OnSendingHeaders-Callback called twice

[kryz]

In my Owin Middleware I subscribe a callback to the OnSendingHeaders. In my UnitTests (see below), the callback is sometimes called twice. Most of the time, the first TestCase succeeds and the second fails. Sometimes both succeed, sometimes both fail. Also on production I have the problem, that the callback is sometimes called more than once.

My expectation would have been, that this callback is only called once. The documentation doesn’t give a hint for this behavior, nor does it exclude it. Therefor my question: Is this behavior on purpose? Or is this a bug?

public class MyMiddleware
{
    public void Initialize(Func<IDictionary<string, object>, Task> next) { }

    public int Count { get; private set; }

    public async Task Invoke(IDictionary<string, object> env)
    {
        IOwinContext context = new OwinContext(env);
        context.Response.OnSendingHeaders(dummy =>
        {
            Count++;
        }, null);

        await context.Response.WriteAsync("something");
    }
}


[TestFixture]
public class MyMiddlewareTests
{
    [TestCase(1), TestCase(2)]
    public async Task TheTest(int dummy)
    {
        var myMiddleware = new MyMiddleware();
        using (var server = TestServer.Create(app => { app.Use(myMiddleware); }))
        {
            var response = await server.HttpClient.GetAsync("/");
            Assert.AreEqual(1, myMiddleware.Count);
        }
    }
}

[kryz]

Update: We realized that we have a different problem on production, i.e. I can only confirm that the callback is called twice with the testserver.

[Aarskin]

FWIW, I am also seeing this - it causes a lot of uncertainty when trying to work with System.Diagnostics.Activity (on the second invocation, Activity.Current may be null).