SmtpClient SendMailAsync with CancellationToken API implementation

[MihaZupan]

3f9f0e7 is the implementation of the API introduced in dotnet/corefx#24475

class System.Net.Mail.SmtpClient
{
    public Task SendMailAsync(string from, string recipients, string subject, string body)
+   public Task SendMailAsync(string from, string recipients, string subject, string body, CancellationToken cancellationToken)
    public Task SendMailAsync(MailMessage message)
+   public Task SendMailAsync(MailMessage message, CancellationToken cancellationToken)
}

The rest of the PR are SmtpClient tests using a mock SmtpServer.

CC @dotnet/ncl

[MihaZupan]

e513058#diff-e2b2d690217e90426b121345ba65abdcR377 is failing in CI as the test now actually checks that NTLM is used.

Looks like it is essentially this: https://github.com/dotnet/corefx/issues/28961.

Stack trace for reference:

System.Net.Mail.SmtpException: Failure sending mail.
 ---> System.ComponentModel.Win32Exception (0x80090020): GSSAPI operation failed with error - An unsupported mechanism was requested. NTLM authentication requires the GSSAPI plugin 'gss-ntlmssp'.
   at System.Net.Security.NegotiateStreamPal.AcquireCredentialsHandle(String package, Boolean isServer, NetworkCredential credential)
   at System.Net.NTAuthentication.Initialize(Boolean isServer, String package, NetworkCredential credential, String spn, ContextFlagsPal requestedContextFlags, ChannelBinding channelBinding)
   at System.Net.Mail.SmtpNtlmAuthenticationModule.Authenticate(String challenge, NetworkCredential credential, Object sessionCookie, String spn, ChannelBinding channelBindingToken)
   at System.Net.Mail.SmtpConnection.SetContextAndTryAuthenticate(ISmtpAuthenticationModule module, NetworkCredential credential, ContextAwareResult context)
   at System.Net.Mail.SmtpConnection.ConnectAndHandshakeAsyncResult.Authenticate()
   at System.Net.Mail.SmtpConnection.ConnectAndHandshakeAsyncResult.SendEHelloCallback(IAsyncResult result)
...

[davidsh]

@stephentoub PTAL the use of CancellationTokenRegistration.

[ManickaP]

/azp run runtime-libraries outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[stephentoub]

The src changes LGTM. Thanks.

[davidsh]

LGTM. You'll want to check the Outerloop CI results to verify the failures aren't related to the PR.

[MihaZupan]

Outerloop failures are unrelated, Mail itself doesn't have any OL tests.

[davidsh]

Outerloop failures are unrelated, Mail itself doesn't have any OL tests.

In that case, you are free to 'Squash and merge' this unless you want additional feedback from others.

[MihaZupan]

Thanks for the reviews!