ClientWebSocket aborts on closes when going thru a proxy

[davidsh]

While working on #24965, I noticed that the following test will fail. There seems to be a problem with the close frame exchange and final TCP socket close when going thru a proxy.

Repro test (not yet in repo):

[OuterLoop] // TODO: Issue dotnet/runtime#18406
[ConditionalTheory(nameof(WebSocketsSupported)), MemberData(nameof(EchoServers))]
public async Task Proxy_ConnectThruProxy_Success(Uri server)
{
    IWebProxy proxy = new WebProxy(new Uri("http://localhost:8080/"));
    using (ClientWebSocket cws = await WebSocketHelper.GetConnectedWebSocket(
        server,
        TimeOutMilliseconds,
        _output,
        default(TimeSpan),
        proxy))
    {
        var cts = new CancellationTokenSource(TimeOutMilliseconds);
        Assert.Equal(WebSocketState.Open, cws.State);

        var closeStatus = WebSocketCloseStatus.NormalClosure;
        string closeDescription = "Normal Closure";

        await cws.CloseAsync(closeStatus, closeDescription, cts.Token);

        // Verify a clean close frame handshake.
        Assert.Equal(WebSocketState.Closed, cws.State); // *** FAILS HERE ***
        Assert.Equal(closeStatus, cws.CloseStatus);
        Assert.Equal(closeDescription, cws.CloseStatusDescription);
    }
}

Fails with error:

    System.Net.WebSockets.Client.Tests.ClientWebSocketOptionsTests.Proxy_SecureConnectThruProxy_Success(server: ws://corefx-net.cloudapp.net/WebSocket/EchoWeb
  Socket.ashx) [FAIL]
        Assert.Equal() Failure
        Expected: Closed
        Actual:   Aborted
        Stack Trace:
           s:\GitHub\corefx\src\System.Net.WebSockets.Client\tests\ClientWebSocketOptionsTests.cs(90,0): at System.Net.WebSockets.Client.Tests.ClientWebSocketO
  ptionsTests.Proxy_SecureConnectThruProxy_Success(Uri server)
           --- End of stack trace from previous location where exception was thrown ---
     System.Net.WebSockets.Client.Tests.ClientWebSocketOptionsTests.Proxy_SecureConnectThruProxy_Success(server: wss://corefx-net.cloudapp.net/WebSocket/EchoWe
  bSocket.ashx) [FAIL]
        Assert.Equal() Failure
        Expected: Closed
        Actual:   Aborted
        Stack Trace:
           s:\GitHub\corefx\src\System.Net.WebSockets.Client\tests\ClientWebSocketOptionsTests.cs(90,0): at System.Net.WebSockets.Client.Tests.ClientWebSocketO
  ptionsTests.Proxy_SecureConnectThruProxy_Success(Uri server)
           --- End of stack trace from previous location where exception was thrown ---

[davidsh]

cc: @dotnet/ncl

[davidsh]

It seems that the behavior differs depending on which proxy is used. Using the test 'HappySockets' proxy server, we get a RST back after the close frame (which causes this test failure). Using Fiddler, we get a FIN.

However, the test passes using .NET Framework and 'HappySockets' proxy server.

Need to investigate further to determine whether or not this is a test bug (proxy server shouldn't be sending back RST) or a difference between .NET Core and .NET Framework.

[stephentoub]

@davidsh, there's still a test disabled against this issue:

runtime/src/libraries/System.Net.WebSockets.Client/tests/ClientWebSocketOptionsTests.cs

Lines 65 to 68 in ccf6aed

	[ActiveIssue("https://github.com/dotnet/corefx/issues/28027")]
	[OuterLoop]
	[ConditionalTheory(nameof(WebSocketsSupported)), MemberData(nameof(EchoServers))]
	public async Task Proxy_ConnectThruProxy_Success(Uri server)

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