Sentry loses its scope inside the ASP.NET pipeline because it uses AsyncLocal for storage

The official Sentry SDK docs for ASP.NET instructs one to start and finish tracing transactions like this in Global.asax.cs:

protected void Application_BeginRequest()
{
    Context.StartSentryTransaction();
}

protected void Application_EndRequest()
{
    Context.FinishSentryTransaction();
}

This approach is buggy, as it will sometimes cause the Sentry scope to be lost inside the (non-Core) ASP.NET pipeline. The underlying problem is that the Sentry SDK stores its scopes in an AsyncLocal field in SentryScopeManager.cs. This is not a safe way to store such state in (non-Core) ASP.NET. The only safe method is likely to use HttpContext.Items. For more information, see:

• https://stackoverflow.com/questions/43391498/asynclocal-value-is-null-after-being-set-from-within-application-beginrequest
• https://devblogs.microsoft.com/dotnet/net-framework-4-7-1-asp-net-and-configuration-features/#asp-net-execution-step-feature
• http://piers7.blogspot.com/2005/11/threadstatic-callcontext-and_02.html

In my use case, I only care about the transaction reference, so I applied the following workaround to Global.asax.cs, based on the above mentioned blog post my Microsoft and how they themselves work around the issue in Application Insights:

public override void Init()
{
    base.Init();
    
    OnExecuteRequestStep((httpContextBase, step) =>
    {
        if (httpContextBase.CurrentNotification == RequestNotification.ExecuteRequestHandler && !httpContextBase.IsPostNotification)
        {
            SentrySdk.ConfigureScope(scope =>
            {
                if (scope.Transaction == null)
                {
                    scope.Transaction = (ITransaction) httpContextBase.Items["__SentryTransaction"];
                }
            });
        }

        step();
    });
}

[Zero3]

@Tyrrrz SentrySdk.GetSpan() now throws a NullReferenceException from deep within Sentry when executed outside a HTTP request. It used to return null. Is this intentional?

The problem is that .Current can return null in the getter here, which is unhandled:

https://github.com/getsentry/sentry-dotnet/blob/28ec35d278b2fa15d356af41079ab81a956fe364/src/Sentry.AspNet/Internal/HttpContextScopeStackContainer.cs#L11-L15

Also see this related issue: https://github.com/getsentry/sentry-dotnet/issues/1157

[Zero3]

Thanks for the fix! I am off for summer holidays, but I might be able to test this in August.