jslater89
commented
2 years ago flutter doctor -v:
$ flutter doctor -v
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• Flutter plugin version 62.0.1
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[✓] Connected device (3 available)
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• No issues found!
maheshj01
pchampio
gspencergoog
github-actions[bot]
jwinarske
I recently started helping with maintenance on the go-flutter-desktop embedder, and have come across an issue (go-flutter-desktop/go-flutter#634) that I can't reproduce in the official Linux embedder, but can't find a clear cause for in the go-flutter-desktop codebase.
The issue occurred in Flutter stable 2.5.x, Flutter beta 2.8, and the new 2.8.
Symptom
If an application is displaying an image and has spawned additional isolates, a segfault occurs on the io.flutter.io thread, before FlutterEngineShutdown returns on the embedder's main thread.
Minimal example
See example repository.
If not on Linux x64, install hover. The repository contains a prebuilt hover executable for Linux x64.
If you hit the Exit button before the image displays, the app exits normally. If you hit it after the image displays, the app segfaults on exit.
Comment out either the Container with key bg1 or the call to
computeininitState, and the issue goes away.Investigation/Speculation
There's a whole lot more of this in the linked issue, but here's the short version, starting with the stack trace from the segfault, but I'll reproduce the current state of my knowledge here.
io.flutter.io segfault
``` #0 sk_spIt appears to be a use-after-free issue. I spent some time digging in, and couldn't identify any obvious differences between go-flutter-desktop and the official Linux embedder modulo the glfw/GTK differences. They both configure the engine in the same way before starting it, and neither seems to do any special preparation before shutting down the engine when glfw/GTK say it's time to close the window.
go-flutter-desktop uses a single platform/render thread like the Linux embedder. The segfault occurs after it calls FlutterEngineShutdown but before that function returns, and before FlutterEngineDeinitialize returns in FlutterEngineShutdown.
The issue seems to be caused by a SkiaGPUObject calling Unref on the io_manager SkiaUnrefQueue after the IO thread teardown code at shell.cc:454. In this case, a flutter::Picture is deleted on a DartWorker thread, after a breakpoint placed at shell.cc:478 (i.e., at the end of the Shell::Shell destructor), shortly before the segmentation fault.
Unref called from DartWorker, late
``` Thread 37 "DartWorker" hit Breakpoint 3, flutter::SkiaUnrefQueue::Unref (this=0x7fffac0108d0, object=0x7fffb83bf860) at ../../flutter/flow/skia_gpu_object.cc:29 29 task_runner_->PostDelayedTask( (gdb) bt #0 flutter::SkiaUnrefQueue::Unref (this=0x7fffac0108d0, object=0x7fffb83bf860) at ../../flutter/flow/skia_gpu_object.cc:29 #1 0x00007ffff74b491e in flutter::SkiaGPUObjectRemoving the call to is torn down on the io.flutter.ui thread during flutter::Picture::dispose, before the breakpoint at shell.cc:478 is hit:
computein main.dart, the SkiaGPUObjectUnref called from UI thread
``` Thread 11 "io.flutter.ui" hit Breakpoint 3, flutter::SkiaUnrefQueue::Unref (this=0x7fffb00108d0, object=0x7fffbc319f80) at ../../flutter/flow/skia_gpu_object.cc:29 29 task_runner_->PostDelayedTask( (gdb) bt #0 flutter::SkiaUnrefQueue::Unref (this=0x7fffb00108d0, object=0x7fffbc319f80) at ../../flutter/flow/skia_gpu_object.cc:29 #1 0x00007ffff74b491e in flutter::SkiaGPUObjectAfter that, the GrGLContext destructor runs, then the breakpoint at shell.cc:478, then the app exits normally. The following breakpoints were used:
Let me know if you need any additional information from me on how the go-flutter-desktop embedder operates, or if there are any caveats about threading/isolates/goroutines we need to keep in mind.
I tried running on the GLFW example in flutter/examples to see if it showed a similar issue (in the hopes of comparing apples to apples), but it failed to start: