Mono & .NET 5 & The Debugger & Exceptions, oh my!

jonpryor commented 4 years ago

Xamarin.Android v10.x uses the mono/mono/2020-02 branch, and uses the System.Diagnostics.Debugger.Mono_UnhandledException() internal call when a debugger is attached, in order to simulate the first chance notification within the debugger.

Debugger.Mono_UnhandledException() does not exist in .NET 5 mono. Consequently, when an exception is thrown in a .NET 5 Xamarin.Android app, the debugger "first chance notification" behavior differs.

As such, when an exception is thrown and not caught, there is no "first chance" notification, nor is there a "last chance" notification. Instead, execution continues, the entire call stack is unwound, and then the exception is visible within the debugger.

This might be a Glorious Opportunity to rework how exceptions work within Xamarin.Android apps when a debugger is attached. xamarin/xamarin-android#4877 describes the current state of affairs, and it's "not good." In particular, current behavior is that when a managed exception is caught while the debugger is attached, when execution continues mono will unwind stack frames, without allowing execution to return to Java code.

This basically means that if you have a Managed > Java > Managed callstack transition, the process is effectively unusable if the rightmost frame throws an exception while the process is being debugged.

(Not using a debugger results in "sane" exception handling, but means you don't have a debugger…)

ghost commented 4 years ago

Tagging subscribers to this area: @tommcdon Notify danmosemsft if you want to be subscribed.

ghost commented 4 years ago

Tagging subscribers to this area: @thaystg Notify danmosemsft if you want to be subscribed.

lambdageek commented 4 years ago

I'm curious what the debugger situation is like on macos/ios - I recall we convert managed exceptions into objective-C exceptions and back at Managed->Native->Managed transitions - and the unwinding in native frames is handled by objC. But I don't know what the debugger behavior is like.

It would be good if Android and ios didn't diverge too much here...

thaystg commented 4 years ago

I'm planning to take a look at this next week, to understand how iOS and Android works to understand which would be the best approach to follow on wasm and on Android using .NET 5 mono. I will do some tests and I can write here everything that I understand. :)

jonathanpeppers commented 4 years ago

There is also another call related to exceptions:

https://github.com/xamarin/xamarin-android/blob/4cae5f5e40896c69b7448cb78cf613cf6327c97c/src/Mono.Android/Android.Runtime/JNIEnv.cs#L251-L262

This allows the AppDomain.UnhandledException event to work, but AppDomain.DoUnhandledException does not exist in .NET 5.

jonathanpeppers commented 3 years ago

Repro here: https://github.com/xamarin/xamarin-android/pull/4927#issuecomment-875876255

jonpryor commented 3 years ago

.NET 6 project: https://github.com/xamarin/xamarin-android/files/6779689/Scratch.JMJMException.zip

Unhandled exception handling is broken on .NET 6 when running within VS 2022: instead of getting a first-chance exception, or any form of unhandled exception dialog, apps crash immediately if they have an unhandled exception. See this adb logcat output:

I DOTNET  : # jon: Should be in a Java > Managed [MainActivity.OnCreate] > Java [Run.tryCatchFinally] > Managed [Run] frame. Throwing an exception...
I DOTNET  :    at Scratch.JMJMException.MainActivity.<>c.<OnCreate>b__0_1() in C:\Users\jopepper\Downloads\Scratch.JMJMException\Scratch.JMJMException\MainActivity.cs:line 28
I DOTNET  :    at Java.Lang.Runnable.Run() in Mono.Android.dll:token 0x601ef04+0x0
I DOTNET  :    at Java.Lang.IRunna
I DOTNET  : bleInvoker.n_Run(IntPtr jnienv, IntPtr native__this) in Mono.Android.dll:token 0x601e900+0x0
I DOTNET  :    at Android.Runtime.DynamicMethodNameCounter.5(IntPtr , IntPtr ) in Mono.Android.dll:token 0x0+0x0
I DOTNET  :    at Java.Interop.NativeMethods.java_interop_jnienv_call_stat
I DOTNET  : ic_void_method_a(IntPtr , IntPtr& , IntPtr , IntPtr , IntPtr ) in Java.Interop.dll:token 0x0+0x0
I DOTNET  :    at Java.Interop.JniEnvironment.StaticMethods.CallStaticVoidMethod(JniObjectReference type, JniMethodInfo method, JniArgumentValue* args) in Java.Interop.dll
I DOTNET  : :token 0x6000593+0x0
I DOTNET  :    at Java.Interop.JniPeerMembers.JniStaticMethods.InvokeVoidMethod(String encodedMember, JniArgumentValue* parameters) in Java.Interop.dll:token 0x600060a+0x0
I DOTNET  :    at Testing.Run.TryCatchFinally(IRunnable p0, ICatchThrowableHandler p1, 
I DOTNET  : IRunnable p2) in C:\Users\jopepper\Downloads\Scratch.JMJMException\Scratch.Try\obj\Debug\generated\src\Testing.Run.cs:line 142
I DOTNET  :    at Scratch.JMJMException.MainActivity.<>c.<OnCreate>b__0_0(Object o, EventArgs e) in C:\Users\jopepper\Downloads\Scratch.JMJME
I DOTNET  : xception\Scratch.JMJMException\MainActivity.cs:line 24
I DOTNET  :    at Android.Views.View.IOnClickListenerImplementor.OnClick(View v) in Mono.Android.dll:token 0x6020b2f+0x0
I DOTNET  :    at Android.Views.View.IOnClickListenerInvoker.n_OnClick_Landroid_view_View_(IntPtr jnienv
I DOTNET  : , IntPtr native__this, IntPtr native_v) in Mono.Android.dll:token 0x6020b2b+0x0
I DOTNET  :    at Android.Runtime.DynamicMethodNameCounter.4(IntPtr , IntPtr , IntPtr ) in Mono.Android.dll:token 0x0+0x0
I DOTNET  : 
W monodroid: MonodroidRuntime::monodroid_pinvoke_override ("java-interop", "java_interop_jnienv_throw")
W monodroid: Found java-interop@java_interop_jnienv_throw in internal p/invoke map (0xcb5a3440)
W monodroid: MonodroidRuntime::monodroid_pinvoke_override ("java-interop", "java_interop_jnienv_throw")
W monodroid: Found java-interop@java_interop_jnienv_throw in internal p/invoke map (0xcb5a3440)
W monodroid: MonodroidRuntime::monodroid_pinvoke_override ("java-interop", "java_interop_jnienv_throw")
W monodroid: Found java-interop@java_interop_jnienv_throw in internal p/invoke map (0xcb5a3440)
--------- beginning of crash
F libc    : Fatal signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x7 in tid 25815 (h.jmjmexception), pid 25815 (h.jmjmexception)
I crash_dump32: obtaining output fd from tombstoned, type: kDebuggerdTombstone
I crash_dump32: performing dump of process 25815 (target tid = 25815)
F DEBUG   : *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
F DEBUG   : Build fingerprint: 'google/sdk_gphone_x86_64/generic_x86_64:9/PSR1.180720.075/5124027:user/release-keys'
F DEBUG   : Revision: '0'
F DEBUG   : ABI: 'x86'
F DEBUG   : pid: 25815, tid: 25815, name: h.jmjmexception  >>> scratch.jmjmexception <<<
F DEBUG   : signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x7
F DEBUG   : Cause: null pointer dereference
F DEBUG   :     eax ffd85a71  ebx e5ff586c  ecx 00000000  edx 00000000
F DEBUG   :     edi ffd86288  esi 00000003
F DEBUG   :     ebp ffd86268  esp ffd86120  eip e5e35618
F DEBUG   : 
F DEBUG   : backtrace:
F DEBUG   :     #00 pc 00532618  /system/lib/libart.so (_ZN3art12StackVisitor9WalkStackILNS0_16CountTransitionsE0EEEvb+584)
F DEBUG   :     #01 pc 00548615  /system/lib/libart.so (art::Thread::DumpStack(std::__1::basic_ostream<char, std::__1::char_traits<char>>&, bool, BacktraceMap*, bool) const+501)
F DEBUG   :     #02 pc 00542c83  /system/lib/libart.so (art::Thread::Dump(std::__1::basic_ostream<char, std::__1::char_traits<char>>&, bool, BacktraceMap*, bool) const+83)
F DEBUG   :     #03 pc 003339d3  /system/lib/libart.so (art::JavaVMExt::JniAbort(char const*, char const*)+1283)
F DEBUG   :     #04 pc 00333ea1  /system/lib/libart.so (art::JavaVMExt::JniAbortV(char const*, char const*, char*)+113)
F DEBUG   :     #05 pc 000db967  /system/lib/libart.so (art::(anonymous namespace)::ScopedCheck::AbortF(char const*, ...)+71)
F DEBUG   :     #06 pc 000da1ae  /system/lib/libart.so (art::(anonymous namespace)::ScopedCheck::CheckPossibleHeapValue(art::ScopedObjectAccess&, char, art::(anonymous namespace)::JniValueType)+1246)
F DEBUG   :     #07 pc 000d92db  /system/lib/libart.so (art::(anonymous namespace)::ScopedCheck::Check(art::ScopedObjectAccess&, bool, char const*, art::(anonymous namespace)::JniValueType*)+827)
F DEBUG   :     #08 pc 000ca8c7  /system/lib/libart.so (art::(anonymous namespace)::CheckJNI::NewString(_JNIEnv*, unsigned short const*, int)+1175)
F DEBUG   :     #09 pc 00046a77  /data/app/scratch.jmjmexception-eDiobdrrJUpLOWxCjyoK6g==/lib/x86/libmonodroid.so (java_interop_jnienv_new_string+55)
F DEBUG   :     #10 pc 00007871  <anonymous:e58ff000>

Also (unrelatedly) odd is the line wrapping for the C# stack trace message, e.g.

I DOTNET  :    at Java.Lang.IRunna
I DOTNET  : bleInvoker.n_Run(IntPtr jnienv, IntPtr native__this) in Mono.Android.dll:token 0x601e900+0x0
I DOTNET  :    at Android.Runtime.DynamicMethodNameCounter.5(IntPtr , IntPtr ) in Mono.Android.dll:token 0x0+0x0

jonpryor commented 3 years ago

dotnet / runtime

Mono & .NET 5 & The Debugger & Exceptions, oh my! #38465