[iOS] App hangs on iOS 27: SimpleThreadSafeCache runs text measurement while holding its lock

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  • #59614 来自 @kasik96 —— 已关闭，未合并

评估

难度
3/5
预计耗时
1-2 天
新手友好度
20/100
Issue 类型
缺陷
描述清晰度
描述清楚
活跃度
停滞
技术栈
cpp, ios, react-native
领域
mobile, performance

调研方向

Read SimpleThreadSafeCache::getMapIterator and the unit test in #59614, which the issue says demonstrates the lock problem without iOS. Trace how TextLayoutManager uses the cache and how a cache miss runs its generator while the mutex is held. The issue links fix #59614, which is closed and unmerged; check that work before starting.

由索引模型根据 Issue 内容生成。

描述

Needs: Attention Needs: Repro
Description

On iOS 27 devices, an app can hang for good when the JS thread and the main thread measure text at the same time.

SimpleThreadSafeCache::getMapIterator holds mutex_ while it runs the generator for a cache miss. TextLayoutManager uses this cache for text measurement, and on iOS the generator creates an NSTextStorage. On iOS 27 devices, -[NSTextStorage processEditing] posts a notification that waits for the main queue (_CFXNotificationPost → -[NSOperation waitUntilFinished]).

If the main thread measures text at that moment, for example in a synchronous commit from Reanimated, it waits for mutex_:

  • JS thread: holds mutex_ and waits for the main queue.
  • Main thread: waits for mutex_.

The observer that the notification waits for comes from iOS, not from app code. The iOS 27.0 simulator does not wait there, so the hang shows only on devices (seen on iOS 27.0.1 and 27.2).

The same code is on main.

Steps to reproduce
  1. Run on a device with iOS 27.
  2. Render a list of about 100 rows with text, and run a Reanimated animation.
  3. Replace the list data while the animation runs.

The app hangs within a few tries.

To see it on the simulator, first add a main-queue observer in lldb:

expr -l objc -- (id)[[NSNotificationCenter defaultCenter] addObserverForName:@"NSTextStorageDidProcessEditingNotification" object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification *n){}]
React Native Version

0.86.3

Affected Platforms

Runtime - iOS

Output of npx @react-native-community/cli info
System:
  OS: macOS 27.0.1
  CPU: (10) arm64 Apple M1 Pro
Binaries:
  Node: 24.14.1
  Yarn: 4.3.1
  npm: 11.11.0
Managers:
  CocoaPods: 1.16.2
IDEs:
  Xcode: 26.6/17F113
npmPackages:
  react: 19.2.3
  react-native: 0.86.3
  react-native-reanimated: 4.5.1
iOS:
  hermesEnabled: true
  newArchEnabled: true
Stacktrace or Logs
Main thread:
  reanimated::ReanimatedModuleProxy::commitUpdates
  facebook::react::ShadowTree::commit
  facebook::react::RootShadowNode::layoutIfNeeded
  ...
  facebook::react::ParagraphShadowNode::measureContent
  facebook::react::TextLayoutManager::measure
  facebook::react::SimpleThreadSafeCache<...>::getMapIterator
  std::mutex::lock

JS thread:
  facebook::react::UIManager::completeSurface
  facebook::react::ShadowTree::commit
  facebook::react::RootShadowNode::layoutIfNeeded
  ...
  facebook::react::TextLayoutManager::measure
  facebook::react::SimpleThreadSafeCache<...>::getMapIterator
  -[RCTTextLayoutManager measureAttributedString:paragraphAttributes:layoutContext:layoutConstraints:]
  -[RCTTextLayoutManager _textStorageAndLayoutManagerWithAttributesString:paragraphAttributes:size:]
  -[NSConcreteTextStorage initWithAttributedString:]
  -[NSTextStorage processEditing]
  -[NSNotificationCenter postNotificationName:object:userInfo:]
  _CFXNotificationPost
  -[NSOperation waitUntilFinished]
Reproducer

No standalone reproducer: the hang needs an iOS 27 device, or the lldb step above. The unit test in #59614 shows the lock problem without iOS.

Fix: #59614

主要语言
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