Http2Connection keeps the ExecutionContext of the first request alive

[MihaZupan]

As part of setting up the Http2Connection we spin up two background tasks for the read/write channel and in doing so we capture the execution context of the request that initiated the new connection.

We should do the SuppressFlow/RestoreFlow dance somewhere around Http2Connection.SetupAsync. This could also keep it alive.

Repro (note that the finalizer doesn't run until you dispose the client):

var client = new HttpClient
{
    DefaultRequestVersion = HttpVersion.Version20
};

await SendRequest(client);

_ = Task.Run(async () =>
{
    await Task.Delay(5_000);
    Console.WriteLine("Disposing the client");
    client.Dispose();
});

for (int i = 1; ; i++)
{
    Console.WriteLine(i);
    GC.Collect();
    GC.WaitForPendingFinalizers();
    await Task.Delay(1000);
}

async Task SendRequest(HttpClient client)
{
    var asyncLocal = new AsyncLocal<Finalizable>();
    asyncLocal.Value = new Finalizable();
    
    await client.GetStringAsync("https://httpbin.org/ip");
}

public sealed class Finalizable
{
    ~Finalizable() => Console.WriteLine("Finalizer");
}

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Issue Details

---

As part of setting up the Http2Connection we spin up two background tasks for the read/write channel and in doing so we capture the execution context of the request that initiated the new connection.

We should do the SuppressFlow/RestoreFlow dance before calling into Http2Connection.SetupAsync.

Repro (note that the finalizer doesn't run until you dispose the client):

var client = new HttpClient
{
    DefaultRequestVersion = HttpVersion.Version20
};

await SendRequest(client);

_ = Task.Run(async () =>
{
    await Task.Delay(5_000);
    Console.WriteLine("Disposing the client");
    client.Dispose();
});

for (int i = 1; ; i++)
{
    Console.WriteLine(i);
    GC.Collect();
    GC.WaitForPendingFinalizers();
    await Task.Delay(1000);
}

async Task SendRequest(HttpClient client)
{
    var asyncLocal = new AsyncLocal<Finalizable>();
    asyncLocal.Value = new Finalizable();
    
    await client.GetStringAsync("https://httpbin.org/ip");
}

public sealed class Finalizable
{
    ~Finalizable() => Console.WriteLine("Finalizer");
}

Author:	MihaZupan
Assignees:	-
Labels:	bug, area-System.Net.Http
Milestone:	-

[MihaZupan]

Triage: This should be easy to fix, moving to 8.0.

[MihaZupan]

Judging by the code, we have the same issue with HTTP/3 as well.

We capture the ExecutionContext of the original request in these tasks inside Http3Connection's ctor:

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http3Connection.cs

Lines 102 to 106 in 6ede02b

	// Errors are observed via Abort().
	_ = SendSettingsAsync();
	// This process is cleaned up when _connection is disposed, and errors are observed via Abort().
	_ = AcceptStreamsAsync();