RemoteAppClientOptions.BackchannelHandler should be a factory method

[duncansmart]

I've called AddRemoteAppClient like so:

services.AddSystemWebAdapters()
    .AddRemoteAppClient(options =>
    {
        options.BackchannelHandler = new MyBackChannelHandler();
    }

Where BackChannelHandler is basically just a HttpClientHandler with some logging. But a few times during testing I've got an ObjectDisposedException: cannot access a disposed object. object name: 'SocketsHttpHandler'. in my HttpClientHandler's SendAsync:

(sorry for the screenshot, didn't grab the plain text)

On investigation It would appear that HttpClientHandler should be a new instance every time, otherwise you eventually hit this ObjectDisposedException. See dotnet/runtime#27610 for another example.

Therefore I suggest that RemoteAppClientOptions.BackchannelHandler be changed so that it's something like a Func<HttpMessageHandler> instead, so that AddRemoteAppClient can call it to get a new instance each time.

For example:

systemweb-adapters/src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/RemoteAppClientExtensions.cs

Line 38 in 9921217

if (options.BackchannelHandler is { } handler)

Change:

if (options.BackchannelHandler is { } handler)
{
    return handler;
}

To something like:

if (options.BackchannelHandlerFactory is { } factory)
{
    return factory();
}

[duncansmart]

Also, according to the ConfigurePrimaryHttpMessageHandler docs remarks "The configureHandler delegate should return a new instance of the message handler each time it is invoked".

My workaround for now is to "manually" build the services. Instead of using AddRemoteAppClient - I call builder.Services.AddOptions<RemoteAppClientOptions>().Configure() and builder.Services.AddHttpClient("remote-client") and in ConfigurePrimaryHttpMessageHandler I can return a new instance of my HttpClientHandler each time.

[mjrousos]

Thanks for the report and investigation, @duncansmart. As you noted, getting a singleton HttpMessageHandler for use with HttpClientFactory configuration is incorrect. In this case, though, the HTTP configuration from RemoteAppClientOptions doesn't really need to come from a factory since it won't change and we can just use a singleton HttpClient. I'll get a fix together and open a PR.