Async APIs should take a CancellationToken

[Tratcher]

There are several async APIs in the feature interfaces or on HttpContext that developers may want granular timeout/cancellation support for. Especially anything that can trigger network IO.

Task<X509Certificate> IHttpClientCertificateFeature.GetClientCertificateAsync();
Task<Stream> IHttpOpaqueUpgradeFeature.UpgradeAsync();
Task<WebSocket> IHttpWebSocketFeature.AcceptAsync(IWebSocketAcceptContext context);
Task IAuthenticationHandler.AuthenticateAsync(IAuthenticateContext context);
Task<IReadableStringCollection> IFormFeature.GetFormAsync();

[Tratcher]

We're just doing GetClientCertificateAsync and GetFormAsync for now, as they can require waiting for client data.

AcceptAsync and UpgradeAsync just send response headers, and internally are not even async at the moment.

AuthenticateAsync's work happens mostly on the middleware's return path, not when invoked from app code.

[davidfowl]

It should be a single optional parameter as well

[Tratcher]

C# won't allow CancellationToken.None as an optional param, optional params must be compile time constants. I've added an overload in the relevant cases.

[Eilon]

Actually you can with default(CancellationToken) - see the engineering doc.

Relevant section:

---

Async method patterns

By default all async methods must have the Async suffix. There are some exceptional circumstances where a method name from a previous framework will be grandfathered in.

Passing cancellation tokens is done with an optional parameter with a value of default(CancellationToken), which is equivalent to CancellationToken.None (one of the few places that we use optional parameters). The main exception to this is in web scenarios where there is already an HttpContext being passed around, in which case the context has its own cancellation token that can be used when needed.
Sample async method:

public Task GetDataAsync( 
    QueryParams query, 
    int maxData, 
    CancellationToken cancellationToken = default(CancellationToken)) 
{ 
    ... 
}

---

[Eilon]

Oh duh now I see it already in the PR... that's what I get for reading stuff in chronological order...