When Do Exception _stackTrace and _stackTraceString Get Populated?

[meriffa]

Hi,

I am investigating exceptions in a couple of Linux core dumps from ASP.NET Core 9.0 applications, running in containers (mcr.microsoft.com/dotnet/aspnet:9.0). The first core dump contains 264 exceptions and the second one 532 exceptions. Between the two dumps, only 24 exceptions have _stackTrace field populated. The rest have 0x0000000000000000 for both _stackTrace and _stackTraceString fields.

I am looking to understand the logic (or rules) that determine when the exception stack trace is populated.

My assumption is that, since stack walk is an expensive operation, exception allocation and stack trace generation are handled by different components.

Any pointers would be greatly appreciated.

Marian.

[jkotas]

The _stackTrace field is populated once the exception is thrown.

try
{
Exception e = new Exception();
// e._stackTrace is null here
throw e;
}
catch (Exception e)
{
// e._stackTrace is non-null here
}

runtime/src/coreclr/vm/excep.cpp

Lines 3134 to 3144 in 02bea82

	if (gc.pKeepAliveArray != NULL)
	{
	_ASSERTE(keepAliveItemsCount > 0);
	gc.pKeepAliveArray->SetAt(0, gc.stackTrace.m_pStackTraceArray.Get());
	((EXCEPTIONREF)ObjectFromHandle(hThrowable))->SetStackTrace(dac_cast<OBJECTREF>(gc.pKeepAliveArray));
	}
	else
	{
	_ASSERTE(keepAliveItemsCount == 0);
	((EXCEPTIONREF)ObjectFromHandle(hThrowable))->SetStackTrace(dac_cast<OBJECTREF>(gc.stackTrace.m_pStackTraceArray.Get()));
	}

is the code that populates _stackTrace.

[meriffa]

@jkotas: Thank you for the excep.cpp reference. This very helpful.

Given that both if & else clauses use SetStackTrace(), is it safe to assume that if an exception instance has _stackTrace = null it was instantiated, but never (or not yet) thrown?

Or maybe I don't quite understand your try/catch example. Is it that, if an exception is thrown and there is a catch block the _stackTrace is populated only inside the catch block, but when the execution leaves the catch block the _stackTrace is cleared (or gets cleared later on)?

The majority of the exceptions that I see are Microsoft.AspNetCore.Connections.ConnectionAbortedException: The Socket transport's send loop completed gracefully. and System.IO.IOException: Unable to read data from the transport connection: Operation canceled.

I am not sure how these exceptions would exist without being thrown. I doubt these are static fields and the number of incoming requests is tiny (I believe there are 1-2 active requests at the time of the core dump capture), so these cannot be exceptions about to be thrown. I believe these exceptions are being handled, since I don't see them in the logs.

[meriffa]

I did some further digging and for ConnectionAbortedException and found this:

_shutdownReason = shutdownReason ?? new ConnectionAbortedException("The Socket transport's send loop completed gracefully.");

So it seems the rest of the empty stack trace instance have similar pattern - either static field, instance field or a parameter of type Exception.

Feel free to close this discussion. I have no further questions.

Thanks.