Purge caches on iOS can happen when unexpected GL context is active

[shorben07]

This issue is a specific case of a more general issue described in #41620. With the exception that the engine's fix flutter/engine#13812 does not fix this case, so it should be reported and fixed separately.

In the summer, 2019 in pr flutter/engine#9491 @dnfield added purge caches on low memory events. On such events, the flutter engine frees gpu resources.

The problem is that flutter engine does not make sure that the correct context is set before freeGpuResources is called.

From Apple's docs:

When you call any OpenGL ES API function, OpenGL ES evaluates it with respect to the calling thread’s current context. Because OpenGL ES functions require a current context, you must use this method to select a context for the current thread before calling any OpenGL ES function

In other words before making something with the context you should make it current for the thread.

• Important comments about threads and context in case platform views are enabled (https://github.com/flutter/engine/blob/master/shell/platform/darwin/ios/framework/Source/FlutterEngine.mm#L428)

Example

Here is an example when flutter and mapbox with MGLOpenGLStyleLayer crashes under low memory conditions:

Flutter engine starts up. <EAGLContext: 0x280d67c20> is flutter's GL context


<EAGLContext: 0x280d5c3e0> is mapbox's GL context

Flutter Engine receives memory warning first. Task is added to GPU task runner to clear cache

  if (_shell) {
    _shell->NotifyLowMemoryWarning();
  }
...
void Shell::NotifyLowMemoryWarning() const {
  task_runners_.GetGPUTaskRunner()->PostTask(
      [rasterizer = rasterizer_->GetWeakPtr()]() {
        if (rasterizer) {
          rasterizer->NotifyLowMemoryWarning();
        }
      });
  // The IO Manager uses resource cache limits of 0, so it is not necessary
  // to purge them.
}

Mapbox receives memory warning second. It set context to free resources

Mapbox set context to nil after it is done with freening resources

Drawing cycle happens in map view. Mapbox set current context

After mapbox drawing phase GrLegacyDirectContext::freeGpuResources() is called and current gl context is still mapbox's

Very soon after that app crashes because mapbox's gl context is corrupted

Example's source code: https://github.com/shorben07/flutter-mapbox-crash

Target Platform: iOS
Target OS version/browser: Any
Devices: any physical device

Logs

flutter doctor -v
[✓] Flutter (Channel stable, v1.12.13+hotfix.5, on Mac OS X 10.14.5 18F132, locale en-UA)
    • Flutter version 1.12.13+hotfix.5 at /Users/shorben/Documents/Projects/flutter
    • Framework revision 27321ebbad (4 weeks ago), 2019-12-10 18:15:01 -0800
    • Engine revision 2994f7e1e6
    • Dart version 2.7.0

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
    • Android SDK at /Users/shorben/Library/Android/sdk
    • Android NDK location not configured (optional; useful for native profiling support)
    • Platform android-28, build-tools 28.0.3
    • ANDROID_HOME = /Users/shorben/Library/Android/sdk
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 11.3)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 11.3, Build version 11C29
    • CocoaPods version 1.8.4

[✓] Android Studio (version 3.5)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 42.1.1
    • Dart plugin version 191.8593
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)

[✓] VS Code (version 1.39.2)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.4.1

[✓] Connected device (2 available)
    • Serhii’s iPhone • 1e6f141526c2c87ea4965c2979a2fa4515f52e2c • ios • iOS 13.3
    • iPhone 5s       • 688BC58B-AEAD-422C-91A7-FAF40E0F08B6     • ios • com.apple.CoreSimulator.SimRuntime.iOS-12-2 (simulator)

• No issues found!

[dnfield]

/cc @cyanglaz this may be helpful with the referenced bug

[shorben07]

Important addendum
Mapbox uses GLKView as canvas for OpenGL drawing. According to the documentation
Before calling its drawRect: method, the view makes its EAGLContext object the current OpenGL ES context and binds its framebuffer object to the OpenGL ES context as the target for rendering commands.
But there is no anything mentioning that after glkView:drawInRect: view set gl context back to previous value or nil. This makes this issue more severe meaning that most likely
GLKView's gl context will be active when GrLegacyDirectContext::freeGpuResources() is called

[shorben07]

flutter/engine/pull/13946 fixes this issue. I compiled the flutter engine from the branch and tested locally
/cc @cyanglaz

[shorben07]

This is no longer an issue for me:

• Flutter switched to Metal on iOS 10+, so gl context corruptions are no longer possible
• Make sure platform views are isolated from the engine's GL context #41620 has been fixed by Rework GLContextSwitch, get rid of RendererContextManager engine#18601

[github-actions]

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.