE, or press "p" in the console), to see the
          // wireframe for each widget.
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text('1111阿发所发生的发生的法师打发谁₍˄·͈༝·͈˄*₎◞ ̑ | ₍˄·͈༝·͈˄*₎◞ ̑ | ᜊ•͈⌔•͈ᜊ | (¯▽¯)ゞ |  ꩰ(՞⸝⸝o ̫ o⸝⸝՞) ᗱ | ٩̋(๑˃́ꇴ˂̀๑)♪(・ω・)ノ૮₍ɵ̷﹏ɵ̷̥᷅₎ა歡囍0123abc你我他哈喽礼啦(っ◕‿◕)😭'),
            Text(
              '',
              style: Theme.of(context).textTheme.headlineMedium,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: '',
        child: const Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

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Flutter Doctor output

Doctor output
Doctor summary (to see all details, run flutter doctor -v):
[!] Flutter (Channel [user-branch], 3.31.0-1.0.pre.84, on macOS 15.5 24F74 darwin-arm64, locale zh-Hans-CN)
    ! Flutter version 3.31.0-1.0.pre.84 on channel [user-branch] at /Users/jingweicai/dev/flutter
      Currently on an unknown channel. Run `flutter channel` to switch to an official channel.
      If that doesn't fix the issue, reinstall Flutter by following instructions at https://flutter.dev/setup.
    ! Upstream repository git@github.com:caijw/flutter.git is not a standard remote.
      Set environment variable "FLUTTER_GIT_URL" to git@github.com:caijw/flutter.git to dismiss this error.
[✓] Android toolchain - develop for Android devices (Android SDK version 36.1.0)
[✓] Xcode - develop for iOS and macOS (Xcode 16.4)
[✓] Chrome - develop for the web
[✓] Android Studio (version 2025.1)
[✓] VS Code (version 1.109.2)
[✓] Connected device (4 available)
[✓] Network resources

! Doctor found issues in 1 category.

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P1High-priority issues at the top of the work listengineflutter/engine related. See also e: labels.has reproducible stepsThe issue has been confirmed reproducible and is ready to work onplatform-webWeb applications specificallyteam-webOwned by Web platform teamtriaged-webTriaged by Web platform team

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