Step becomes a "go" in .NET 9

[gregg-miskelly]

Description

.NET 9 contains a regression where ICorDebugStepper's are not always firing. The issue is non-deterministic, and causes intermittent failures in our debugging tests, and we have had a few customers reports of the problem.

Reproduction Steps

A Microsoft employee can reproduce this problem by:

1. Enlisting in the 'Concord' repo
2. Following instructs in '/glass.md' for setting up a local test environment
3. Open Test Explorer
4. Enter “CSAsyncStepOverinTryBlock-NetCore-Latest-x64” in the filter box
5. Navigate to the test
6. Right click and invoke “Run Until Failure”

Expected behavior

Stepper should complete

Actual behavior

Stepper sometimes fails to detect when it is complete and this the debugger behaves as if there was no stepper

Regression?

This regressed between .NET 8 and .NET 9

Known Workarounds

None

Configuration

So far all of the reports are from Windows, but I don't know if this is actually a causal connection.
We have at least mostly seen this on x64, but again, I am not sure if that is actually a causal connection or just because that is the most frequent configuration

Other information

No response

[dotnet-policy-service]

Tagging subscribers to this area: @tommcdon
See info in area-owners.md if you want to be subscribed.

[f2bo]

I'm seeing this all the time. I hadn't seen this issue before. I reported the problem here a few days ago: https://developercommunity.visualstudio.com/t/Single-stepping-is-broken-when-targettin/10802855.

I can reproduce it with the following simple steps:

• Create a new Console App project that targets .NET 8.

• Replace the contents of Program.cs with the following code:

  foreach (Range range in new Range[5])
  {
     Console.WriteLine($"{range.Start}, {range.End}");
  }

• Execute in the debugger by using Step Over (F10) or Step Into (F11) to verify that the debugger is able to single-step through each line in the program correctly.

• Change the project file to target .NET 9.

• Execute again with Step Over (F10) or Step Into (F11) . Notice that single-stepping no longer advances to the next line as expected, but that execution suddenly continues without breaking, as if F5 had been pressed.

NOTE: This behavior is erratic, so you may need to repeat a few times if it seems to work correctly.

[geoffrey-gaspari]

Same problem here.

[thaystg]

Hi @f2bo!
Every time that you run this app this "resume" happens?
It happens any time in the foreach loop or it's always in the last loop for example?
Can you share a video with the bug happening?

[f2bo]

Hi @thaystg,

Every time that you run this app this "resume" happens?

Almost every time (see video below).

It happens any time in the foreach loop or it's always in the last loop for example?

In this example, it usually happens on the first iteration of the foreach loop. I should mention that I've had this problem in other code and it's not always associated with a loop. In fact, in one of my tests, I saw it happen on a Console.WriteLine that I temporarily added before the loop using the code above.

Can you share a video with the bug happening?

Sure. Here it is.

The video shows the program being executed with F10, 3 times in succession. The first time, it run through the 5 iterations without failing, though I have to say that this is much less frequent. The remaining 2 times, instead of single-stepping, it resumes execution on the first iteration.

[f2bo]

Interesting fact. After disabling CET in the project file, I have been unable to reproduce the problem so far, at least in this simple example.

<CETCompat>false</CETCompat>

[tommcdon]

Reactivated for backport to 9.0

[IamIC]

Same problem here.

[Ollhax]

I'm getting this all the time, both on my main machine and laptop.

[tommcdon]

Fixed via #110533