`Process.KillTree` takes long

[Youssef1313]

I'm seeing a situation in microsoft/testfx repo where Process.KillTree appears to take so long.

In the above case, the process itself has already exited before Kill(true) was called. Based on the code comment here:

runtime/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.Win32.cs

Lines 378 to 380 in e8812e7

	// If the process has exited, the handle is invalid.
	if (handle.IsInvalid)
	return null;

I'm really surprised this condition isn't hit and we return immediately.
I guess it's not a supported scenario to kill whole tree if the parent process already exited?

Note: I haven't tried to create a minimal repro yet.

[Youssef1313]

Note that even if the process itself hasn't exited, something is still going on.

And seems somehow related to whether or not debugger is attached?

Run the following with debugger attached in VS:

using System.Diagnostics;

if (args.Length == 1 && args[0] == "1")
{
    var processStartInfo = new ProcessStartInfo(Process.GetCurrentProcess().MainModule!.FileName)
    {
        Arguments = "2",
        UseShellExecute = false,
        RedirectStandardOutput = true,
        RedirectStandardError = true,
    };

    Process.Start(processStartInfo);

    while (true)
    {
        Thread.Sleep(10000);
    }
}
else if (args.Length == 1 && args[0] == "2")
{
    while (true)
    {
        Thread.Sleep(10000);
    }
}

var processStartInfo1 = new ProcessStartInfo(Process.GetCurrentProcess().MainModule!.FileName)
{
    Arguments = "1",
    UseShellExecute = false,
    RedirectStandardOutput = true,
    RedirectStandardError = true,
};

var process = Process.Start(processStartInfo1);
Thread.Sleep(1000);
var timestamp = Stopwatch.GetTimestamp();
process.Kill(true);
Console.WriteLine(Stopwatch.GetElapsedTime(timestamp));

prints ~ 00:00:23.6890791

If I run without debugger, I get ~ 00:00:00.1698470.

23seconds seems like A LOT.

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