crash can not be captured on windows

[chenxinhua2021]

I am using flutter to develop app. and use sentry to capture crash. C PlusPlus code is called by ffi which is manged by dart in flutter. Now, The crash is not captured in c plusplus code, try to use crashpad/breakpad/inproc backend, fail to capture the crash.
Found The issue which is in the dart website. dart-lang/sdk#39140
FFI is only a C-based. Maybe c plus plus exceptions can not be captured. Is that why Crash can not be captured by sentry ?
If i want to capture the crash, which methods can be used ?

• OS: [e.g. Windows 10, 64-bit]
• Compiler: [e.g. MSVC 22]
• CMake version and config: [e.g. 3.17.2, SENTRY_BACKEND=inproc]

[supervacuus]

Hi @chenxinhua2021! I need more context because I am not well-versed in Flutter. Especially the following questions immediately pop up in my head:

• do you only initialize the Native SDK in a C++ FFI module?
• (or) are you using the Dart SDK for your flutter application too?
• how do you initialize the Native SDK? does the FFI library keep a global sentry context?
• does the initialization work for you (would you see any error when you enable the debug output)?
• can you send normal events successfully?

What I can tell you:

• There is an open ticket to integrate the Native SDK into the Dart SDK to support Flutter Desktop applications on Windows: Support native crash reporting for Windows sentry-dart#896. This means there is currently no support for native crashes on Windows in the Dart SDK.
• The Native SDK doesn't handle C++ exceptions on a language level (i.e., we do not hook into std::terminate), but every uncaught C++ exception will lead to a low-level exception which we will capture.
• The issue you linked to talks about safely handling exceptions through the FFI boundary. This is only partially related: you must provide a C wrapper to your C++ code when using the Dart FFI to guarantee defined behavior in an error case. If the VM crashes due to letting a C++ exception escape through the FFI boundary, then our crash handler should be able to capture this too. But:
• It is entirely unclear to me how Dart handles (SEH) exceptions on Windows and how FFI modules might be sandboxed in a way that will prevent the useful operation of unhandled exception filters (which our backends would need to function correctly). A core task of Support native crash reporting for Windows sentry-dart#896 is figuring out how (and if) Dart allows FFI code to set this up correctly.

[chenxinhua2021]

Thank you very much for your fast reply @supervacuus . I use dart sdk and native sdk in my flutter application.
Native sdk Initialize:
sentry_options_t* options = sentry_options_new();
sentry_options_set_dsn(options, "https://@sentry.io/");
sentry_init(options);
I can get dump file in the path of my project by making some crashes which is not in ffi module.
In addition, normal events can be successfully sent. and other crashes can be captured, only the crash that c++ is called by ffi module can not be captured.

[supervacuus]

When you say "other crashes can be captured", do you mean dart crashes or other native crashes (like an access violation) from an FFI module? I guess any other SEH exception from an FFI module would also not be reported. I cannot tell you when I will have time to investigate this further, but it is on our agenda.

[chenxinhua2021]

Yes, any other SEH exception from an FFI module is not reported. Other crashes Which is maked by c++ plugin to be called by flutter can be captured. I will try to do a simple demo to present the problem.

[supervacuus]

Ok, thanks for verifying. So the C++ FFI aspect seems to be a red herring in this issue since C++ exceptions on Windows are just SEH exceptions.

If SEH exceptions fail to report, the problem lies with the registration or the execution environment of the unhandled exception filter in our backend because Dart installs one itself during the startup of the runtime.

This filter would normally print a stack trace if there was a native crash. The Native SDK in turn would overwrite that filter with its backend-specific implementation. Can you compare whether crashing from the FFI module prints a stack trace without initializing the Native SDK?

[chenxinhua2021]

I try to use windows system api (SetUnhandledExceptionFilter), crash can not be captured which is maked c++ code which is called FFI module.

Stack information can be printed in visual studio tool when crash from ffi module.

[chenxinhua2021]

Hi @supervacuus we make a simple demo to present the problem. this is url. https://github.com/Arctuition/dart-ffi-crash
There are two ways to verify crash. Same program call ffi interface and c++ plugin. Crash in Ffi module( c++ code be called) can not be captured. Crash in c++ plugins can be captured.

[supervacuus]

Thank you for preparing a repro that covers the FFI and plugin case, @chenxinhua2021. As I said before, I cannot say when I will have the time to dive into this.

@marandaneto, I think you will be involved in the dart/native evaluation, right? @chenxinhua2021 prepared a repro-repo, that fails at capturing SEH from FFI modules but is successful when crashing in Flutter plugins. Even if this is not actionable for you rn, I think the Dart-SDK team should be aware of this issue. I will try to figure out where in the FFI boundary the exception propagation fails and if we can even do something about it from the Native SDK.

[chenxinhua2021]

Thank you very much for your reply @supervacuus . I have consulted on drak sdk, some reply may be is worthy for you.

[marandaneto]

Thank you for preparing a repro that covers the FFI and plugin case, @chenxinhua2021. As I said before, I cannot say when I will have the time to dive into this.

@marandaneto, I think you will be involved in the dart/native evaluation, right? @chenxinhua2021 prepared a repro-repo, that fails at capturing SEH from FFI modules but is successful when crashing in Flutter plugins. Even if this is not actionable for you rn, I think the Dart-SDK team should be aware of this issue. I will try to figure out where in the FFI boundary the exception propagation fails and if we can even do something about it from the Native SDK.

That's correct, it's not a supported use case yet, and apparently, there's a dependency with dart-lang/sdk#51726 (comment) and its exception filters.
If @chenxinhua2021 is setting up sentry-native directly in the Flutter app, that should work, that's exactly what we'd do when tackling getsentry/sentry-dart#896 (but automatically) so if there's a problem, we'd need to consider that as well.

[supervacuus]

Hi @marandaneto, yes, that is why I hooked you in. You will experience the same issue with any native code called from Dart-generated code (it happens with JIT and AOT). As mentioned here, it is not specific to FFI modules (though it will mostly be an issue there) but related to unwinding the Dart code regions in case of a crash on Windows.